US3377506A - Electromagnetic current control for a hollow cathode - Google Patents

Description

21g-121 SR www mm@ Melzo@ ma 3,377,506.

April 9, 196s C. M. ANAS ETAL 3,377,506

ELECTROMAGNETIC CURRENT CONTROL FOR A HOLLOW CATHODE Filed March 50, 1966 2 Sheets-Sham l April 9, 1968 c. M. BANAS TAL 3,377,506

ELECTROMAGNETIO CURRENT CONTROL FOR A HOLLOW CATHODE Filed March 50, 1966 l 2 Sheets-Sheet 2 a [ffii United States Patent O ABSTRACT F THE DISCLOSURE A magnetic current control device is described for a hollow cathode structure which operates in a glow dis charge and produces a collimated beam of electrons from the volume enclosed by the cathode structure. The number of electrons produced in the beam is controlled by subjecting the plasma within the hollow cathode structure to a magnetic field and varying the intensity of the field to produce the desired beam current.

This invention relates to a device for controlling the current in a beam of charged particles emanating from an aperture in a hollow electrode. More specifically, it relates to a device for controlling the magnitude of the current in a beam of charged particles produced from a plasma in a hollow electrode with a magnetic lield acting on the plasma.

Conventional means for producing electron beams inn volve the liberation of electrons from the surface of a heated cathode by thermionic emission. Recently, electron beams have been produced from an apertured cathode as a result of the volume production of electrons within a hollow chamber enclosed by the cathode. In glow discharge devices various operating modes may be end countered if one, for instance, varies the potential diiference between the cathode Vand the anode and gas density of the environment One of these modes produces a well delined electron beam which may predictably and advan tageously be used to work materialsu These apertured hollow cathodes have hollow chambers and are fabricated from a wire mesh or solid metal with a single aperture in one end. When the cathode is subjected to a high negative potential with respect to an anode and with the proper cathode geometry and pressure level in the hollow chamber, a Well-defined pencil-form beam of high current density, high energy electrons emanates from the aperture. An example of the versatility of the configurations possible with the apertured hollow cathode may be found in the copending application Ser. No. 417,399, filed Dec. l0, 1964, entitled Annular Holn low Cathode Discharge Apparatus by Fernand I. Fer-1 reira, and assigned to the same assignee.

The aperture size has a strong influence on the operatd ing mode and the characteristics of the beam. Its dimensions must be approximately equal or larger than the cathode fall. This dimensional requirement will vary with different gases and gas densities.

A large fraction of the electrons for the beam are obtained from a plasma generated by an intense discharge within the chamber enclosed bythe cathode. Various controls have been proposed to vary the current obtainable from the hollow cathode chamber and to maintain the electron beam current within precisely controlled limits. One such control, for instance, would involve varying the anode-to-cathode potential to maintain the desired current. 'One disadvantage of varying this potential is that the focal length of an electron lens depends upon it and compensation would have to lbe made in order to maintain a fixed beam spot size on a workpiece. This is especially so 3,377,506 Patented Apr. 9, 1968 ICC at the low pressure end of the cathode operating regime in which a small variation in the potential may even result in extinguishment of the discharge.

Another approach suggested to obtain the current control has been to vary the gas pressure in the hollow cathode. This approach involves complicated electromechanical devices. Generally, the mass liow characteristic of a diffusion pump slopes downward with increasing pressures. With operating pressures, for instance, within the range from one to microns an increase in the gas ,tiow also causes an increase in pressure due to a decrease in the pumping speed and unstable operation results. Hence, a complicated throttling device on the diifusion pump or a mechanical pump having a high-speed response is needed. In operations such as welding, a significant amount of material is evaporated and the pressure of the environment is affected. Hence, a pumping system with a fast response is needed to control the gas pressure and maintain the beam current within the desired limits.

Still another approach involves the use of a control grid which operates adjacent to the emitting surfaces of the hollow cathode and suppresses electron ow from the cathode to the plasma when a suitable negative potential is applied. The disadvantage of this type of current control is that contact with the high density plasma leads to disintegration of the control grid. v

It is therefore an object of this invention to provide a current control for a beam of charged particles emanating from a hollow electrode.

It is a further object of this invention to provide a device for controlling the magnitude of the current in a beam of charged particles produced from a plasma in a hollow electrode with a magnetic lield acting on the plasman It is still another object of this invention to provide a magnetic current control for a iiow discharge hollow cathode operating in the electron beam mode.

These and other objects will becomev more readily apparent upon a review of the drawings and specification wherein:

FIGURE l shows a cross-sectional view of the current control.

FIGURE 2 shows the linearity of the current control device at different anode-to-cathode potentials.

FIGURE 3 shows a block diagram of an automatic control system which is used to maintain the electron beam current from the hollow cathode at a preselected reference value.

FIGURE 4 shows the eifectiveness of this current control with varying pressure conditions.

In this invention, current control of a hollow cathode operating in the electron beam generating mode is obtained by applying a magnetic field to the chamber 15 enclosed by the cathode wherein the plasma producing the beam of electrons is active. In FIGURE 1 a cathode 10, shown in sectional form, comprises a cylindrical hollow member which is Amade of a non-magnetic material such as tantalum. The back wall 11 and the peripheral wall 13 define the cylindrical chamber 15. In addition, the peripheral wall is shaped to form an aperture 12. substantially concentric therewith and opposite the back wall. The cathode 10 is electrically connected to a highvolt age supply 18 via a supporting conductor 14 and a lead 16. The aperture dimensions are approximately 1A ythat of the face in which it appears. Surrounding the cathode 10 is a discharge suppressor shield 20 which extends back to the wall of the chamber 22 and surrounds the conductor 14 as well as the nonemitting surfaces of the cathode 10. A typical hollow cathode cylindrical structure having an insulator shield is described in the copending application tiled by Conrad M. Banas and Clyde O. Brown 3 entitled Insulator Shielded Cathode, Ser. No. 506,237, filed Now 3, 1965, and assigned to the same assignee.

Surrounding and coaxial with the cathode and the shield 20 is a solenoid-type coil 24 located generally at the center of the cathode and provides a magnetic field that is substantially parallel to the cylindrical axis of the cathode 10. The coil 24 is so located withr respect to the cathode that its magnetic field acts predominantly on the plasma generated within the chamber 15. The field may also diverge from the cylindrical axis in the manner of solenoids. The current for the control coil 24 is provided by a control power supply 26 that is located outside of the chamber 22. Spaced from the cathode aperture 12 is an additional magnetic coil 30 designed to further focus the beam emanating from the hollow cathode 10 at the workpiece 32., The workpiece in this instance operates as the anode but as described in the previously-mentioned patent application to Ferreira, the anode may be located anywhere within the chamber 22 provided it is sufficiently distant from the cathode 10 The current for the focusing coil 30 is supplied from an external supply 34. Interposed between the control coil 24 and the workpiece 32 is a shield 28 to protect the control coil from the heat generated by the working of the electron beam on the workpiece $2.` This shield 28 may be of the same material as shield 20.

The control coil 24 produces an electromagnetic field that acts through the shield 20 and the wall of the cathode 10 on the plasma enclosed -by the hollow cathode 10. Contrary to what prior experience in plasma physics might lead one to expect, application of a magnetic field within the hollow cathode increases the current from the hollow cathode through the aperture 12., As is shown in FIGURE 2, in the absence of any current flowing through the control coil 24, the hollow cathode operates in its normal fashion producing various currents for different anode-to-cathode potential differences. As the magnetic field of the control coil 24 is increased, the electron beam current increases as well. The effect of the magnetic field on the discharge is similar to that obtained by variation in chamber pressure.. Hence, a substantial simplification of current control is possible with change in operating pressures. Such pressure variations arise, for instance, during welding or from irregularities in the pumping system and may be readily compensated for. This control is simple, since, as can be seen from FIGURE 2, the current varies practically linearly with the strength of the magnetic field applied by the control coil 24 The advantage of this invention becomes clearly evident from FIGURE 4 In this figure the operating lines of a typical hollow cathode are shown for different magnetic field strengthso Although current values indicate the discharge current, its values reflect the beam current since an advantage of this type of control is its preservation of the efficiency. Thus for example, if a hollow cathode operates at 20 kilovolts and the field strength from control coil 24 as fixed by the field references 58 is 20 gauss, the current in the electron beam is about 190 milliamperes with a pressure of 8 microns (.008 mm.) of mercury. Under these conditions the cathode operates at point C in FIGURE 4.. If the pressure in the chamber rises to 9 microns, the operating point of the device will shift to point A along the 20 gauss line and to compensate for this increase in current without changing the pressure the control current. through coil 24 is reduced to lower the magnetic field strength to that for curve B1 where the corresponding current level is again 190 milliarnperesu Similarly, corrections are made for reductions in pressure levels but now the magnetic field intensity from the control coil 24 is increased so that operation of the cathode proceeds from C to D to E.

The excursion of the cathode from desired operating current has been exaggerated in FIGURE 4 for clarityD Actually, with a quickly responsive control feedback circuit of the type shown in FIGURE 3, essentially constant current may be maintained.. A very small change in. current as a result of a change in chamber pressure is observed and the current in the control coil 24 is immediately changed to compensate for the change. As a result of a one micron increase in the chamber pressure, the hollow cathode will reach the new operating point B essentially along the line CB with only very small excursions.

The curve of FIGURE 4 .is for a constant voltage source. A three-dimensional model could be envisioned if voltage is also a variable. The design operating conditions are in general a function of various requirements such as imposed by the material, stability of the beam, type of work to be accomplished, etc.,

The hollow electrode may be used to generate a plasma therein from which a beam of ions is extracted. This invention although described in relation with an electron beam is also applicable to beams of ion particles and control the current magnitude of the ion beam. Higher field strengths are needed to control the heavier ion particles and this is simply provided by increasing the coupling between the coil 24 and the cavity or cham-ber 15G An automatic control as shown in FIGURE 3 operates as follows. The resistor 50 is shown in series with the high voltage supplier 18 so that the voltage developed across it will have a direct relationship with the current emanating from the hollow cathode 10. This resistor may actually ybe the current sensing element of the electron beam power supply. The voltage developed across resistor 50 is fed to a difference amplifier 52 to generate an output signal that refiects the difference 4between the current sensed by the resistor 50 and a preselected reference value set by the circuitry 54. The reference value indicates the desired beam current from the hollow cathode, The amplifier 52 output signal is then applied through a magnetic amplifier 56 to the control coil 24 to provide the desired magnetic field strength. The initial operating point of the magnetic amplifier 56 corresponding to point C is determined by the field reference circuit 58. The error signal from differential amplifier 52 is superimposed on the field reference signal, By selecting the polarity of the current sensed by the resistor 50 on the difference amplifier 52 current may be appropriately increased or decreased and a fast and reliable current control for a hollow cathode discharge may be provided by driving the error signal to a minimum.

It is to be understood that the invention is not limited to the specific embodiments herein illustrated and described but may be used in other ways without departure from its spirit as defined by the following claims.

We claim:

1. A device for controlling the flow of charged particles from a hollow cathode operating at a high potential difference with respect to an anode in a gaseous environment comprising:

a hollow cathode structure having a chamber evacuated to a predetermined gaseous pressure,

said chamber being provided with an aperture,

means establishing a glow discharge within said chamber and producing a beam of charged particles from said aperture, and

an electromagnetic coil adjacent the chamber for generating a magnetic eld within said chamber and vary ing the number of charged particles in the beam Vfrom said cathode chamber aperture as a function of the strength of the magnetic field 2. A device for controlling the flow of electrons from a hollow cathode operating at a high negative potential difference with respect to an anode in a gaseous environment comprising:

a hollow cathode structure having a chamber evacuated to a predetermined gaseous pressure,

means producing a glow dischar'ge within said chamber,

said chamber being provided with an aperture having dimensional characteristics for establishing a beam of electrons therefrom, and

means for generating a magnetic field within said cham-1 ber, said magnetic field increasing the electron current in the beam with an increase in magnetic field strength and decreasing the electron current in the beam with a decrease in magnetic eld strength.

3. A device as recited in claim 2 wherein the hollow cathode structure comprises:

a cylindrical chamber having nonmagnetic walls and provided with said aperture in one end, said aperture being substantially coaxial with the cylindrical axis of the chamber,

and where said magnetic field generating means comprises.

a solenoid externally adjacent to the chamber and substantially coaxial therewith for producing a magnetic field in the chamber that is substanm tially parallel with said cylindrical axis.

4. A device as recited in claim 3 and further comprislng:

a nonmagnetic glow discharge suppressor shield intenl posed between the solenoid and the chamber,

where the shield is selectively spaced from and substantially encloses the cylindrical wall of said chamu ber to suppress the glow discharge therebetween.

5. A device as recited in claim 4 and `further comprislng:

a heat shield interposed between the solenoid and the lbeam of electrons.

6. A device for controlling the magnitude of the current in a beam of electrons produced by a hollow cathode operating in a glow discharge to perform work on workn pieces comprising:

a hollow cathode structure,

said cathode structure having a back wall,

a peripheral wall extending away from said back wall to define a cavity of sufficient size to establish said glow discharge therein,

said peripheral wall further forming an aperture from which the beam of electrons emerges,

an electromagnetic coil wrapped about said peripheral wall to produce a magnetic eld in said cavity, and

means supplying current through said electromagnetic coil to vary the strength of the magnetic field and control the magnitude of the current in the beam.

7. A device as recited in claim 6 and further comprisa nonmagnetic glow discharge suppressor shield interposed .between the electromagnetic coil and said peripheral wall,

said shield being selectively spaced from and substan-x tially enclosing the peripheral wall to suppress the glow discharge there-between, and

a heat shield extending outwardly from said peripheral wall in between the beam of electrons and said electromagnetic coil toshield the coil from the heating effects of said beam of electrons upon the workpieces.

8. A device as recited in claim 6 and further comprising:

means for generating a first signal indicative of the actual current in the beam,

means for generating a second reference signal indicative of the desired current in the beam, and

means responsive to said first and second signals for producing an error signal and applying said error signal to said electromagnetic coil current supply means with a polarity to vary the intensity of the magnetic field and control the current in the beam to the desired value.

9. A device as recited in claim 6 wherein the magnetic field in said cavity is oriented substantially transverse to the aperture.

10. A device as recited in claim 9 wherein said peripheral wall comprises:

a single cylindrical wall, and

wherein said magnetic coil is a solenoid to produce a magnetic field substantially coaxial with said single cylindrical wall.

11. A device as 4recited in claim 2 wherein said magnetic field generating means comprises an electromagnetic coil adjacent the cathode chamber and varying the numn1 ber of electrons in the beam from said cathode chamber aperture as a function of the strength of the magnetic field.

References Cited UNITED STATES PATENTS 1,954,025 4/1934 Reynolds 313-84 X 2,940,010 6/1960 Kenny 315-107 2,945,160 7/1960 Burk N 315-107 X 3,152,238 1-0/1964 Anderson BSO- 49.5 X 3,243,570 3/1966 Boring 219-121 3,320,475 5/1967 Boring u.. 315-108 JAMES W. LAWRENCE, Primary Examiner.

C. R. CAMPBELL, `Assistant Examiner.

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