WO2013116787A1 - Method and apparatus for lifetime extension of compact surface plasma source (csps) - Google Patents

Abstract

In a particular embodiment, a device is disclosed that includes means for providing a negative ion source comprising a compact surface plasma source (CSPS) with a significantly increased average beam current, increased brightness, and an extended operation time. The device also includes means for operating the negative ion source comprising the CSPS with the significantly increased average beam current, increased brightness, and the extended operation time for high current, high brightness negative ion beam production. In another particular embodiment, a method is disclosed that includes steps for providing a negative ion source comprising a compact surface plasma source (CSPS) with a significantly increased average beam current, increased brightness, and an extended operation time. The method also includes steps for operating the negative ion source comprising the CSPS with the significantly increased average beam current, increased brightness, and the extended operation time for high current, high brightness negative ion beam production.

Description

METHOD AND APPARATUS FOR LIFETIME EXTENSION OF COMPACT

SURFACE PLASMA SOURCE (CSPS)

INVENTOR:

Vadim Dudnikov, Ph.D.

J. Cross-Reference to Related Application

[0001] This application claims the benefit of U.S. Provisional Patent Application No.

61/593,498, filed February 1, 2012, which is hereby incorporated by reference in its entirety, as if set out below.

//. Field of the Disclosure

[0002] The present disclosure is generally related to high current, high brightness negative ion beam production and, in particular, to a negative ion source comprising a compact surface plasma source (CSPS) with a significantly increased average beam current, increased brightness, and an extended operation time.

///. Summary

[0003] In a particular embodiment, a device is disclosed that includes means for providing a negative ion source comprising a compact surface plasma source (CSPS) with a significantly increased average beam current, increased brightness, and an extended operation time. The device also includes means for operating the negative ion source comprising the CSPS with the significantly increased average beam current, increased brightness, and the extended operation time for high current, high brightness negative ion beam production.

[0004] In another particular embodiment, a method is disclosed that includes steps for

providing a negative ion source comprising a compact surface plasma source (CSPS) with a significantly increased average beam current, increased brightness, and an extended operation time. The method also includes steps for operating the negative ion source comprising the CSPS with the significantly increased average beam current, increased brightness, and the extended operation time for high current, high brightness negative ion beam production.

IV. Brief Description of the Drawings

[0005] The following figures form part of the present specification and are included to further demonstrate certain aspects of the present invention. The present invention may be better understood by reference to one or more of these drawings in combination with the description of embodiments presented herein.

[0006] Consequently, a more complete understanding of the present disclosure and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which the leftmost significant digit(s) in the reference numerals denote(s) the first figure in which the respective reference numerals appear, wherein:

[0007] Figure 1 is a diagram illustrating an embodiment of an apparatus including means for providing a negative ion source comprising a compact surface plasma source (CSPS) with a significantly increased average beam current, increased brightness, and an extended operation time and means for operating the negative ion source comprising the CSPS with the significantly increased average beam current, increased brightness, and the extended operation time for high current, high brightness negative ion beam production; and

[0008] Figure 2 is a flow diagram of an illustrative embodiment of a method including steps for providing a negative ion source comprising a compact surface plasma source (CSPS) with a significantly increased average beam current, increased brightness, and an extended operation time and steps for operating the negative ion source comprising the CSPS with the significantly increased average beam current, increased brightness, and the extended operation time for high current, high brightness negative ion beam production.

V. Detailed Description

[0009] Illustrative embodiments of the present invention are described in detail below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of the present disclosure.

[0010] Particular embodiments of the present disclosure are described with reference to the drawings. In the description, common features are designated by common reference numbers.

[0011] Referring to Figure 1, a diagram illustrating an embodiment of an apparatus is depicted and indicated generally, for example, at 100. The apparatus 100 includes means for providing a negative ion source comprising a compact surface plasma source (CSPS) with a significantly increased average beam current, increased brightness, and an extended operation time 110 and means for operating the negative ion source comprising the CSPS with the significantly increased average beam current, increased brightness, and the extended operation time for high current, high brightness negative ion beam production 120.

[0012] Referring to Figure 2, a flow diagram of an illustrative embodiment of a method is depicted and indicated generally, for example, at 200. The method 200 includes steps for providing a negative ion source comprising a compact surface plasma source (CSPS) with a significantly increased average beam current, increased brightness, and an extended operation time 210 and steps for operating the negative ion source comprising the CSPS with the significantly increased average beam current, increased brightness, and the extended operation time for high current, high brightness negative ion beam production 220.

[0013] Attached herewith as an Appendix to this specification is a document describing more details about various illustrative embodiments, which Appendix to this specification is incorporated by reference as if set forth below. More details about various illustrative embodiments may be found by referring to the Appendix. [0014] The present invention is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as those that are inherent therein. While the present invention has been depicted, described and is defined by reference to exemplary embodiments of the present invention, such a reference does not imply a limitation of the present invention, and no such limitation is to be inferred. The present invention is capable of considerable modification, alteration, and equivalency in form and function as will occur to those of ordinary skill in the pertinent arts having the benefit of this disclosure. The depicted and described embodiments of the present invention are exemplary only and are not exhaustive of the scope of the present invention.

Consequently, the present invention is intended to be limited only by the spirit and scope of the appended claims, giving full cognizance to equivalents in all respects.

[0015] The particular embodiments disclosed above are illustrative only, as the present

invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of composition or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and intent of the present invention. In particular, every range of values (of the form, "from about a to about b," or, equivalently, "from approximately a to b," or, equivalently, "from approximately a-b") disclosed herein is to be understood as referring to the power set (the set of all subsets) of the respective range of values, in the sense of Georg Cantor. Accordingly, the protection sought herein is as set forth in the claims below.

[0016] The particular embodiments of the present invention described herein are merely

exemplary and are not intended to limit the scope of this present invention. Many variations and modifications may be made without departing from the intent and scope of the present invention. Applicants intend that all such modifications and variations are to be included within the scope of the present invention as defined in the appended claims and their equivalents.

[0017] While the present invention has been illustrated by a description of various

embodiments and while these embodiments have been described in considerable detail, it is not the intention of the Applicants to restrict, or any way limit the scope of the appended claims to such detail. The present invention in its broader aspects is therefore not limited to the specific details, representative apparatus, methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the scope of Applicants' general inventive concept.

Lifetime extension of compact surface plasma source (CSPS).

Vadim Dudnikov

The problem of intense, high brightness negative ion beam production for accelerators (including nuclear polarized negative ions) and for neutral beam injection was solved, in general, by the discovery of the "cesiation effect", whereby FT emission is enhanced while co-extracted electron current is simultaneously decreased below the FT current by a small admixture of cesium vapor into the gas discharge ion source [ 1 ] , In subsequent experiments [2-5] it was demonstrated that cesium adsorption decreases the surface work function, enhances secondary emission of negative ions, and catalyzes the surface plasma generation (SPG) of negative ions caused by the interaction of the plasma with the electrode surface. Ion sources based on this process have been named surface-plasma negative ion sources (SPS). The high brightness SPS with Penning discharge (PD) was developed and described in [4] . During the last 30 years, proton accelerators with charge exchange injection have used some versions of the compact SPS (cesiated magnetron SPS and cesiated Penning discharge SPS) or Large Volume (LV) SPS (surface converter SPS). Reviews of SPS development and using in accelerators and in neutral Beam injectors are presented in Refs [3, 5-9] . For new accelerator projects it is necessary to develop negative ion sources with significantly increased average current, increased brightness, and extended operation time [9] . In response to these challenges, advanced versions of magnetron SPS and PD SPS are proposed and considered.

II. COMPACT SPS

A. Magnetron (Planotron)

The cesiated magnetron (planotron) SPS was invented by Dudnikov [1] in BINP, Novosibirsk and developed with Belchenko and Dimov up to 1 A of FT current [5] . The magnetron design, used in FNAL, BNL, ANL, and DESY was developed and adapted by C. Schmidt, where it has been operational in the Tevatron accelerator complex since 1978 [6] . The efficiency of FT generation and lifetime were improved significantly by geometrical focusing developed by Dudnikov [3, 10] . The peak current of the FT ion beam at the exit of the 750 keV_accelerator column is ¾, -70 mA with U_ex = 25 kV with a beam pulse length T= 0.075 ms at 15 Hz. The design of the magnetron with spherical focusing (developed by J. Ales si in BNL) is shown in Fig. 1 (recently it was improved by machining a cylindrical groove around the entire cathode.). This design is optimized for low duty factor (df) operation with a low average discharge power of P - 50 W.

In these magnetron designs, the optimum cathode temperature T_c - 500°C, and the anode temperature T_a - 250° C are relative high. For this reason, the cathode and anode are thermally insulated from the air-cooled base plate by Macor machinable ceramic. The cathode is supported by insulators made of machinable ceramic Macor with very low thermal conductivity (-0.014 W/cm-K) and with a weak contact between parts. The anode cover (plasma plate), which contains the emission aperture, is very thin and it is thermally insulated. The small cathode anode gap (-1 mm) is enough to hold the discharge voltage without short circuiting during long term operation. This design performs very well for production of up to 100 mA pulses with dfup to 0.5%. In discharge without cesium, the discharge voltage is ~ 600 V and sputtering is very strong. With low voltage discharge (Ua -100 V) and good optimized cesiation, electrode sputtering caused by the discharge is very low. A more significant limitation of this CSPS lifetime is due to cathode sputtering caused by back accelerated positive ions, which stems from the use of a simple diode extraction system without back acceleration of positive ions suppression.

Fig. 1. Cathodes sputtering of the FNAL magnetron after long time operation. Flat cathodes are in first left column, cathodes with cylindrical focusing in second column. Assembling of magnetron is on right bottom. The sputtering by back accelerated positive ions through the emission slit is dominating.

Nevertheless, this very compact and simple version of the magnetron has worked sufficiently well for high voltage pre-injectors and RFQs. With noisy discharge the efficient transverse ion temperature is relatively high -10 eV. The normalized rms emittance with a 10 mm long slit is s_ny~ 0.5 (π mm mr). The magnetron has many years of operational accelerator experience. In all cases, magnetron sources are able to satisfy the requirements of the accelerators, and in most cases run at the space-charge limit for extraction.

All magnetron sources manage to provide beams for up to 9 months (up to 3.6x10 pulses ~ 2 A hrs) per year before requiring dismounting and cleaning. In the HERA Linac, a cesiated magnetron was tested for continuous operation for 32 months with low cesium consumption [7] . The duty factor can be increased by replacing the Macor insulators by A1N ceramic, which has much higher thermal conductivity. The magnetron with cathode cooling by compressed air for larger average current production is described by J. Peters in [7].

The compact SPS lifetime is determined by flakes formation cased the cathode sputtering. Example of cathode sputtering are presented in Fig. 1. The sputtering is produced by ions of discharge and by back accelerated positive ions penetrating through an emission slit. A sputtering rate by back accelerated positive ions is higher then by discharge ions.

Microscopic photo of the sputtered cathode surface is shown in Fig. 2. The sputtered surface is very rough with a roughness size - 0.1 mm because cathode's material is powder sintered Molybdenum which was not melted. The preliminary surface processing by melting should decrease a sputtering rate and produce smooth emitting surface necessary for high efficiency of negative ion emission and good geometrical focusing.

Fig. 2. Microscopic photo of the sputtered cathode surface opposite to the emission The roughness size - 0.1 mm.

Examples of anode sputtering and flakes formation in penning discharge SPS (ISIS RAL version) are shown in Figs. 3-8

Fig. 5. Examples of cathode sputtering deposition in PD SPS after long time operation. Sputtering by discharge is not limited SPS operation because it can be balanced by material deposition.

at o e ano e assem ng ater ong tme operation (24 days)

Fig.6. Examples of the cathode anode assembling of the PD SPS after long time operation. Flakes formation can be a reason of short circuit between cathode and anode.

Fig. 7. Example of flakes formation on the plasma electrode internal ( flakes deposition, melting). This flakes can close emission slit and distort the negative ion emission.

Fig. 8. Plasma electrode external (ion bombardment around the slit). Actual emission slit is 0.6x10 mm .

Flakes formation on anode surface of CW PD SPS after long time operation is shown in Fig.9.

Fig.9. Flakes on anode surface of CW PD SPS after long time operation.

These flakes can be evaporated by high current pulsed discharge during SPS operation.

Fig. 10. INP PD SPS after long time operation.

CSPS need be cleaned after long time operation and can be used for further operation after replacing of Cesium source (cesium chromate pellets or cesium ampoule). Photo of PD SPS after cleaning is shown in Fig. 10.

Sputtered material is deposited to hot plasma bombarded cathode surface as solid bulk deposit. In Fig. 11 are shown examples of cross sections with boundary of deposited materials which demonstrates high adhesion of deposit to the cathode surface. This solid deposition should not limited SPS lifetime.

Fig. 11. Examples of cross sections with boundary of deposited materials which demonstrates high adhesion of deposit to the cathode surface. Fig. 12 below from Tan et al, "The 750 keV injector upgrade plan" FNAL report demonstrate closing of the channels for gas and cesium delivery into discharge chamber by flakes created by cathode sputtering (mainly by cesium ions).

Fig. 12. This figure compares the postmortem of a broken source to a new

source. High arc current operation causes the cathode to erode and to deposit

some of it onto the anode which blocks the cesium inlet.

The lifetime relative the closing of these channels and emission slit by flakes

extended by using of special shape of cathode shown in Fig. 13.

Fig. 13. Modified magnetron SPS. Left-cross section perpendicular to the magnetic field, right is cross section along the magnetic field.

1- cathode; 2-anode; 3-cylindrical grove; 4- spherical dimple; 5-flux of negative ion;

6- negative ion beam;7-extractor. Yellow is insulator.

In this version of the magnetron a discharge is supported in the cylindrical grove 3 as in semiplanotron. Emitted negative ions are focuses by the cylindrical surface of this grove to surface of anode. The ions sputter deposit preventing the flakes and deposit formation. Sputtering of deposit by focused negative ion was observed during long time operation of semiplanotrons SPS with a spherical focusing as shown in Fig. 14.

Fig. 14. Anode of the spherical semiplanotrons SPS after long time operation. A surface around the emission aperture is clean because sputtered by focused fast particles from the cathode.

The surface of the anode around of emission aperture is cleaned by focused negative ion 9and fast neutral) flux. A part of grove connected with the spherical dimple has conical shape with decreased deepness and radius near the dimple for decrease the gas flow to the extraction area.

For efficient geometrical focusing of the emitted negative ions the emission surface should be mirror smooth because surface irregularity can increase of angle spread of emitted negative ions. The smooth emission surface can be produced by melting of the emission surface by pulsed electron beam. For smooth surface production it is need to have a cathode material of high purity because small impurities can induce surface structures during the melt solidification. Examples of such structures are shown in Fig. 15.

Fig. 15. Wave structure on the surface after melt solidification. Copper with Tin 0.5%. Another possibility to decrease rate of cesium channel closing by the sputtered material is a drilling of hollow cathode channel (8) as shown in Fig. 16.

Fig. 16. Schematic of magnetron SPS with spherical focusing and hollow cathode opposite of the cesium inlet.

In magnetron with cylindrical grove (3) can be higher efficiency of negative ion generation because discharge is concentrated on the smaller surface and the necessary discharge current density can be produced with lower discharge current. Efficiency of negative ion generation can be increased by using for discharge only part of the cathode circumference by fabrication of the cylindrical grove on this part and by decrease a cathode perimeter with discharge.

The magnetron beam quality can be improved by suppressing discharge noise, which will increase beam brightness. One possibility for noise suppression is to use a hollow cathode as shown in Fig. 16. This technique has been successfully used in semiplanotrons [3, 8, 10,11] for discharge noise suppression.

Examples of magnetron cathode sputtering by back accelerated positive ions (left); plasma plate sputtering(blistering) by back accelerated positive ion beam (middle); extractor damage by co-extracted electrons (left). From report of Lettry et all. (CERN, BNL) are shown in Fig. 17. The beam of back accelerated positive ions is well focused and drill narrow, deep channel. The diameter of this channel is relative small (~1 mm2) and it should have relative small influence to the negative ion emission. Sputtering of material Mo, processed by surface melting, could be uniform and can keep the good geometrical focusing. A preliminary drilling of sputtering channel can prevent the flakes formation from sputtered material because this material can be deposited inside of this channel. The sputtering by back accelerated positive ions can be decreased with using of two stage extraction/acceleration with lower voltage on the first extraction gap and by suppression of positive ion from negative ion beam by suppression electrode with the positive voltage. Cathode of Spherical semiplanotron after long time operation ( ~ 6 weeks) continuously is shown in Fig. 18. The cathode is well polished through the sputtering by hydrogen ions from discharge with a hollow cathode in the crossed ExB fields.

Fig. 17. Examples of magnetron cathode sputtering by back accelerated positive ions (left); plasma plate sputtering(blistering) by back accelerated positive ion beam (middle); extractor damage by co-extracted electrons (left). From report of Lettry et all. (CERN, BNL). Counters below are lines with desired deepness of erosion.

In the semiplanotrons plasma can drift in the crossed fields though a small cathode anode gap to the insulator which can initiate a surface discharge along the insulator surface. Design of magnetron SPS with the spherical focusing of emitter negative ions and forced cathode and anode cooling is shown in Fig. 19. This new magnetron SPS is capable for DC operation with high average negative ion current generation.

Fig. 18. Cathode of Spherical semiplanotron after long time operation ~ 6 weeks continuously. Emission current density was increased up to 1 A/cm in DC mode of operation.

Fig. 19. Cross sections of magnetron SPS with cathode cooling, (top-median transverse to the magnetic field; bottom- section along the magnetic field).

1- cathode disc; 2-anode; 3-insulators; 4-magnetic poles; 5-cooling tube; 6-spherical dimple (negative ion emitter, R=4 mm); 7-cylindrical grove (discharge channel, r=3mm); 8-flux of focused negative ions; 9-negative ion beam extracted through emission aperture (2 mm diameter); 10- gas inlet; 11-cesium inlet; 12-extractor; 13- magnet. Cross sections of new magnetron are shown in Fig. 19. A disc shape cathode (1) has 18 mm diameter D and 12 mm thickness H. A surrounded anode (2) is separated from the cathode by insulators (3). A vacuum gap between cathode and anode is d~l mm.

Cathode is cooled by liquid or gas flux flowing through the cooling tube (5) with ID~4 mm. The magnetron is compressed by ferromagnetic poles (4).

A working gas is injected to the discharge chamber through a channel (10). Cesium is added to discharge through second channel (11). Magnetic field, created by magnet (13) and formed by magnetic poles (3) has direction along axis of cooling tube (5). The discharge in the crossed ExB fields is localized in the cylindrical grove (7) as in the semiplanotrons SPS. The cylindrical grove focus emitted negative ions to the anode surface and fast particles keep anode surface clean by sputtering the flakes and deposit. A plasma drift in the discharge can be closed around the cathode perimeter or can be bracket by shallow cylindrical grove. For beam formation are used negative ions emitted from the spherical dimple (6), geometrically focused to the emission aperture made in anode (2). These ions are extracted by electric field applied between anode (2) and extractor (12). Emission aperture of ~2 mm diameter has a conical shape. The spherical dimple with a curvature radius R~4 mm has a working surface S-12 mm . For the emission current H- of 0.1 A it is necessary to have the emission current density on the cathode surface Je~

1 A/cm , which is acceptable for pulsed operation. The emission current density of H-

-0.1 A/ cm necessary for 10 mA extraction is acceptable for DC operation.

Anode (2) is cooled by gas or liquid flow flowing through the cooling tube attached to the anode front.

Material of cathode and anode for H- beam production is Molybdenum. The surface of spherical dimple should by mirror smooth for efficient negative ion emission and sharp focusing into the emission aperture. For smoothing the surface and improve the sputtering resistance it is possible to use the surface melting by pulsed electron beam.

For heavy negative ion production it is possible to use some compound with necessary elements and necessary emission and discharge properties such as LaB₆.

Two stage extraction/acceleration is preferable for operation with high average beam current for collection of co-extracted electrons to the electrode with low potential.

B. Penning discharge SPS

The Penning Discharge SPS (shown in Fig. 20) uses a discharge with an anode window, surrounded by cathodes at each end, aligned along the magnetic field. Extraction of the ions is through a slit in the anode perpendicular to the magnetic field. The Penning Discharge SPS was invented by Dudnikov in BINP [4]. It has had a long history of development at LANL [9, 13, 14]. Now it is successfully used at ISIS RAL [15, 16] and is under development for the Chinese SNS. The fundamental difference between the magnetron and Penning sources is that in the magnetron, FT ions produced at the cathode are directly extracted, while in the Penning source the ions must undergo a charge- exchange process or scattering on atomic hydrogen to reach the emission aperture since there is no direct path from the cathode to the aperture. Discharge noise can be eliminated in a cesiated Penning SPS by optimizing the magnetic field and gas density or using a small admixture of heavier gas (N₂ in [14]). In this regard, emittance measurements have shown the Penning SPS always has higher brightness than the magnetron (and other ion sources). The effective ion temperature can be as low as Tj~ 1 eV.

The LANL IX Penning and ISIS Penning SPS have essentially the same discharge chamber dimensions as in the first version of the Dudnikov type SPS [4] . The RAL Penning SPS that is in use at the ISIS facility delivered 35 mA (discharge df~2.5 %, beam df ~l%) after 650 kV pre- acceleration for a period up to 50 days [9,16], (the lifetime is 25 days with 50 mA).

This version of SPS shown in Fig. 18 has limited cooling because the prototype was optimized for low (^operation. The cathode cooling was improved by contact with a water cooled flange through a mica layer. But this layer has low thermal conductivity and limited heat transfer. The anode is cooled by air flow. However, the thin plasma plate that includes the emission slit has low thermal conductivity and is easily overheated. This ion source is currently under redevelopment at RAL for possible use on the European

Spallation Source (ESS). The development goals are 70 mA H^" current with a short pulse of 1.2 ms at 50 Hz, and 70 mA H^" current with a long pulse of 2.5 ms for 50/3 Hz. The design emittance (ms, normalized) is < 0.3 (π mm-mr) with lifetime greater than 20 days [9,16] .

A Penning SPS for higher average current was built and tested at BINP by V. Dudnikov and co-authors [17] . Photo of this PD SPS is shown in Fig. 10. Operation with beam current above 100 mA in 0.25 ms pulses with repetition rate of 100 Hz has been demonstrated for >300 hours (df= 2.5%). Integral of the generated beam charge is ΓΓ= 7.5 A hours ~2 k Coulomb. Operation with a repetition rate of up to 400 Hz (df 10%) has been tested. Distinctive features of this Penning SPS compared with the ISIS source are its slightly larger discharge cell and more massive anode cover (plasma plate) with forced air or water cooling. The cathode has a strong pressed contact with a copper cooler. It is cooled by strong flow of water. A fast (0.1 ms) gas valve [12] is used to inject gas at a repetition rate up to 500 Hz. Stable support of noiseless discharge has been established which is important for high brightness beam production.

Efficiency of negative ion generation can be increased by increase of emission aperture because a large part of the generated negative ions can be extracted. However, the size of emission aperture is limited by the minimal gas density in the discharge chamber because negative ion stripping on the escaping gas target become significant. The minimum gas density necessary for stable discharge can be lower by increase the distance between cathodes in the Penning discharge SPS.

At LANL, PD SPSs were designed and constructed applying plasma scaling laws and increasing two of the source dimensions by a factor 4 (the 4X source) and by the factor 8. This reduced the cathode power load from 16.7 to 2.24 kW/cm while increasing the FT current from 160 mA (0.5x10 mm² slit) to 250 mA (2.8x10 mm² slit) [9, 16]. Cathodes of these SPS are shown in Fig. 21. Actual size of Small Angle Source (SAS) source is similar to the original Dudnikov type source [4].

Fig. 21. A photograph of the SAS, 4X, and 8X source cathodes (top line) and cathodes with anodes (bottom line). The 4X source cathode-cathode gap, 17 mm, is four times the SAS cathode-cathode gap, 4.3 mm. The 8X source cathode-cathode gap, 34 mm, is eight times the SAS cathode-cathode gap.

The measured rms normalized emittance is 0.15 π mm-mrd in the narrow slit dimension (2.8 mm). Emittance in the long slit dimension (10 mm) is 0.29 π mm mr for an un- optimized slit extraction system at 29 keV extraction energy. It is possible that the last emittance increase, which affects only a small part of the beam, is connected with end effects of the slit. In this case, it can be improved by collimation. Increased emission current density (up to 0.35 A/cm ) of FT was produced in the

Dudnikov type source with LaB₆ cathodes without Cs after electrode activation by a high current discharge [18]. Later, increased emission current density was produced in a multicusp FT source with a hot LaB₆ cathode [19,20], where surface plasma FT generation was enhanced by high current discharge with negative biasing of the plasma electrode during electrode activation. This enhancement can be explained as due to the decrease of the surface work function connected with La deposition.

However, it was observed in many cases including [20] that in discharges with LaB₆ cathodes, the "cesiation effect" is not as strong as with W or Mo hot or cold cathodes. Therefore, in [20] it was concluded that the work function of the surface coated with B, La, and O cannot be decreased by seeding Cs. Insufficient deoxidization of the Mo-PE surface, caused by a temperature lower than 300 °C due to the thermal conduction to the copper plate, is probably the cause of the lower I _H · It is well known from work function research that smaller work function can be produced by cesiation of the surface with larger work function. By this reason the cesiation of surfaces with low work function deposited by lanthanum can produce only relative small improving of work function.

It is possible to increase cathode -cathode surface gap up to 10-11 mm by remove a cylindrical volume of cathode as shown in Fig. 1.

Some simple modification of existing PD SPS can be used for increase the SPS lifetime.

1. The use of a Tungsten anode should improve the lifetime of the source because

Tungsten has a better sputter resistance.

2. Changing the aperture plate design by removing the ribs, adding a 45° champher on the inside edge of the aperture slit and making the plate thicker to improve thermal

conductivity. This should increase the output current, improve source lifetime and decrease a cesium escaping. Proposed modifications ISIS PD SPS cathode, anode and plasma plate are shown in Fig. 22.

3. Introducing magnetic inserts in the extract electrode to shape the magnetic field in the extraction region to minimise penning discharge in the extraction gap. This will reduce extraction current and reduce extraction breakdowns.

4. To make a scaled source the only dimension that needs to be scaled is the distance between the cathode jaws. A larger source should produce a higher output current.

5. Moving the position of the H₂ gas pulse should minimise output beam current droop during the beam.

6. Old cathode version with a larger diameter is more suitable for higher duty factor operation. Increase of cathode slit size and increase of anode insert dimension can be used for reusing of all old used cathodes.

7. Introducing of suppression electrode for reflection of positive ions from the beam ( as shown in presentation) can be useful for decrease of ion source damage by back

accelerated positive ions.

Fig. 22. Proposed modifications of cathode, anode and plasma plate with possibility for reusing of used cathodes of ISIS PD SPS.

C. Advanced Penning discharge SPS

In design of the scaled PD SPS are repeated a design of Dudnikov-type source with cylindrical cathode which needs expensive large pieces of Mo for fabrication. More convenient and less expensive design of the scalable PD SPS is presented below.

The FT ion temperature Ti in Penning discharges can be ~1 eV [4,9] . Accordingly, for an emission slit with length 21, the normalized rms emittance εη=0.045 x / (Ti)^1/2 π mm mrd can be -0.25 π mm mr. The effective ion temperature in the transverse direction is about 3 times larger but the transverse slit size is smaller and so this emittance is smaller than 0.25 π mm- mr. The schematic of the proposed modified PD SPS is shown in Fig. 23. The design and operation of the proposed source are clear from the captions to Figs. 20, 21,22,23 and 24.

This PD SPS can be assembled from Mo plates with thickness of -10 mm and can have direct intense cooling of cathode and anode by flow of gas or liquid. Heat pipes (or thrmo siphons) also can be used for SPS thermo stabilization. With a cathode-cathode gap L-10 mm it is possible to have emission slit width d~2-2.5 mm instead d~0.5-l mm for SPS with L-4-5 mm because gas density is inverse proportional to L. With a large emission aperture it is possible to have lower emission current densityj~0.5 A/cm2 and lower discharge current density with increase the operating lifetime. This lifetime extension can be significant because lifetime has strong nonlinear dependence on intensive parameter near threshold level. The SPS lifetime can be increased by using of the high current pulses for evaporation of the internal short circuit caused by flakes formation and with using of flakes gasification by discharges in NF₃ or by XeF₂ as in ion implantation

Fig. 23. Cross sections of Penning Discharge SPS with cathode cooling, (left-mediane section along magnetic field; right- section transverse to the magnetic field

I- cathode plate (cathode-cathode distance L 10 mm); 2-anode; 3-insulator plates; 4- magnetic poles plates; 5-cooling tube; 6-emission slit (2x10 mm ); 7-anode window with discharge plasma (discharge cross section 3x15 mm ); 8-channel for Cesium inlet; 9-negative ion beam extracted through emission slit (2x10 mm ); 10- gas inlet;

I I- extractor; 13- magnet.

Cross sections of Penning Discharge SPS with cathode cooling are shown in Fig. 23 (at left is median section along magnetic field; right- section transverse to the magnetic field). This PD SPS consist of: cathode plates (1) with a cathode-cathode distance L-10 mm; anode (2) with anode window (7) with discharge cross section 3x15 mm ; cathode and anode connected by insulators plates (3) with attached magnetic poles (4); cathode is cooled by cooling tube (5). Insulators (3) is made from stress resistance ceramic as A1N and pyrolitic BN. The plasma is generated by discharge with electron oscillation in the magnetic field between cathode surfaces and anode window. The working gas (hydrogen) is injected into discharge through channel (10) by pulsed valve or mass flow controller. Cesium is delivered from small oven through channel (8). The negative ion beam is extracted through emission slit (2x10 mm ) by extractor (11). Magnetic field is formed by the magnetic poles (4) and by magnet (13).

In discharges with high plasma density and increased distance between cathode surfaces (1) and emission aperture, the FT ions from the cathode can't reach the emission slit without destruction. In this case, the surface plasma generation of FT on the plasma electrode (anode SPG) around the emission aperture is most important. In previous experiments it has been demonstrated that this anode SPG is efficient (-15 mA/kW). The cesium admixture decreases the work function of the cathode and anode to increase the secondary emission of electrons and negative ions. For stability of the optimal cesium film it is important to maintain the optimal surface temperature, which is easier for larger sources. The cesium concentration and conditions for SPG should be optimized on the plasma plate surface around the emission aperture. A more detailed drawing of the extraction/post- acceleration system is shown in Fig. 24.

FIG. 24. Extraction system of DT SPS. 1-cathode; 2-anode; 3-source body; 4-cooled plasma plate; 5-anode cooling; 14- extractor; 15-magnet; 18-negative ion beam; 19- suppressor/deflector; 20-acceleration electrode; 21-electron flux; 22-reflector.

Slit extraction is very adequate for FT production by an anode SPG. Low ion

temperature is preserved very well during slit extraction. The observed increased emittance along the slit [14], is due to aberrations that affect a small fraction of the beam extracted from the ends of the slit and can be decreased by collimation.

Beam acceleration and focusing by transaxial lenses is shown in Fig. 25 is sutable for ribbon beam formation.

Scale ~ 1: 1 emission slit is 1x10 MM . Magnetic field is created by permanent magnets (violet) abd by magnetic poles (5, green).

A ribbon beam of FT (3) is extracted from PD SPS (1) through emission slit of 1x10 mm (2) by extactor (4) in magnetic field of SPS magnet (5) as in previous versions of PD SPS [24].

Extracted beam (3) further accelerated in the cylindrical gap between electrode (4) and electrode (6) serving as the transaxial electrostatic immersion lens. This lens has many parameters which can be used for independent optimization of the focusing properties in perpendicular transverse directions. By variation of radiuses and centers location of cylindrical gap is possible to change the focusing force strength and sign along the emission slit with relative low change to the beam focusing in the orthogonal direction.

Fig. 25: PD SPS with transaxial lens focusing (left- cross section along magnetic field; right-cross section perpendicular to magnetic field).

1- PD SPS; 2-emision slit; 3-ion beam; 4-extraction electrode; 5-magnet of SPS; 6- grounded electrode of transaxial immersion lens; 7-compensating magnet; 8-first transaxial lens.

The shape of accelerating (decelerating) gap can be used for correction of aberrations. Shifting of lens plates can be used for correction of beam direction and position. The voltage between plates can be used for beam direction correction and for beam deflection. The compensating magnet (7) is used for restoring beam direction distorted by SPS magnet (5). In design of extraction and accelerating, focusing gaps it is necessary to avoid an electron trapping and conditions for high vacuum discharges.

In discharges with high plasma density and increased distance between cathode surfaces (1) and emission aperture, the FT ions from the cathode can't reach the emission slit without destruction. In this case mainly the surface plasma generation of FT on the plasma electrode (anode SPG) around the emission aperture is important. In previous experiments it has been demonstrated that this anode SPG is efficient (-10 mA/kW with slit 0.5x10 mm ). The cesium admixture decreases the work function of the cathode and anode, to increase the secondary emission of electrons and negative ions. For stability of the optimal cesium film it is important to maintain the optimal surface temperature, which is easier for larger sources. The cesium concentration and conditions for SPG should be optimized on the plasma plate surface around the emission aperture. A more detailed drawing of the extraction/post- acceleration system is shown in Fig. 24.

Slit extraction is very adequate for FT production by an anode SPG. Low ion temperature is preserved very well during slit extraction. The increased emittance along the slit, observed in work [27], is due to aberrations that affect a small fraction of the beam extracted from the ends of the slit and can be decreased by collimation.

A three or four electrode extraction system will be optimized to produce beam optics with minimum aberrations and low co-extracted electron current. The electron flux (21) is collected along magnetic field lines to the electron dump. It is important to suppress the secondary emission of FT and cesium ions from the extractor. Suppression electrode (19) is used for collection of slow positive ions to prevent their acceleration into the discharge chamber which is important for suppression of electrode sputtering by these positive ions. Positive ions in the acceleration gap should be defocused by the electric field and collected. The reflector (22) is used to reflect positive ions generated in the FT beam.

All these comments are applicable to the DC Penning SPS discussed in publication [13]. The combination of proposed improvements can deliver high quality FT beam with a pulsed intensity - 100 mA and with average (CW) current up to 20 mA at the discharge power < 2 kW. Lifetime of this SPS can be extended to ~ 7 A hours (~ 700 hours for 10 mA average beam current).

Cesium atom excitation by a resonant laser beam (16) will be used in further for effective suppression of cesium loss from the discharge chamber as disclosed in [33].

Ion source and implanter cleaning and recovery of

discharge gap short circuit.

The operation time of ion source, plasma sources, magnetron deposition system with a discharge is limited by cathode erosion in plasma, deposition of conductive films to the insulators and flakes formation with a short circuit of a discharge gap between insulated electrodes. For removing of deposited films and flakes is proposed to use a discharge in Fluorides such as NF3. Fluorine radical interacts with deposited films and flakes and form a high vapor pressure compound. These compound are pumped by forevacuum pump and can be transformed to harmless materials. C4F8 can be used as less expensive alternative to NF3. For embodiment of this discharge cleaning a cleaning gas should be delivered into Ion Source and into implanter by corresponded gas delivery system. Relative high gas pressure P-l-5 Torr can be produces with not to high gas flow by using a slow pumping by fore vacuum pump. Cleaning discharge can be supported by voltage, applied between chamber wall and any existing insulated electrodes, as cathode, extractor, Faraday cup, for discharge support can be used existing power supply such as discharge power supply, or can be used special DC or RF power supply. With gas pressure p~l Torr discharge voltage should be 400-600 V . Discharge current -0.1 A can be enough for ion source or implanter cleaning during -10 min. Schematic of implanter with gas delivery to ion source, for beam neutralization and cleaning is shown in Figure 1.

Figure 1. Schematic of implanter with gas delivery to ion source, for beam neutralization and cleaning. 1- pumping; 2- ion source; 3- extraction; 4 magnet analyzer; 5- swiping magnet; 6- will; 7- processing chamber; 8- Faraday cup; 9- gas cylinder; 10- gas valve; 11-pressure regulator; 12-gas lines; 13-mass flow control; 14- gas nozzle for beam neutralization; 15- beam neutralizer; 16- plasma source for wafer neutralization; 17- mass slit.

Gases with high probability of negative ion formation and low cross section of charge exchange used for low energy ion beam neutralization such as NF3, BF4, CF4,

H20,....Some of these gases can be used for implanter cleaning.

Cleaning processes using plasmas under remote (downstream) conditions are now being commercially implemented. The processes use NF3 as the fluorine source; some limited studies on fluorocarbon gases suggest significantly lower cleaning rates versus NF3. A study of various fluorocarbon gases under remote plasma conditions were studied. The first results reveal a number of differences versus in-situ plasma cleans. The optimum (for maximum oxide etch rate) gas composition of the mixtures with oxygen is significantly different between in-situ and remote plasmas, and the maximum etch rates between the two fluorocarbon compound are significantly different as well. A second set of tests, conducted under somewhat different conditions, show differences in the exact values of the etching rates, but generally agree with the results of the first study.

Laboratory studies of remote plasma clean processes presents greater challenges than for in-situ plasmas. With in-situ plasmas, active cleaning species (principally atomic fluorine) are being actively created in the same region as the film to be removed. In contrast, active species in remote processes are transported from the plasma-containing volume to another region for film removal. In this case, active species recombination (in the case of NF3, to F2; in the case of CxFy/Cte, to CF4 and COF2) - which occurs mainly on reactor surfaces - becomes an important element of process performance. Thus, reactor parameters such as surface-to-volume ratio, and the surface composition in the downstream region, can have important influences on the relative and absolute results seen. Consistent with these details, we have found differences in the relative etch rates of fluorocarbons versus NF3 in our tests, compared to commercial units. Thus, in the latter case, NF3 etch (cleaning) rates are typically about twice as fast for fluorocarbons, while in our studies, the NF3 etch rates (not shown) are five times or more fast. We are continuing to study the details of fluorocarbon behavior and process performance in the remote reactor configuration.

Some Practical Considerations

The boiling point of C-C4F8 (-6 °C) is higher than for other widely used chamber cleaning gases such as C2F6 (-78 °C) or NF3 (-129 °C). As a result, the vapor pressure is relatively low (25 psig), presenting a potential issue for pressure drop at the higher gas flows used in chamber cleaning, compared to the previous applications in dry etching. A substantial pressure drop could result in process control issues, if the pressure were to move below the ca. 10 psig normally required as the minimum input pressure to commonly used gas MFCs. We have examined this issue from both an experimental and theoretical viewpoint, and find that the pressure drop is in fact minimal under the conditions expected for chamber cleaning applications. A summary of the data is presented in Figures 5 and 6, with complete details available elsewhere [5]. Note that for the Novellus and Applied Materials processes described earlier, flow rates of C-C4F8 for optimized processes are less than 0.5 slm. Thus, pressure drop considerations should not be an issue for drop-in replacement of C-C4F8 into gas handing systems currently used for other chamber cleaning gases.

For discharge support in ion source usually used power supplies with a current

stabilization with current limitation for sparking prevention. A typical discharge current Id---1-10 A is small enough for long time conducting by these short circuit. It was observed, that during operation of pulsed discharge with a low impedance forming line a flakes formation is significantly suppressed and short circuit, created by DC discharge could be recovered. Short circuit created by conductive film deposition to the insulator or flakes can curry a low discharge current but can be evaporated by high pulsed current.

Evaporated material create a dust, accumulated in any pockets in gas discharge chamber without disturbing of discharge. It is proposed to use a special power supply for increase an ion source lifetime. This power supply for supporting a discharge consist of a limited current current power supply (1) with a voltage and current stabilization and limitation and a parallel charging through resistor (3) the low impedance forming line (2) with a switch (7) for connection of this forming line to the discharge gap, between cathode (6) and anode (5) if discharge gap is closed by

insulator deposition of by flakes. Serial diode (4) could be used in line between power supply and discharge gap for prevention of power supply from forming line current.

Diagram of advanced power supply with device for recovery of a discharge gap is shown in Fig. 2.

Fig.2. Schematic of advanced power supply for long time discharge operation.

1- power supply with a limited current (DC or pulsed); 2- high current pulsed power supply; 3- charging resistor; 4- diode; 5- discharge anode; 6- discharge cathode; 7- pulsed switch.

[I] V. Dudnikov, SU Author Certificate, C1.H01 3/04, No. 411542, 10 March,1972, http://www.fips.ru/cdfi/reestr_rupat.htm; patent number 411542.

indico. cern. ch/conferenceOtherViews.py ?showSession...

Jun 8, 2011 - 17:09 CESIATION PATENT, Dudnikov

Theory and design o charged particle beams - Google Books Result

books.***.com/books ?isbn=3527407413... Martin Reiser - 2008 - Science - 647 pages

A cesium catalysis of negative ion formation in gas discharge was discovered by

Dudnikov in 1971 during the work for H- source development for the larger ...

[2] V. Dudnikov, Doctor Thesis, INP, Novosibirsk, 1977.

[3] V. Dudnikov, Rev. Sci. Instrum. 63(4), 2660 (1992); Rev. Sci. Instrum. 73(2), 992 (2002).

[4] V. Dudnikov, in Proceedings of 4th All-Onion Conf. on Charged Part. Accel., Moscow, Moskva, Nauka, 1974, V.l, p.323; English Translation, LASL. LA-TR-75- 4, 1975,

www.brookhaventech.com/pdf/dtpsps 1.pdf .

[5] Yu. I. Belchenko, G. I. Dimov, and V. G. Dudnikov, BNL 50727, 79 (1977).

[6] Charles W. Schmidt, Linac 1990 (LA-12004-C), 259 (1990).

[7] J.Peters, Rev. Sci. Instrum., 71(2), 1069 (2000); APS Conf. 1097, pp. 236 (2008).

[8] Zhang Hua Shun, Ion Sources, Springer, 1999.

[9] D. P. Moehs, J. Peters, and J. Sherman, IEEE Transactions on Plasma Science, 33, 6, 1786 ( 2005).

[10] Yu. Belchenko and V. Dudnikov, Journal de Physique, v. 40, 501-502 (1979).

[I I] Dudnikov et al., Rev. Sci. Instrum., V. 73, 989-991 (2002).

[12] G.E. Derevyankin, V.G. Dudnikov, P. A. Zhuravlev, Pribory i Tekhnika

Eksperimenta, 5, 168-169 (1975); English translation in INSTRUMENTS AND

EXPERIMENTAL TECHNIQUES.

[13] P. Allison, "Experiments with Dudnikov-type H- ion source", IEE Trans. Nucl. Sci., NS-24, 3, 1594 (1977); Rev. Sci. Instrum., 58 (2), 235 (1987).

[14] H. V. Smith et al, Rev. Sci. Instrum., 65 (1), 123 (1994).

[15] R. Sidlow, et al. EPAC 96, 1996, THP084L.

[16] J. Thomason, et al., Rev. Sci. Instrum., 75 (5), 1735 (2004), Rev. Sci. Instrum., 75 (5), 1738 (2004); 20th ICFA Advanced Beam Dynamics Workshop, April 8 - 12, 2002; Dan Faircloth, et al, Rev. Sci. Instrum., 81, 02A721 (2010).

[17] G. Dimov, G. Derevyankin, V. Dudnikov, IEE Trans. Nucl. Sci., V NS - 24, n.3, 1545 (1977).

[18] K. Leung, K. Ehlers, P.Alisson. V. Shmidt, AIP Conf. Proc.158, p.356, 1986.

K.Leung, K. Ehlers, P.Alisson,V. Shmidt, Rev.Sci.Instrum., 58 (2), 235 (1987).

[19] A. Ueno, K.Ikegami, Ya. Kondo, Rev. Sci. Instrum., 75 (5), 1714 (2004); H. Oguri,

A.Ueno, K. Ikegami, Rev. Sci. Instrum., 79 (2), 02A506 (2008).

[20] A. Ueno, H. Oguri, K. Ikegami, Y. Namekawa, and K. Ohkoshi, Rev. Sci. Instrum.,

81 (2), 81, 02A720-1 (2010).

[21] Belchenko et al. APS Conf. 1097, pp. 214-222. (2008).

[22] D. P. Moehs, AIP Conf. Proc. CP925, pp. 361-365 (2007).

[23] A. Dudnikov et al, Rev. Sci. Instrum., 81, 02A714 (2010). [24] Martin P. Stockli, B. Han, S.N. Murray, T.R. Pennisi, M. Santana, R.F. Welton, AIP

CP 1097, edited by Surrey and Simonin, p. 223 (2009).

[25] V. Dudnikov, RoUand P. Johnson, Martin P. Stockli, B. Han, S.N. Murray, T.R.

Pennisi, M. Santana, R.F. Welton, IPAC 2010, THPEC073, Kyoto, Japan, 2010.

[26] U. Fantz, R. Gutser, and C. Wimmer, Rev. Sci. Instrum. 81, 02B102 (2010)

[27] V.Z.Kaibyshev, V.A.Koryukin and V.P.Obrezumov, Atomnaya Energiya, 69, No.3,

196 (1990).

[28] A.S. Belov, V.G. Dudnikov, B.B. Woitsekhovskii, et.al., AJP Conf. Proc. 287, ed.

J. Alessi, and A. Hershkovitch, p.485 (1994).

[29] A. Belov, PSTP-2007, BNL, USA, AJP conf. Proc. 980, pp. 209-220 (2008).

[30] A. S. Belov and V. P. Derenchuk, Jefferson Lab., April 2003 ICFA Beam Dynamics Newsletter, (2003).

[31] Yu. Bel'chenko, V. Davydenko, G. Dimov, V. Dudnikov, Rev. Sci. Instrum. 61, 378-384 (1990).

Potential for Improving of the Compact Surface

Plasma Sources

V. Dudnikov^a,

a Muons, IncBatavia, IL 60510, USA; ^b FNAL, Batavia, IL 60510, USA ;

Abstract. Factors limiting the operating lifetime of Compact Surface Plasma Sources (CSPS) are analyzed and possibilities for lifetime enhancement are considered. Noiseless discharges with lower gas and cesium densities are produced in experiments with a modified discharge cell. With these discharge cells it is possible to increase the emission aperture and extract the same beam with low enough emittance from a lower current discharge with a corresponding increase in source lifetime. A design of advanced CSPS for production of H- beam with average current up to 20 mA (with pulsed current up to 0.1 A) and extrapolated integrated beam lifetime -10 A hours is presented.

Keywords: surface plasma source, negative ion, cesiation, Penning discharge, semiplanotron.

PACS: 07.77.Ka, 29.25.Ni,52.80.Pi, 41.85.Ja. Work Supported in part by STTR Grant DE-SC0006267.

INTRODUCTION

Compact Surface Plasma Sources (CSPS) with cesiation [1-4] such as the magnetron, semiplanotron, and Penning Discharge (PD) SPS can have high plasma density (up to 10¹⁴ cm^"3), high emission current densities of negative ions (up to 8

A/cm ), and have small (1-5 mm) gaps between cathode emitters and a small extraction aperture in the anode. They are very simple, have high energy efficiency up to 100 mA per kW of discharge (-100 times higher than a modern large volume RF SPS [1-3]) and have a high gas efficiencies (up to 30%) using pulsed valves [5,6]. CSPSs are very good for pulsed operation but electrode power densities are often too high for cw operation. CSPSs have been the main "work horses" in accelerators with charge exchange injection for the last 30 years. However, for low current DC operation volume sources with low emission current densities Je~10 mA/ cm were used mainly [7]. Hollow cathode Penning discharge SPS [8-9] and spherical focusing semiplanotron SPS were used for cw operation [4,10-11] with extracted current densities of Je~100 mA/cm . Physical principles of SPS operation were presented in Refs. 12 and were reproduced in many books and reviews.

Flakes from electrode sputtering and blistering induced by the discharge and by back accelerated positive ions are the main reasons for ion source failure [10-13]. Suppression of back accelerated positive ions and flake evaporation by pulsed discharge can be used to significantly increase the operating lifetime of CSPS. Noiseless discharges with lower gas and cesium densities are produced recently in modified discharge cells. With these discharge cells it is possible to increase the size of the emission aperture and extract the same beam with low enough emittances from a lower current discharge with a corresponding increase source lifetime. Design of an advanced CSPS is presented. Extrapolated H^" beam parameters are: up to 100 mA pulsed current and up to 20 mA average current with an integrated lifetime up to 10 A hours.

The operational ISIS PD SPS [2,13] has very small discharge cell (5x2x11 mm ) and for noiseless discharge production it is necessary to use a high gas and cesium density. A narrow 0.6 mm emission slit is needed to prevent extraction voltage arcing.

Very high emission current densities of 1.5 A/cm and high discharge current densities of J-150 A/cm are destructive for the discharge cell electrodes. However, it can operate for up to 48 days (1100 hours) with pulsed beam current -50 mA after bending magnet with discharge duty factor df=2.5 . Pulsed beam with current 60 mA at df=5 was produced recently [14].

To use a more relaxed discharge parameter and corresponding increased lifetime it is necessary to produce the noiseless discharges with low gas and cesium density (like in large volume SPS [15]). Experiments with modified discharge cells for noiseless discharge production are described below.

NOISELESS DISCHARGE WITH MODIFIED DISCHARGE

CELLS

The Penning Discharge SPS uses a discharge caused by electron oscillations in a magnetic field that are sustained in anode window capped with cathodes at each end along the magnetic field, as shown in Fig. 1. Ion extraction is provided through a slit in the anode perpendicular to the magnetic field. The PD SPS was invented in INP [16]. The original design of the PD SPS was reproduced in Los Alamos and has had a long history of development at LANL [17]. A Los Alamos replica of this PD SPS can produce H^" beam with intensity up to 160 mA ( pulsed ~1 ms, 10 Hz) and up to 4 mA in CW mode of operation. The PD SPS shown in Figs. 1 a and b is now used in ISIS at RAL [2, 18] and has been successfully reproduced for the Chinese SNS [19]. High brightness PD SPS was developed for technology applications [20]. Beam intensity in PD SPS with hollow cathode for DC operation was increased recently to 25 mA [9].

FIGURE 1. Proposed modification of the ISIS PD SPS for reuse of old cathodes (a); Right- original configuration; left-proposed modification. L-cathode gap; PP-plasma plate; Modification of ISIS PD SPS: (b) right- standard cathode with a discharge gap L=5 mm; left-modified cathode with gap L=7 mm; central-PD SPS assembling with modified cathode gap L=9 mm.

Unfortunately, this beam has high levels of fluctuation [8], which degrades the beam space charge compensation and brightness. In the magnetrons and in the planotrons [4] the emission surfaces are bombarded by the back accelerated positive ions, which are not good for very long lifetime extension, but it can be long enough for many applications. In ISIS PD SPS the cathodes are deposited by molybdenum, sputtered from the anode insert. A new cathode is installed each time the source is refurbished. A big stock of used cathodes is accumulated. To explore the possibility for reusing the old cathodes, a small processing modification was proposed to the ISIS PD SPS. Namely, the deposited parts of cathode would be removed and the plasma plate thickness would be increased for better cooling of the H^" emitting area around the emission slit as shown in Fig. 1 a.

Several ISIS PD SPS cathodes were sent from RAL to Fermilab and processed as shown in Fig. 1 b.

The cathodes were assembled with the PD SPS body as shown in Fig. 1 b and the discharge and beam extraction were tested in the test stand. As a first step, a discharge supported by a DC power supply with hydrogen injection by piezoelectric valve with frequency of 50 Hz was tested and used for PD SPS conditioning and cesiation.

Pulses of discharge current starting at voltage Ud=600 V are shown in Figs. 2. The discharge current is high at high gas density and decreases with decreasing gas density until the discharge stops. A time dependence of gas density in discharge cell is similar to a time dependence of discharge current in Fig. 2 (a).

Fig. 2 (a) shows the discharge current signal in the PD with a standard gap of L=5 mm, with DC voltage Ud=600 V and pulsed gas 50 Hz. The discharge is noisy during the entire pulse with the standard conditions: vacuum p=7.3 10^"5 Torr; magnetic coil current Im=13 A. This experiment shows that in a standard cell with L=5 mm cathode gap, the discharge is noisy and a special attempt for noiseless discharge production is needed by increasing gas flow and volume cesium density.

Fig. 2 (b) shows the discharge current in the PD with gap L=7 mm, in similar conditions.

a b c

FIGURE 2. (a) Discharge current in PD with gap L=5 mm, with DC voltage Ud=600 V, pulsed gas 50 Hz; (b) shows the discharge current in the PD with gap L=7 mm, in similar conditions; (c) Discharge current in PD with L=9 mm cathode gap . Noiseless discharge achieved for ~3 ms With lower gas density then those with L=7 and 5 mm.

The discharge is noiseless for ~2 ms, which is very favorable for high brightness H^" beam production at low cesium density, which is also favorable for stable extended operation. By the increasing the cathode gap to L= 9 mm, the discharge stability was further improved at lower gas density as shown in Fig. 2 (c). With the same operation conditions the discharge current is noiseless for an even longer duration until the gas density decreases near the end of discharge. The noise generation increases the discharge current as well the increase the gas density because transverse electron mobility in a magnetic field increases with increased scattering frequency.

FIGURE 3. Diagram of dependence of the effective transverse electron mobility μ on the ratio of scattering frequency v to cyclotron frequency ω (a) and. diagram of the magnetron and Penning discharge stability as a function of magnetic field B and gas density n (b).

The dependence of the effective transverse electron mobility μ on the scattering frequency v and the electron cyclotron-frequency ω is expressed by:

μ = ev/m (v² + ω²) (1) and is shown in Fig. 3 a. The effective transverse electron mobility μ increases at low scattering frequency v below the cyclotron frequency ω and decreases at higher v. Transverse mobility can be increased through electron scattering by plasma fluctuations connected with plasma turbulence. For this reason the plasma instability development is thermodynamically "profitable" at low gas density and strong magnetic field and "non-profitable" at high gas density n and low magnetic field B [12].

A diagram of the magnetron discharge stability as a function of magnetic field B and gas density, n, [12] is shown in Fig. 3 b. The diagram for Penning discharges has a similar shape with different parameters. For discharge triggering at low gas density, the magnetic field B is used to prevent direct collection by the anode of electrons emitted by the cathode. Higher magnetic field is necessary for lower gas density. The boundary of gas discharge triggering in the diagram B, n of Fig. 3 b can be presented by expression:

(n-n_min) x (B-B_min) = C (2) where C depends on the discharge cell configuration. In this diagram, B_mi_n is the smallest magnetic field necessary for discharge triggering. The gas density, n, also should be higher than n_mi_n. Gas density for different discharge conditions can be calibrated using a measured gas density supported in the vacuum chambers with ion source.

At gas density below a critical density, n*, it is possible to have only a noisy discharge. At gas densities above n* the discharge becomes noisy at high magnetic field and noiseless at lower B. For reliable ion beam extraction, the gas density in the discharge cell should be lower than the level n_m at which the probability of extraction voltage breakdown becomes high or FT stripping become significant. With n higher than n_m it is need to use a small emission aperture. With slit emission aperture it is possible to have a larger total emission area because the gas target thickness after the emission aperture is determined by the small dimension of the slit and higher perveance for beam extraction.

For the standard ISIS PD SPS discharge cell with L=5 mm cathode gap and anode window WxT=2xl l mm shown in Fig. 1 b (right), the condition for normal beam extraction corresponds to a vacuum gauge reading p=7.3 10^~5 Torr and a magnetic coil current of Im=13 A.

With these parameters, the discharge without cesium is noisy during the entire gas pulse as shown in Fig. la. In this discharge cell the noiseless discharge can be produced only at the end of the high current discharge pulse with a high source body temperature of Tb~460 C. For this cell, n_m <n* for hydrogen and only adding a large density of cesium into the volume shifts n* below n_m and permits noiseless discharge production.

This circumstance significantly increases cesium consumption, increases electrodes sputtering by back accelerated cesium ions, decreases the efficiency of negative ion production, and decreases ion source lifetime several times.

Discharge properties changed dramatically by increasing the cathode gap to L=7 mm. With the same standard discharge conditions, p=7.3 10^" Torr, Im =13 A, with the modified cathode shown in Fig. b (left), the discharge is noiseless without cesium during the first 2 ms (see Fig. 2 b). In this situation there is no need to have cesium in the cell volume and one only needs to have half of a monolayer of cesium film on the plasma electrode plate around the emission aperture for enhanced FT production. A noiseless beam with good emittance and high brightness can be extracted from the entire discharge pulse. This decreases cesium consumption, decreases electrodes sputtering, increases efficiency of FT generation and extraction, and should increase SPS lifetime up to several times. Increasing the cathode gap to L=9 mm as shown in Fig. 1 b (center), the discharge stability was further improved as shown in Fig. 2 c. The discharge without cesium is noiseless for 3 ms and the gas density can be decreased without loss of the noiseless part of the discharge. In terms of expression (2), the increase of the cathode gap decreases the minimal gas density n_min and minimal magnetic field B_mi_n, needed for triggering the Penning discharge. At this the gas density, n*, is shifted below n_m and the large noiseless area for beam production becomes available.

With lowered gas and cesium density in the discharge cell it is possible to increase the emission slit width and produce the required beam current with low emittance at a lower discharge current and increase the SPS lifetime significantly.

The possibility to increase the emission slit was tested using scaled versions of a PD SPS at LANL [21]. The transverse normalized emittance was below 0.2 π mm-mr with the emission slit width increased up to 2.8 mm. With such increase of the slit width it is possible to produce the necessary beam current with a discharge current up to 5 time less than with the existing 0.6x10 mm slit. In the scaled versions of a PD SPS at LANL larger anode windows were needed, meaning they had to increase the discharge current, which decreased the efficiency of negative ion generation. A small increase of anode window size up to 4x14 mm would have been useful for easy discharge triggering and reducing the necessary gas density, yet without the decrease of the FT generation efficiency.

ADVANCED CSPS

For achieving best FT beam and ion source performance advanced PD SPS is really most promising candidate because noiseless discharge is possible and back accelerated positive ions does not bombarding FT production surfaces directly.

The FT ion temperature Ti in Penning discharge SPS can be -0.2 eV for low discharge currnt (<10 A) and ~leV for high discharge current. Accordingly, for emission slit with length 21=10 mm a normalized rms emittance εη=0.045 x / (Ti) 1/2

π mm mrad can be ~ 0.25 π mm mrad [17, 21]. An efficient local angle spread in transverse direction is ~ 3 times larger through the aberrations, but transverse slit width is smaller and this emittance is smaller than 0.20 π mm mrad [12, 16, 17, 21] . The general schematic of proposed modification PD SPS is shown in Fig. 5. The design and operation of the proposed source are clear from the explanations and captions to Figs.l, 2 and 5.

Ion beam with equal emittances can be produced with a slit extraction system, used in PD SPS. Ribbon ion beams were used successfully in many pre-injectors with high voltage pre-accelerators and with RFQ. In first versions of CSPS for beam focusing and SPS separation from high voltage accelerator was used bending magnet and "cold box" for decrease a Cs penetration to high voltage structure [16-18]. By minimizing of Cs escaping from SPS become possible to use high voltage post- acceleration and electrostatic focusing if H- beams in CSPS, which decrease a size and mass of CSPS. CSPSs without bending magnet are used in BNL [1, 3] in BINP [8-10] and in Fermilab [22]. One version of beam formation with electrostatic transaxial lenses focusing is shown in Fig. 4. A ribbon FT beam (3) is extracted from PD SPS (1) through emission slit of 2x10 mm (2) by extractor (4) in magnetic field of SPS magnet (5) as in previous versions of PD SPS [1, 2, 16-19]. Extracted beam (3) is further accelerated in the cylindrical gap between electrode (4) and electrode (6) serving as the transaxial electrostatic immersion lens. This lens has many parameters which can be used for independent optimization of the focusing properties in perpendicular transverse directions. By variation of radiuses and centers location of cylindrical gap is possible to change the focusing force strength and sign along the emission slit with relative low change to the beam focusing in the orthogonal direction.

FIGURE 4. PD SPS with transaxial lens focusing (left- cross section along magnetic field; right-cross section perpendicular to magnetic field).

1- PD SPS; 2-emision slit; 3-ion beam; 4-extraction electrode; 5-magnet of SPS; 6-grounded electrode of transaxial immersion lens; 7-compensating magnet; 8-first transaxial lens.

The shape of accelerating (decelerating) gap can be used for correction of aberrations. Shifting of lens plates can be used for correction of beam direction and position. The voltage between plates can be used for beam direction correction and for beam deflection. The compensating magnet (7) is used for restoring beam direction distorted by SPS magnet (5). In design of extraction and accelerating/focusing gaps it is necessary to avoid an electron trapping and conditions for high vacuum discharges. Addition transaxial lenses such as gap between electrodes (6) and (8) can be used for addition beam focusing, defocusing for optimal matching with followed accelerator structures such as RFQ.

In design of the scaled PD SPSs [21] it was repeated a first design of Dudnikov- type source [16] with cylindrical cathode which needs expensive large pieces of Mo for fabrication. More convenient and less expensive design of the scalable PD SPS is presented in Fig.5 (at left is median section along magnetic field; right- section transverse to the magnetic field). This design used module technology has been successfully applied to development of ion sources for ion implantation. This PD SPS consist of: cathode plates (1) with a cathode-cathode distance L-10 mm; anode (2) with anode window (7) with discharge cross section 4x14 mm ; cathode and anode connected by insulators plates (3) with attached magnetic poles (4); cathode is cooled by cooling tube (5). Insulators (3) is made from stress resistance ceramic as A1N and pyrolitic BN. The plasma is generated by discharge with electron oscillation in the magnetic field between cathode surfaces and anode window. The working gas (hydrogen) is injected into discharge through channel (10) by a pulsed valve or mass flow controller. Cesium is delivered from small oven through channel (8). The negative ion beam is extracted through emission slit (2x10 mm ) by extractor (11). Magnetic field is formed by the magnetic poles (4) and by magnet (13).

This PD SPS can be assembled from Mo (and W) plates with thickness of -10 mm and can have direct intense cooling of cathode and anode by flow of gas or liquid.

FIGURE 5. Cross sections of Penning Discharge SPS with direct cathode cooling: (a)-median section along magnetic field; (b)- section transverse to the magnetic field); 3D drawing (c); 1 -cathode plate (cathode-cathode distance L^v 10 mm); 2-anode; 3-insulator plates; 4-magnetic poles plates; 5-cooling tube; 6-emission slit (2x10 mm²); 7-anode window with discharge plasma (discharge cross section 3x15 mm²); 8-channel for Cesium inlet; 9-negative ion beam extracted through emission slit (2x10 mm²); 10- gas inlet; 11- extractor; 13- magnet; 14-Anode cooling channel; 15-Pocket in node insert for collection of sputtered material.

Heat pipes (or thermo siphons) also can be used for SPS thermo stabilization. With a cathode-cathode gap L-10 mm it is possible to have emission slit width d~2-2.5 mm instead of d~0.5 mm for SPS with L-4-5 mm because gas density is inverse proportional to L and noiseless operation with lower gas density was produced. With a large emission aperture it is possible to have lower emission current density J-0.1

A/cm and lower discharge current density with increase the operating lifetime. This lifetime extension can be significant because lifetime has strong nonlinear dependence on intensive parameter near threshold level. The SPS lifetime can be increased by using of the high current pulses for evaporation of the internal short circuit caused by flakes formation and with using of flakes gasification by discharges in NF₃ or by XeF₂ as in ion implantation.

In discharges with high plasma density and increased distance between cathode surfaces (1) and emission aperture, the FT ions from the cathode can't reach the emission slit without destruction. In this case, the surface plasma generation of FT on the plasma electrode (anode SPG) around the emission aperture is most important. In previous experiments it has been demonstrated that this anode SPG is efficient (Ef~50 mA/kW with emission aperture 0.5x 10 mm ) [12, 16, 21]. With emission aperture

2x10 mm it is possible to have Ef~200 mA/kW and 20 mA of FT can be produced with discharge power < 0.2 kW, discharge current Id<2A, which can be compatible with 10 hours of operation. The cesium admixture decreases the work function of the cathode and anode to increase the secondary emission of electrons and negative ions. For stability of the optimal cesium film it is important to maintain the optimal surface temperature, which is easier for larger sources. The cesium concentration and conditions for SPG should be optimized on the plasma plate surface around the emission aperture.

FIGURE 6. Simulation of beam extraction and focusing by transaxial lens with code IBSimu [23].

Simulation of beam extraction and focusing with transaxial lenses using a code IBSimu [23] shown in Fig. 6 demonstrates a possibility for using this extraction system.

SUMMARY

Optimization of the discharge cells in a Penning FT ion source is a viable method for increasing the phase space of the stable region for noiseless discharge production. With this method, cesium usage would be decreased, potentially resulting in longer source lifetimes. Proposed advanced PD SPS should be capable produce up to 20 mA of average FT beam current with extrapolated integrated beam lifetime -10 A hours.

ACKNOWLEDGEMENTS

This work has been supported in part by grant DE-SC0006267, and STFC JAI grant ST/G008531

REFERENCES

1. V. Dudnikov , "Forty Years of Surface Plasma Source Development", Rev. Sci. Instrum. 83, 02A708 (2012); http://www.adams=institute.ac.uk/lectures/?scheme=l&id=68.

2. D. Faircloth, " Negative Ion Sources (Magnetron, Penning)"; CERN Accelerator School , Slovakia, 2012; http://cas.web.cern.ch/cas/Slovakia-2012/Lectures/FairclothNegative.pdf, http://www.adams=institute.ac.uk/lectures/?scheme=l&id=55.

3. Jim Alessi, "Recent Developments in Hadron Sources", IP AC 2011, St. Sebastian, Spain, 2011 ; accelconf.web.cern.ch/accelconf/IPAC201 l/talks/frxba01_talk.pdf.

4. V. Dudnikov, "Features of Semiplanotron Surface Plasma Sources", Rev. Sci. Instrum., 83, 02A724 (2012).

5. A.Appolonskiy, Yu. Belchenko, G. Dimov and V. Dudnikov, Sov. Tech. Phys. Lett. 6, 86 (1980).

6. G. Derevyankin, V. Dudnikov and P. Zhuravlev, "ELECTROMAGNETIC SHUTTER FOR A

PULSED GAS L LET INTO VACUUM UNITS", Pribory i Technika Experimenta, 5, 168 (1975).

7. T. Kuo, D. Yuan, K. Jayamanna, M. McDonald, R. Baartman, W. Z. Gelbart, N. Stevenson, P.

Schmor, and G. Dutto, "Further development for the TRIUMF H2/D2 multicusp source", Rev. Sci. Instrum, 69, 2, 959 (1998).

8. Yu. Belchenko, A. Sanin, A. Ivanov, AIP Conf. Proc. 1079, p.214 (2009). Yu. I. Belchenko, A. I. Gorbovsky, A. A. Ivanov, A. L. Sanin, V. Ya. Savkin, M. A. Tiunov, "Upgrade of CW Negative Hydrogen Ion Source", Report P-314, These Proceedings.

A.A. Bashkeev, V.G. Dudnikov, "Continuously operated negative ion surface plasma source", AIP, Conf. Proc. 210, 329 (1990).

V. Dudnikov , C.W. Schmidt, R. Hren, J.Wendt, "Direct current surface plasma source with high emission current density", Rev. Sci. Instrum. 73, 989 (2002).

Yu. I. Belchenko, G. I. Dimov, and V. G. Dudnikov, Report BNL 50727, 79 (1977); 3Y. Belchenko and V. Dudnikov, "Surface negative ion production in ion sources," in Production and Application of Light Negative Ions, 4^th European Workshop, edited by W. Graham (Belfast University, Belfast, 1991), pp. 47-66; Y. Belchenko, Rev. Sci. Instrum. 64, 1385 (1993). D. C. Faircloth, S. Lawrie, A. Letchford, C. Gabor, M. Perkins, M. Whitehead, T. Wood, O. Tarvainen, J. Komppula, T. Kalvas, V. Dudnikov, H. Pereira, J. Simkin, S. Elliot,

"Developing the RAL FETS Source to Deliver a 60 mA, 50 Hz, 2 ms H- Beam", report O- 308, These Proceedings.

D. C. Faircloth et al., "Optimizing the front end test stand high performance H^" ion source at RAL", Rev. Sci. Instrum., 82 (2, Part 2) 02A701 (2012).

R. F. Welton, V. G. Dudnikov, K. R. Gawne, B. X. Han, S. N. Murray, T.R. Pennisi, R.T. Roseberry, M. Santana, M.P. Stockli, M.W. Turvey, "H- radio frequency source development at the Spallation Neutron Source", Rev. Sci. Instrum. 83, 02A725 (2012).

G. Dimov, G. Derevyankin, V. Dudnikov, IEE Trans. Nucl. Sci., INS - 24, n.3, 1545 (1977); V. Dudnikov, Proc. 4th All-Union Conf. on Charged Part. Accel., Moscow, 1974, V. l, p.323. Moskva, Nauka, 1974.

P. Allison, "Experiments with Dudnikov-type H- ion source", IEE Trans. Nucl. Sci., NS-24, 3, 1594 (1977). P. Allison et al., Rev. Sci. Instrum., 58 (2), 235 (1987).

R. Sidlow, et al. EPAC 96, THP084L (1996).

H.F. Ouyang, Zhao, D.C. Faircloth , et al., "The Development of the H- Ion Source Test Stand for CSNS", THP115, Linear Accelerator Conference LINAC2010.

S.K. Guharay, W. Wang, V.G. Dudnikov, et al., "High-brightness ion source for ion projection lithography", Journal of Vacuum Science & Technology B, 14 (6) 3907 (1996).

P. Allison, H. V. Smith and J. Sherman, "H- and D- scaling laws for Penning Surface-Plasma Sources", Review of Scientific Instruments, Vol. 65 (1), pp. 123-128 (1994); J. Sherman et al., EPAC 2002, Paris, France, 202.; 20^th ICFA BDW, 288, FNAL, Batavia, IL, 2002.

D. S. Bollinger, "H- Ion Source Development for the FNAL 750keV Injector Upgrade", Report O-303, this Proceedings.

Taneli Kalvas, "Beam Extraction and Transport I, Π" , CERN Accelerator School, Slovenia, 2012; http://cas.web.cern.ch/cas/Slovakia-2012/Lectures/Kalvas.pdf.

Muons, Inc. Highly Reliable Negative Ion Sources Topic 32e

DoE SBIR/STTR Phase I, FY2013 Release I

PROJECT TITLE: Highly Reliable Negative Ion Sources

PRINCIPAL INVESTIGATOR: Dr. Vadim Dudnikov Tel: 631-807-9960

TOPIC: 32e

STATEMENT OF THE PROBLEM OR SITUATION THAT IS BEING ADDRESSED

Negative ion sources are used for large accelerator facilities such as SNS at ORNL, LANSCE at LANL, RHIC at BNL, and for many medical and industrial accelerators. Improved FT ion sources are essential for the next generation of high power proton accelerators. For example, Project X at Fermilab requires a CW FT Ion Source with 10 mA CW beam current with 0.2 π mm-mrd normalized transverse emittance, fast chopping capability, fast intensity variation, and high availability for months of operation. No FT source available today satisfies these demands.

STATEMENT OF HOW THIS PROBLEM OR SITUATION IS BEING ADDRESSED

In this project, we are developing novel modifications of FT source designs, which will satisfy these requirements. The new source will be an advanced version of a Compact DT SPS (Dudnikov-Type Penning Surface Plasma Source) with high efficiency, up to 15 mA average current with improved electrode thermal stabilization, reduced cesium loss, longer lifetime through suppression of electrode sputtering and immunity to electrode shorting by flakes, and turn-key operation.

WHAT WILL BE DONE IN PHASE I

The design of the advanced DT SPS will be improved by using an optimized discharge cell, temperature stabilization, beam extraction and formation. The suppression of discharge noise with suppressed electrode sputtering will be experimentally tested in CW operation of PD SPS prototype.

WHAT IS PLANNED FOR PHASE II

The advanced version of the DT SPS FT source with an improved plasma generator designed in Phase I will be developed further with emphasis on higher availability and improved beam parameters: average FT beam current of 15 mA with integrated beam lifetime -7-10 A hours and push-button operation.

COMMERCIAL APPLICATIONS AND OTHER BENEFITS

The developed source will be an essential component of proton drivers to be used for further colliders and spallation neutron sources. The source is also an upgrade path for many other existing and planned medical applications, including the large number of existing cyclotrons with external injection for isotope production and cancer therapy, and high current tandem accelerators for Boron Neutron Capture Therapy (BCNT). Homeland Security applications include production of resonant gamma rays to detect explosives and special nuclear materials.

KEY WORDS: negative ion source, FT production, Penning discharge, Project X, CW.

SUMMARY FOR MEMBERS OF CONGRESS

A negative ion source needed for the Fermilab Project X linear accelerator is being developed to have higher intensity, better reliability, and improved efficiency. It has many applications in particle accelerators needed for medicine, industry, homeland defense, and discovery science. Muons, Inc. Highly Reliable Negative Ion Sources Topic 32e

NARRATIVE SECTION

32e Highly Reliable Negative Ions Sources

Table of Contents b. Proprietary Data Legend - Not Applicable

Project Overview

c. Identification and Significance of Problem or Opportunity, and Technical Approach Identification and Significance of the Problem or Opportunity

Recent progress in development of advanced negative ion sources was connected with improving of cesiation (small admixture of cesium) in surface plasma sources (SPS) [1,2,3,4 ] . Cesiation is accepted as indispensable for high intense high brightness negative ion beam production. SPSs with cesiation are the work-horses in almost all high intense proton accelerators. Operation of these accelerators were not limited or compromised by SPS operation during last 30 years. New SPS's with a higher duty factor (DF) -6% at pulsed current -40 mA, normalized emittance <0.3 π mm mrad and lifetime up to -1000 hours (Integrated beam lifetime (IBLT) -2.5 A hours) were developed for new facilities such as SNS (ORNL). Cs consumption was reduced from -1 mg per hour to -1 mg per week, (however, some operation SPS has Cs consumption -1 g per day).

Intensity of Penning Discharge SPS was increased up to 60 mA at 5% DF. Reliable production of noiseless discharge in PD SPS is developed. Efficiency of Semiplanotron SPS was increased to -100 mA/kW, that similar to the best proton sources. However, for internal and external cyclotron injection until now are used Cesium-less hot cathode discharge negative ion sources with low energy and gas efficiency and high electron current.

The lastest developments of SPS are promising to increase the lifetime of SPS with average current -10 mA to -700- 1000 hours (IBLT -7- 10 A hours).

Project X at Fermilab needs a CW H^" Ion Source that satisfies demanding performance specifications: 10 mA CW beam current with normalized rms emittance -0.2 π mm mrad, ability for fast chopping, fast intensity variation and long (month) lifetime (-720 hours; 7.2 Ahours). Currently, there is no H^" source with demanding performance.

One source that can be used for CW H^" beam production is the TRIUMF multicusp, Cs-free, filament-driven source [5] which is now being commercialized by D-Pace, Inc. [6] . They can run at 10 mA H^" DC beam output with filament power -1 kW, discharge power -2.5 kW, ratio of e/H^" -4 and H₂ flow~32 seem. Relative high efficiency of H^" generation -4 mA/kW is produced with a large emission aperture 13 mm of diameter (emission current density -9 mA/cm ). 4-rms normalized emittance is 0.28 π mm mrad for 10.6mA. Announced lifetime of cathodes is -300 hours (-13 days; 3.5 Ahours). Replacing-conditioning time is -5-6 hours. High gas flow needs high pumping speed -3500 1/second.

In the volume sources the volume production process is the dissociative attachment of low energy electrons to ro-vibrationally excited H₂ molecules [1, 2, 5] . If the H₂ molecule is

-21 2 vibrationally cold, the dissociative attachment cross section is extremely small (10^" cm ). However when the H₂ molecule is highly ro-vibrationally excited, the cross section increases with decreased threshold energy for dissociative attachment. Thus, low energy electrons can be effective in generating H^" ions by dissociative attachment to highly vibrationally excited Muons, Inc. Highly Reliable Negative Ion Sources Topic 32e molecules. The ro-vibrationally excited molecules are produced mainly in the plasma volume by fast electron impact.

In both volume and surface-plasma sources there are many processes that can destroy FT ions. As well as being fragile and easily destroyed, the FT ion is much more likely to be stripped. The aim of the source designer is to minimize the FT destruction processes by controlling the geometry, temperature, pressure and fields in the source.

The rate of FT generation is proportional to the molecular density Ng and electron density Ne near the emission aperture. After extraction, FT ions can lose electron in collision with molecules. The cross-section of this stripping process is low at low energy but increases to σ ~10^~

15 cm 2 for energy greater than 100 eV. By this stripping process, the extracted beam current density J is attenuated exponentially with increased gas thickness. The thickness is proportional to the gas density Ng in the discharge chamber and to diameter d of the emission aperture (or the width of an emission slit d). The dependence of J as a function of Ng increases linearly at low Ng and exponentially decreases for large Ng with a maximum at Ng=l/ σ B d (or B σ d Ng=l). The maximum emission current density J_max=A Ne/ σ B d exp (1) is inversely proportional to d. The extracted beam intensity I=J_max π d /4= i d A Ne/ σ B exp (1) is proportional to d. Thus the emission current density of FT ions from volume sources can be relative high at small emission aperture, but cannot be higher than J~ 10 mA/cm with aperture d~l cm diameter. The extracted intensity is at exp(l) times lower than the emitted beam intensity just after emission aperture because the remaining part [l-exp(-l)] of the extracted FT ions is stripped after extraction. The extrapolated intensity without stripping is linear function of Ng. The difference between this line and the real intensity is the intensity of fast neutrals generated by FT stripping on the gas target. This stripping creates a large flux of fast neutrals, which can influence further acceleration of FT ions. For example, this uncontrollable flux can produce intense secondary electron emission in the high voltage accelerator or in the RFQ. It is useful to separate the FT beam from this neutral flux. The TRIUMF (D-Pace) volume source has relatively high energy-efficiency (I- 10 mA at 2.5 kW discharge power) because of its large emission aperture d=13 mm, but it has low emission current density J<10 mA/cm and needs high pumping speed.

Volume generation of FT always exists in the plasma but it can deliver only - 10 mA/cm of emission current density (-10 mA of FT with -15 mA of fast neutrals and a high current of co- extracted electrons). Enhanced extracted FT current can be increased up to 40, 50, 80 mA in discharges without Cs by surface plasma generation that is enhanced by accumulation of impurities with low ionization potential (mainly potassium) on the collar. This accumulation is enhanced by ionization of these impurities in the plasma and by transportation by the electric field to the negative collar. In these conditions, the efficiency of FT generation corresponds to a thick layer of potassium on the collar surface with work function -2.2 eV.

Any low concentration of such impurities is enough for significant enhancement of surface plasma generation relative to volume generation. In a discharge system well cleaned to remove alkali impurities, the efficiency of FT generation can be much lower with increased co-extracted electron current. This phenomena was observed in the CERN replica of the DESY RF source fabricated with a clean vacuum technology [7]. The corresponding efficiency can be -5-7 times lower than the efficiency with optimized cesiation with work function -1.6 eV.

The energy efficiency, gas efficiency, beam brightness and e/FT ratio can be improved by "Cesiation" a small addition into discharge of cesium or other substances lowering of the electrode work function [1,2,3,4,8]. There were reported several declarations for conduct a "cesiation" of the TRIUMF multicusp, filament-driven source [5,9] but there was no Muons, Inc. Highly Reliable Negative Ion Sources Topic 32e improvement report. However, for very similar sources optimized for surface-plasma H^" generation these parameters were improved to several times by proper cesiation. RF-driven FT sources without cesiation have low efficiency <1 mA/kW, high e/H^" ratio~20-50 which is difficult to extrapolate for 10 mA CW beam production [3, 7] . RF Surface plasma Sources with cesiation have efficiency -1.5 mA/kW and e/H^" ratio ~ 2. This efficiency is also not enough for comfortable production of 10 mA CW beam. It is possible to hope that CW RF driven SPS can be developed after significant increase of efficiency in RF SPS with saddle antenna.

The following sections describe the design of some of the most successful H^" ion sources. Compact Surface Plasma Sources (CSPS) [1,2,4,8, 10] with cesiation such as semiplanotrons, magnetron have energy efficiency up to 100 mA/kW and have life time up to several month with H^" current - 100 mA at duty factor ~ 2% (continuous operation was conducted up to 32 month for HERA-DASY magnetron [11]). It is not too complex to improve a magnetron cooling several times for production the CW H^" beam current -10 mA. Unfortunately, in these CSPS cathode- emitter is directly bombarded by a back accelerated positive ions with sputtering of this surface. For this reason it is difficult to hope for very long time operation and very low cesium consumption in CW mode with 10 mA. However with a suppression of this sputtering the lifetime can be acceptable for some applications of CW beams.

Better opportunity for very long time operation in CW mode with low cesium consumption has CSPS with Penning discharge (known as Dudnikov-type source) [1,2,10, 12] . In PD SPS the back accelerated positive ions bombard anode insert which do not participate directly in H^" emission and can be fabricated with a "pocket" for keeping sputtering flakes.

A perspective of this approach is supported by experience of the BINP-BNCT Penning type H^" CW source [ 13] which has been in operation producing -10 mA for -5 years at the Budker Institute of Nuclear Physics with run intervals of -200-300 hours (for 15 mA; 4.5 Ahours) with source conditioning taking <1 hours (0.5% down time). Some source parameters are: Emission aperture: d=3 mm; discharge parameters: 80 V x 4.5 A = 360W for 10 mA H^" beam (efficiency 30 mA/kW); 1-rms normalized emittance for 15 mA is <0.2 π mm mrad. This source does employ Cs -1 mg/hr. Gas flow is -7-8 seem.. This source [13] uses the Penning glow discharge with the hollow cathodes.

The distance between cathode surfaces is 8 mm. The maximum emission current density of H^" ions is 280 mA/cm . Hydrogen and cesium are injected into the Penning discharge region through the plasma of the hollow cathode or through the anode.

Cesium seeding is supplied by heating an external oven loaded with Cs pellets (Cs₂Cr0₄+Ti). A cesium delivery system using an ampoule of metallic cesium, similar to that used now in the Fermilab magnetron SPS, is available as an option. Cesium is needed for the hollow cathode low voltage discharge and for effective H^" production on the discharge electrodes [1-4,12,13,14]. The magnetic field in the ion source is produced by external permanent NdFeB magnets with additional coils for field control. Since the H^" beam is deflected by this field, a second correction magnetic system is used in order to direct it back coaxially. Built-in ohmic heaters are used for fast start up the cesium discharge. Automatic programmable heating are used for push-button starting of discharge and extraction. Negative ions are extracted through the emission hole in the anode bottom cover. A triode ion-optical system is used for beam extraction at 4-5 keV and post-acceleration to 32 keV by the grounded accelerating electrode. Recently, H^" beam intensity was increased up to 25 mA by increase the emission aperture diameter to d=5 mm. Unfortunately, the discharge in this PD SPS with hollow cathodes is very noisy [13] and this destroys a space charge compensation and degrade a beam performances. Muons, Inc. Highly Reliable Negative Ion Sources Topic 32e

Technical Approach

In this project we will develop PD SPS with close FT beam intensity -15 mA CW but with noiseless discharge and improved beam performances. In this section we have shown that the Project-X ion source requirements are in line with several existing ion source technologies (RF multicusp, filament multicusp and Penning) but will clearly require some development work to demonstrate all requirements can simultaneously be met by a single source. In this project we propose to develop novel modifications of FT source designs which should satisfy these requirements. The new source will be an advanced version of a Penning DT SPS (Dudnikov- Type Penning Surface Plasma Source) with noiseless discharge, high efficiency, deliver up to 15 mA average current with improved electrode thermal stabilization using new materials, fast chopping capability, and reduced cesium loss, have longer lifetime through suppression electrode sputtering by back accelerated positive ions, by developing an immunity to electrode shorting by flakes and by discharge chamber cleaning without vacuum opening.

CW noiseless discharge operation will be tested with ion source prototypes in the Fermilab' s Test Stand in Phase I. Advanced version of PD SPS with extended lifetime and push-button operation will be developed and tested in Phase II of the project.

In this proposal we are starting from previous studies demonstrating that in some other SPS such as Semiplanotrons with Hollow cathodes and in Penning discharge SPS (known as Dudnikov- type source) a transition to noiseless discharge increases the beam brightness up to 3-5 times which is enough for significant improvement of accelerator performance (lower beam loss and lower activation). Steady operation of a Penning discharge SPS with average beam current up to 25 mA has been demonstrated. This is a solid confirmation that a DT SPS is capable of long term operation with high CW current (-10 mA).

Nowadays, new science projects, including spallation neutron sources (SNS) and proton drivers, commercial tandem accelerators and commercial cyclotrons need to have FT beams with a higher intensity, duty factor (df) and brightness B. For this requirement, new research projects to improve the SPS are underway at many (at least 20) Laboratories around the world. All these groups have made progress in improving FT beam current but other parameters including lifetime, power and gas efficiency, beam brightness and stability of operation still need improvement. Some of these organizations can be potential users of commercially available FT /D^" sources with improved characteristics.

Programs and developments of several of these laboratories are discussed in recent reviews [1- 4,15, 16] . The most recent development in this field was presented at ICIS2011, PAC2011 and in NIBS 2010, 2012, AIP Conf. Proa, 1097, 2009. For SNS upgrades and for intense Proton Drivers, FT beams are needed with pulsed current up to 70 mA, average current up to 10 mA (df > 6%), with normalized rms emittance < 0.25 π mm mrad. An acceptable lifetime for high df operation is >500 hrs (> 3 A hrs). For high current tandem accelerators (BNCT, explosive and nuclear material detection, positron diagnostics) and for high current cyclotrons H7D^" beams with DC current > 10 mA, beam emittance < 0.2 π mm-mrad and lifetime > 600 hrs (-6A hrs) are needed. Minimizing cesium loss is important for long periods of operation.

An additional important requirement for H^" sources for further SNS and FNAL accelerator development is a flexible beam chopping system with nanosecond resolution. A possible solution is to use an ion source that allows fast beam current modulation. This modulation may provide chopping of the main portion of the beam so that other systems only have to clean out a relatively small amount of beam. This fast chopping requirement will be addressed below. Muons, Inc. Highly Reliable Negative Ion Sources Topic 32e d. Anticipated Public Benefit

The primary applications of the new source to be developed in this project are the next upgrade of the Fermilab accelerator complex, and the Project X proton Driver. The source would also be a component of other proton drivers that might be used for muon colliders and for ADS. The source would be an upgrade path for many other existing and planned applications such as medical treatments (including cyclotrons with external injection for cancer therapy, and high current tandem accelerators for Boron Neutron Capture Therapy), and homeland defense to produce resonant gamma ray techniques to detect explosives.

Other FT users around the world also need help to improve sources for particular applications, a situation that offers special commercial opportunities to provide services (R&D, engineering, maintenance, or consulting) or devices (components or complete systems). For commercial applications, compact SPS are preferable because they are more flexible in applications and have lower manufacturing and ownership costs than Large Volume RF-driven SPS. e. Phase I Technical Objectives

We believe that the Muons, Inc., SNS, and FNAL teams can collaborate to increase the chances of achieving the required parameters of the accelerator upgrade project.

In Phase I, the design of advanced DT SPS with Penning discharge will be developed using previous experience and computer simulation, including optimization of cooling, beam extraction, formation. Operation of PD SPS with noiseless CW discharge and suppression of the electrode sputtering will be tested with ion source prototypes in the Fermilab' s Test Stand.

The operation of a CSPS with Penning discharge has been extended for CW operation with FT beam intensity of -15 mA.

Status of CSPS with Penning discharge (Dudnikov type Source)

The Penning Discharge SPS uses a discharge with an anode window capped with cathodes at each end, along the magnetic field. Extraction of the ions is through a slit in the anode perpendicular to the magnetic field. The Penning discharge SPS was invented in BINP [12]. It has had a long history of development at LANL [17,18]. Now it is successfully used at ISIS RAL [19,20] and is under development for the Chinese SNS. The fundamental difference between the magnetron and Penning sources is that in the magnetron, FT ions produced at the cathode are directly extracted, while in the Penning source, the cathode has no line of sight and so ions must undergo a charge-exchange process on atomic hydrogen or scattering to reach the emission aperture. Discharge noise can be eliminated in the cesiated Penning SPS by optimizing the magnetic field and gas density or using a small admixture of heavier gas (N₂ in [18])). In this regard, emittance measurements have shown the Penning SPS always has higher brightness than the magnetron (and other ion sources). The effective ion temperature can be as low as Tj~ 0.2 eV for low discharge current and ~1 eV for high discharge current [18]. Improving brightness and suppressing discharge noise will be some of the key items focused on in this proposal.

The LANL IX Penning and ISIS Penning SPS have essentially the same discharge chamber dimensions as in the first version of the SPS [12]. The ISIS operational PD SPS routinely produces 55 mA of FT ions during a 200-250 / s pulse after a bending magnet at 50 Hz (beam DF is -1%, discharge DF is -2.5%, because only noiseless part of discharge is used for beam extraction) for uninterrupted periods of up to 50 days (1200 hours). Muons, Inc. Highly Reliable Negative Ion Sources Topic 32e

The operating time of CSPS is limited by flake formation through electrode sputtering and blistering by back accelerated positive ions. This version of SPS has limited cooling because the prototype [12,17] was optimized for low <i operation. The cathode cooling was improved by contact with a water-cooled flange through a mica layer. But this layer has low thermal conductivity and limited heat transfer. The anode is cooled by airflow. However, the thin plasma plate that includes the emission slit has low thermal conductivity and is easily overheated. Thermal stabilization of the system with the goal of mitigating this failure mode will be one of the key items investigated in this proposal.

Trace of back accelerated ion bombardment is visible on the cathode/anode assembly of the PD SPS (after long period of operation -48 days). Cathode' s surfaces are not sputtered but solid deposited. Flake formation can be a reason of a short circuit between cathode and anode. Some metallic drops are formed by anode's cones melting by electrons of pulsed Penning discharge. This pulsed overheating can be avoided during CW operation. Flake formation on the inner plasma plate surface of a PD SPS after extended operation is visible, but the emission surface around the emission slit remains clean. The external surface of the plasma plate around the emission slit is also clean. Back-accelerated positive ions are a reason of electrode sputtering but it can clean flakes. Fortunately, cathode sputtering by discharge is compensated by material deposition as solid layer.

A Penning SPS for higher average current was built and tested at ΒΓΝΡ [21] . Operation with beam current above 100 mA in 0.25 ms pulses with repetition rate of 100 Hz has been demonstrated for >300 hours (emission slit 0.5x10 mm ; df = 2.5%; 0.75 Ah). Operation with repetition rate of up to 400 Hz (df 10%) has been tested. Distinctive features of this Penning SPS compared with the ISIS source are its slightly larger discharge cell and more massive anode cover (plasma plate) with forced air or water cooling. The cathode has a strong pressed contact with a copper cooler. It is cooled by strong flow of water. A fast (0.1 ms) gas valve [22] is used to inject gas at a repetition rate up to 500 Hz (valve was tested for 10⁹ pulses). Stable support of noiseless discharge has been established which is important for high brightness beam production. In SW mode of operation H^" beam with current 4 mA was extracted from discharge 1A, 80 V.

At LANL, PD SPS sources were designed and constructed applying plasma scaling laws and increasing two of the source dimensions by a factor 4 and 8. This reduced the cathode power

2 - 2 load from 16.7 to 2.24 kW/cm while increasing the H^" current from 160 mA in IX (0.5x10 mm slit) to 250 mA in 4X (2.8x10 mm slit) [15, 23, 24] . The measured rms normalized emittance is 0.15 π mm-mrad in the narrow slit dimension (2.8 mm). Emittance in the long slit dimension (10 mm) is 0.29 π mm mrad for an un-optimized slit extraction system at 29 keV extraction energy. It is possible that the last emittance increase, which affects only a small part of the beam, is connected with end effects of the slit. In this case, it can be improved by collimation.

Advanced Penning discharge SPS

For achieving best H^" beam and ion source performance advanced PD SPS is really most promising candidate. The H^" ions temperature Ti in Penning discharge SPS can be <leV. Accordingly, for emission slit with length 21=10 mm a normalized rms emittance εη=0.045 x /

1/2

(Ti) π mm mrad can be ~ 0.25 π mm mrad. An efficient local angle spread in transverse direction is increased to ~ 3 times by aberrations, but transverse slit width is smaller and this emittance is smaller than 0.20 π mm mrad. Muons, Inc. Highly Reliable Negative Ion Sources Topic 32e

The general schematic of proposed modification PD SPS is shown in Fig. 1. In discharges with high plasma density and increased distance between cathode surfaces (1) and emission aperture, the FT ions from the cathode cannot reach the emission slit without destruction. In this case, mainly the surface plasma generation of FT on the plasma electrode (anode SPG) around the emission aperture is important. In previous experiments, it has been demonstrated that this anode SPG

Fig. 1: Schematic of the upgraded version of the Penning discharge SPS for pulsed and CW chopped FT beam production. 1- cathode; 2-anode; 3-source body; 4-cooled plasma plate; 5- anode cooling; 6- cathode insulator; 7-cathode cooler; 8- thermal conductive insulator (A1N); 9- cooled flange; 10- base plate; 11-high voltage insulators; 12- gas delivery system (pulsed valve); 13- cesium delivery system;14- extractor; 15- magnet (SmCo) + coils; 18-negative ion beam; 19- suppressor/deflector; 20-accelerating electrode.

The cesium admixture decreases the work function of the cathode and anode, to increase the secondary emission of electrons and negative ions. For stability of the optimal cesium film, it is important to maintain the optimal surface temperature, which is easier for larger sources. The cesium concentration and conditions for SPG should be optimized on the plasma plate surface around the emission aperture. Conceptual design of the discharge cell and extraction system of advanced PD SPS developed in Phase I of this project is based on the previous experience discussed above. An important feature of this design is a magnetic insert that shapes the magnetic field in the extractor gap (creating arc shaped magnetic field lines). With this "correct" curvature of the magnetic field, the co-extracted electrons can be removed very fast to the extractor by an electric field along the magnetic field lines. A preliminary engineering design of an advanced PD SPS was prepared using the experience discussed above and can be used for prototyping.

Slit extraction is very adequate for FT production by an anode SPG. Low ion temperature is preserved very well during slit extraction. The increased emittance along the slit, observed in work [27] is due to aberrations that affect a small fraction of the beam extracted from the ends of the slit, and can be decreased by collimation. A three or four electrode extraction system will be optimized to produce beam optics with minimum aberrations and low co-extracted electron current. It is important to suppress the secondary emission of FT and cesium ions from the extractor. Suppression electrode (19) is used for collection of slow positive ions to prevent their acceleration into the discharge chamber, which is important to suppress electrode sputtering by these positive ions. Positive ions in the acceleration gap should be defocused by the electric field and collected. The reflector is used to reflect positive ions generated in the FT beam. Muons, Inc. Highly Reliable Negative Ion Sources Topic 32e

All these comments are applicable to the DC Penning SPS discussed in publication [17]. The combination of proposed improvements can deliver high quality H^" beam with an average (CW) current up to 10 mA at the discharge power < 1 kW. Lifetime of this SPS can be extended to IBLT ~ 7-10 A hours (~ 700-1000 hours for 10 mA average beam current).

Cesium atom excitation by a resonant laser beam (16) will be used in further for effective suppression of cesium loss from the discharge chamber as disclosed in [25].

The attractiveness of PD SPS for many applications can be increased by removing the heavy bending magnet and using an Einzel lens (or transaxial lenses) for beam focusing. In the first version of PD SPS a bending focusing magnet was used for one-dimensional focusing and for better protection from Cesium penetration to high voltage pre-accelerators or RFQ. In further developments, a "cold box" was added for better Cesium collection. However, reliable operation of the SPS directly connected (without bending magnet) to the high voltage pre-accelerators and RFQ was demonstrated in several H^" pre-injectors (BNL, LANL, KEK, SNS). One version of the Fermilab SPS accommodation to the injector without bending magnet is under testing now. These compact and convenient designs with optimized pumping can be used as prototypes for the design of a commercial version of SPS. Computer simulation of DC H^" beam focusing by electrostatic lens is shown in Fig. 2 a. Stable operation with good simulation agreement was confirmed in experiments with H^" beam intensity up to 17 mA (at energy -32 keV).

Many pre-injectors used SPS with axisymmetric emission apertures and extraction. However, emittances of formed H^" beams were different for transverse directions parallel and perpendicular to the magnetic field of the ion source. Ion beams with equal emittances can be produced with the slit extraction system used in a PD SPS. Ribbon ion beams were used successfully in many pre-injectors with high voltage pre-accelerators and with a RFQ. Beam acceleration and focusing with transaxial lenses are shown in Fig. 2 b, c, where a ribbon beam of H^" (3) is extracted from the PD SPS (1) through emission slit of 1x10 mm (2) by extractor (4) in the magnetic field of SPS magnet (5) as in previous versions of the PD SPS. The extracted beam (3) is further accelerated in the cylindrical gap between electrode (4) and electrode (6), which serves as the transaxial electrostatic immersion lens. This lens has many parameters, which can be used for independent optimization of the focusing properties in perpendicular transverse directions. By variation of radiuses and center locations of the cylindrical gap it is possible to change the focusing force strength and sign along the emission slit with relatively little change to the beam focusing in the orthogonal direction. The shape of the accelerating (decelerating) gap can be used for correcting aberrations. Shifting the lens plates can be used for correcting beam direction and position. The voltage between plates can be used for beam direction corrections and for beam deflection.

Fig. 2: Computer simulation of the beam extraction and focusing by electrostatic lens. DC SPS with Penning discharge. Emission aperture is 3 mm diameter. H^" beam intensity up to 17 mA, Muons, Inc. Highly Reliable Negative Ion Sources Topic 32e beam energy 32 keV (a); and PD SPS with transaxial lens focusing (b) - cross section along magnetic field; (c)-cross section perpendicular to magnetic field. 1-PD SPS; 2-emision slit; 3-ion beam; 4-extraction electrode; 5-magnet of SPS; 6-grounded electrode of transaxial immersion lens; 7-compensating magnet; 8-first transaxial lens.

The compensating magnet (7) is used for restoring the beam direction distorted by the SPS magnet (5). In the design of the extraction and acceleration focusing gaps, it is necessary to avoid electron trapping and conditions for high vacuum discharges. Addition transaxial lenses such as the gap between electrodes (6) and (8) can be used for additional beam focusing or defocusing for optimal matching to a downstream accelerator structure such as an RFQ.

In the design of the scaled PD SPS in LANL [25], the first version of the PD SPS [16] is repeated with a cylindrical cathode, which needs large, expensive pieces of Mo for fabrication. A more convenient and less expensive design of the scalable PD SPS is presented in Fig.3.

Fig. 3: Cross sections of Penning Discharge SPS with direct cathode cooling: (a)-median section along magnetic field; (b)- section transverse to the magnetic field); 3D drawing (c); 1-cathode plate (cathode-cathode distance L 10 mm); 2-anode; 3-insulator plates; 4-magnetic poles plates;

5-cooling tube; 6-emission slit (2x10 mm ); 7-anode window with discharge plasma (discharge cross section 3x15 mm ); 8-channel for Cesium inlet; 9-negative ion beam extracted through emission slit (2x10 mm ); 10- gas inlet; 11- extractor; 13- magnet; 14- Anode cooling channel; 15-Pocket in node insert for collection of sputtered material.

This design used module technology has been successfully applied to development of ion source for implantation. With easy shapes of using parts it is possible to use some hard material such as tungsten without fabrication problems, as in ion implantation industry. Schematics of the proposed modified PD SPS are shown in Fig. 3 a, b. 3D assembling of PD SPS is shown in Fig. 3 c. This PD SPS can be assembled from Mo plates with thickness of -10 mm and can have direct intense cooling of the cathode and anode by a flow of gas or liquid. Heat pipes (or thermal siphons) also can be used for SPS thermal stabilization. With a cathode to cathode gap L-10 mm it is possible to have the emission slit width d~2-2.5 mm instead of d~0.5-l mm for a SPS with L-4-5 mm because gas density is inverse proportional to L and noiseless operation with lower gas density was produced in experiments with L increased. With a large emission aperture, it is possible to have lower emission current density J-0.5 A/cm and lower discharge current density with increased operating lifetime. This lifetime extension can be significant because lifetime has strong nonlinear dependence on intensive parameters near threshold. The SPS lifetime can be increased by using high current pulses for evaporation of the internal short circuit caused by flake formation and by using of flakes gasification by NF₃ or by XeF₂ discharges as in ion implantation. The working gas (hydrogen) is injected into the discharge through channel (10) by Muons, Inc. Highly Reliable Negative Ion Sources Topic 32e a pulsed valve or mass flow controller. Cesium is delivered from a small oven through channel (8). The negative ion beam is extracted through emission slit (2x10 mm ) by extractor (11). H^" beam will be accelerated in the second accelerating gap up to final energy. The magnetic field is formed by the magnetic poles (4) and by the magnet (13). Channels (14) attached to anode are used for anode thermal stabilization. In this design, the thermal conductivity between discharge surfaces heated by the plasma and the well cooled cathode, anode, and emission aperture surfaces, are up to 10 times larger than in ISIS RAL PD SPS. Optimized temperature distribution and fast temperature optimization can be reached by using heated air and liquid flow cooling as used in SNS H- RF SPS and in PD SPS [17] with push button starting and operation. Thermo Stabilization of Electrodes is very important. Electrodes temperature control is the key condition for efficient H^" production and long-time operation. Active feedback T control of cesiated surfaces of Plasma Electrode, Extractor Electrode and Accelerating Grounded electrode are necessary. Plasma electrode should be at the temperature 150 - 250°C to distribute cesium and to keep the optimal cesium coverage. It is important to prevent the accumulation of cesium on parts, bombarded by fast particles from plasma or from extraction sytem. Active cooling can be provided by Heat Transfer Fluid circulation through the channels bored in the electrodes. High- temperature Heat Transfer Fluid (MarloTherm) can be used for electrodes preliminary heating up to 250°C and for cooling during operation.

CSPS Lifetime Extension:

Failures of the CSPS are not irreversible as LV SPS (hot cathode breaking, internal antenna leaking) but it is as a rule a slow degradation of performance that can be treated and usually attributed to build up of cesium hydride near the Cs inlet or on the pulsed gas inlet. This can be eliminated by stabilization and creating theproper temperature distribution. Additionally, flaking of the electrode material caused by sputtering of the electrode surface by back accelerated positive ions, either shorting the anode and cathode, or blocking the source outlet can lead to failure. This can be mitigated by suppression of back acceleration of positive ion by design of extraction system; decreasing of electrode sputtering by choosing of low sputtering material; suppression of blistering by proper temperature; design of special pockets for flakes collection; it is better to have the emission aperture above the cathode to prevent flakes from closing the emission aperture; use high current pulses for flakes evaporation; use discharge with fluorine for flakes gasification.

Realization of these treatments can increase CSPS lifetime up to 2-3 times necessary to meet requested lifetime -7-10 A hours.

For increase of average beam current with extended lifetime it is necessary:

• The electrode cooling/heating and temperature control must be improved. Cathode

cooling can be improved significantly by using aluminum nitride (A1N) ceramic with very high conductivity instead Macor. Other possibility is using of heat tubes.

• The electrode work function must be stabilized by controlling the cesium coating.

• The discharge noise must be suppressed for high brightness production.

• Back accelerated positive ion must be suppressed to decrease electrode sputtering (Multi- electrode extractor). f. Phase I Work Plan

The goals of the Phase I project are:

• Design of PD SPS with extended lifetime optimized for project X injector: Muons, Inc. Highly Reliable Negative Ion Sources Topic 32e o Review specific design features that limit performance of Penning SPS and decide means to improve them,

o Determined availability of optimal materials and technologies, develop a general design for DT SPS.

o Perform theoretical calculations and experimental measurements that will demonstrate feasibility of the conceptual design,

o Perform computer simulations of thermal stability, ion production attenuation, extraction and beam formation,

o CW noiseless operation of prototype CSPS will be tested in the test stand.

• Report preparation. Plan for Phase II.

In addition to achieving these specific objectives, the Phase I research program will examine the total system cost and manufacturability and decide the requirements for power supplies, vacuum equipment, and diagnostics needed to perform the Phase II research.

Responsibilities

Muons, Inc.: The direction of the project is the responsibility of the company and Dr. Vadim Dudnikov, the PI.

FERMILAB: Dr. Daniel Bollinger will be responsible for assistance in the design of the SPS adaptation into the Fermilab Test Stand and RFQ at Fermilab.

g. Phase I Performance Schedule

3 months after start of funding:

• Investigate specific design features that limit performance of the Penning SPS and decide means to improve them.

• Preparing of Prototype of CSPS for testing of noiseless CW operation and electrode sputtering and sputtering suppressing in the Fermilab Test Stand.

6 months after start of funding:

• Design of Penning discharge SPS adaptable to the Fermilab RFQ injector.

• Determine availability of materials, develop a general design DT SPS, and prepare general design drawings.

• Perform theoretical calculations and experimental measurements that will demonstrate feasibility of the conceptual design. Perform computer simulation of thermal stability, ion production attenuation, extraction and beam formation.

• Plan for phase II effort formed.

9 months after start of funding:

• Phase II proposal prepared

h. Related Research or R & D

The Muons, Inc. program is summarized in the Commercialization History and Plan document that is part of this proposal. Related projects that are associated with muon cooling and muon colliders are described there. The particular interest in the H^" Source development project proposed here is in the proton driver that will be required to produce the copious numbers of muons that are needed for muon colliders, neutrino factories, and muon studies such as the mule experiment. Muons, Inc. Highly Reliable Negative Ion Sources Topic 32e i. Principal Investigator and other Key Personnel

Muons, Inc. Principal Investigator: Dr.Vadim Dudnikov is an internationally recognized expert in the invention and development of ion sources, ion beam systems, and accelerators. He received a M. Sci. from the Novosibirsk State University, Novosibirsk, Russia and started to work for development of charge-exchange injection. While a student at the Institute of Nuclear Physics (INP), he completed this important work which is still considered to be the best solution to the problem of charged particle injection. At INP, Dudnikov received two Ph.D. degrees: one in Accelerator Physics and a second (Doctor of Physical - Mathematical Science) in Experimental Physics. While he was at INP, Dr. Dudnikov developed many versions of negative ion sources for application in charge exchange injection. In 1971, he discovered the enhancement of negative ion generation in gas discharge by cesium admixture and invented the method of cesium catalysis to enhance negative ion formation in a gas discharge. This method, which dramatically enhances negative ion emission, is now the most widely used method for production of intense, high brightness, negative ion beams. There are currently many versions of the Dudnikov type surface plasma negative ion source (SPS) with cesium catalysis. These sources hold the record for efficiency, intensity, brightness, and optimized design. Dudnikov' s discovery of the physical basis of cesium catalysis has led to the development of SPS in all the National Laboratories of the USA, Japan, and other countries. Cesium catalysis is used in neutral beam injectors for nuclear fusion devices. His innovations in ion beam formation, transportation, space charge neutralization, separation, accumulation, and diagnostics are widely used in accelerators, ion beam science, and technology. In the USA, Dr. Dudnikov has worked in National Laboratories (BNL, ORNL, FNAL), at Universities (UMD, MIT), and in industry (SDI, SCI).

Fermilab Subgrant Principal Investigator: Daniel Bollinger is the Pre-accelerator Group Leader of the Proton Source Department, responsible for the daily operations, maintenance, and improvements of the FNAL pre-accelerator. He is also is the co-manager of 750 keV injector upgrade project, which consists of a BNL style ion source and RFQ that will replace the existing Cockcroft- Walton accelerators. He has had fifteen years of experience in the physics, engineering, and technology of both high-energy particle accelerators and nuclear power plants. His specializations include design and development of negative ion sources, electronic, mechanical, and vacuum systems for accelerators, and power upgrades for nuclear reactor feedwater systems. Recent experience has included the development of a new negative ion source for the FNAL injector upgrade project.

j. Facilities/Equipment

Muons, Inc. currently shares facilities with MuPlus Inc. This includes our corporate headquarters, a building of approximately 4000 square feet of floor space in Batavia, Illinois, a short drive from Fermilab, which is used as office space, conference rooms, workshop area, and living quarters as needed. We also have office space in Wilson Hall at Fermilab (Batavia, IL) and in the ARC building at Jefferson Lab (Newport News, VA). We have several high-performance personal computers and workstations with high-speed net access and sufficient computing power to perform simulations and CAD work. FNAL has its own facilities located in Batavia IL. Two test stands and four preinjectors are used for ion source testing and development. These test stands have all the equipment necessary for ion source characterizations, including emittance scanners and beam profile measurements. We anticipate that the construction of the mechanical devices such as the ion source components will be done by our outside vendor and the tests of Muons, Inc. Highly Reliable Negative Ion Sources Topic 32e the new configuration will start at the FNAL source test stand. We anticipate that the new Illinois Accelerator Research Center being built on the Fermilab site will be part of our longer-range plan for ion source development. This new building will have room for offices and laboratory space, where we expect to develop test stands and support infrastructure.

k. Consultants and Subcontractors

Our partner Research Institution is Fermi National Accelerator Laboratory. The certifying official is:

The certifying official is:

Dr. Jack W. Anderson

Fermi National Accelerator Laboratory Directorate

Mail station: 200 (WH 2W)

Phone: 630-840-3930

Email: [email protected]

There are no consultants expected to participate in the Phase I program.

References:

[I] J. Alessi , "Recent Developments in Hadron Sources",

accelconf.web.cern.ch/accelconf/IPAC2011/talks/frxba01_talk.pdf

[2] D. Faircloth, " Negative Ion Sources (Magnetron, Penning)"; CERN Accelerator School ,

Slovakia, 2012; http://cas.web.cern.ch/cas/Slovakia-2012/Lectures/Faircloth Negative.pdf http://www. adams = institute, ac. uk/lectures/? scheme = 1 &id=55.

[3] M. Stockli, "Volume and Surface-Enhanced Negative Ion Sources"; CERN Accelerator

School , Slovakia, 2012; http://cas.web.cern.ch/cas/Slovakia-2012/Lectures/Stockli

Negative.pdf

[4] V.Dudnikov, , Rev. Sci. Instrum., 83 (2) pt. 2, 02A708(2012); 02A724 (2012).

[5] T. Kuo et al., 1996, Rev.Sci.Instrum. 67(3), 1314-1316, (March 1996).

[6] http://www.d-pace.com/products_hion.html.

[7] J. Lettry et al., Rev. Sci. Instrum. 81, 02A723 (2010).

[8] V. Dudnikov, "The Method of Negative Ion Production", SU Author Certificate, CI. HOI 3/04, No. 411542, n, 10 March,1972; Yu. I. Belchenko, G. I. Dimov, and V. G. Dudnikov, Report BNL 50727, 79 (1977).

[9] Xianlu Jia et al., "Emittance improvement efforts on the 15-20 mA dc H- multicusp

sourcea ", Rev.Sci.Instrum. 81, 02A712 (2010).

[10] V. Dudnikov, Rev. Sci. Instrum. 63(4), 2660 (1992): V. Dudnikov, Rev. Sci. Instrum.

73(2), 992 (2002).

[I I] J. Peters, AIP CP 1097, edited by Surrey and Simonin, p. 234 (2009).

[12] V. Dudnikov, Proc. 4th Ail-Union Conf. On Charged Part. Accel., Moscow, 1974, V. l, p.323. Translated to English, LANL.

[13] Belchenko et al., CP 1097, edited by Surrey and Simonin, p. 214 (2009).

[14] Zhang Hua Shun, Ion Sources, Springer, 1999.

[15] Joseph Sherman and Gary Rouleau, AARAI 02, Denton, TX 2002.

[16] D. P. Moehs, AIP Conf. Proc. CP925, pp. 361-365 (2007). Muons, Inc. Highly Reliable Negative Ion Sources Topic 32e

[17] P. Allison, "Experiments with Dudnikov-type H- ion source", IEE Trans. Nucl. Sci., NS-

24, 3, 1594 (1977). P. Allison et al., Rev. Sci. Instmm., 58 (2), 235 (1987).

[18] H. V. Smith et al, Rev. Sci. Instrum., Vol 65 (1), pp. 123-128 (1994).

[19] R. Sidlow, et al. EPAC 96, 1996, THP084L.

[20] J. Thomason, et al. Rev. Sci. Instrum., 75 (5), 1735 (2004), Rev. Sci. Instrum., 75 (5), 1738 (2004). 20th ICFA Advanced Beam Dynamics Workshop, High Intensity High Brightness Hadron Beams, Fermilab, April 8 - 12, 2002. Dan Faircloth, S. Lawrie, A. P. Letchford, C. Gabor, P. Wisea,et al, "The Front End Test Stand High Performance H^" Ion Source at RAL", report N-16, ICIS 2009, Galtinburg, TN, 2009.

[21] G. Dimov, G. Derevyankin, V. Dudnikov, IEE Trans. Nucl. Sci., V NS - 24, n.3, 1545

(1977).

[22] G. Derevyankin, V. Dudnikov, P. Zhuravlev, Pribory i Technika Experimenta, : 5,168- 69(1975).

[23] H. V. Smith et al, Rev. Sci. Instrum., , Vol 65 (1), pp. 123-128 (1994).

[24] J. D. Sherman et al, Proc. EPAC 2002, 3 June, Paris, France, p.284.

[25] A. Dudnikov, V. Dudnikov, P. Chapovsky, Rev. Sci. Instrum. 81, 02A714 (2010)

Claims

1. A device comprising:

means for providing a negative ion source comprising a compact surface plasma source (CSPS) with a significantly increased average beam current, increased brightness, and an extended operation time; and means for operating the negative ion source comprising the CSPS with the significantly increased average beam current, increased brightness, and the extended operation time for high current, high brightness negative ion beam production.

2. A method comprising:

steps for providing a negative ion source comprising a compact surface plasma source (CSPS) with a significantly increased average beam current, increased brightness, and an extended operation time; and

steps for operating the negative ion source comprising the CSPS with the significantly increased average beam current, increased brightness, and the extended operation time for high current, high brightness negative ion beam production.