US3551728A - High intensity linear accelerators - Google Patents

Description

United States Patent lnventor Petric Croitoru.

Bucharest, Rumania Appl. No. 719,963 Filed Apr. 9, 1968 Patented Dec. 29, I970 Assignee lnstitutul de Fizica Atomica,

Bucharest, Rumania, an Institut of Rumania Priority Apr. 10, 1967 Rumania No. 53,518

HIGH INTENSITY LINEAR ACCELERATORS 12 Claims, 11 Drawing Figs.

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[56] References Cited UNITED STATES PATENTS 2,901,628 8/1959 Lamb 3l3/63X 3,133,227 5/1964 Brown et a1... 3 l5/5.42 3,171,054 2/1965 Dong et al. 315/3.5 3,214,632 10/1965 Harman 3l5/5.41X 3,343,101 9/1967 l-laimson 3l5/3.5X

Primary Examiner-Herman Karl Saalbach Assistant Examiner-Saxfield Chatmon, Jr. Attorney-Karl F. Ross ABSTRACT: A particle accelerator of the electrostatic, wave guide or cavity-resonator type in which a tubular electron or ion beam is introduced at one side of the elongated evacuated evacuated chamber at at annular-beam source, may be focused by an intermediate electrostatic lens, and accelerated by particle-accelerator means of annular construction through the evacuated chamber. At the output end of the accelerator, an annular axially symmetrical deflector arrangement is provided to focus the beam upon the axis of symmetry of the device.

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INVENTOR.

Pefrica Croiforu HIGH INTENSITY LINEAR ACCELERATORS My present invention relates to high-intensity direct or resonant linear accelerators and, more particularly, to improvements in particle as accelerators for the generation of high-intensity, easily controlled beams of charged particles.

A linear accelerator for the increase in the intensity and energy and/or momentum of a beam of charged atomic or subv atomic particles, generally comprises an elongated evacuated chamber having accelerator means which electrically and/or magnetically accelerate the particles of the beam at a number of spaced-apart locations along the. accelerator. The two general types of accelerators commonly in use are those which utilize drift tubes and those which make use of an electric field propagated in a waveguide. Waveguide accelerators are most generally employed for the acceleration of electrons while drift tube accelerators may be employed for the acceleration of ionic particles (e.g. protons).

In linear accelerators of the latter type, a plurality of socalled drift tubes constitute electrodes spaced along the accelerator chamber and particles of relatively low energy enter these drift tubes while radiofrequency power is applied to the tube electrodes, generally surrounding the access of the accelerator, so that alternate drift tubes are in opposite phase. The radiofrequency phase relationship and the velocity of the ingoing particles are established such that, on reaching a gap between any two electrodes, the particles emerging from the upstream drift tube are accelerated in the direction of downstream drift tube. In a proton accelerator, the downstream tube at each gap is relatively negative while the upstream tube may be relatively positive while, in electron accelerators, the downstream tube may be positive and the upstream tube negative. Thus, the particles pass with increased energy into the downstream drift tube with a travel at constant speed since they are there not exposed to an electric field. By the time the following gap between electrodes is reached, the radiofrequency voltages have reversed so that again a downstream tube is at a polarity opposite that of the charged particle whereas the upstream tube is of opposite polarity. In most cases, the drift tubes are connected to a radiofrequency generator or oscillator but recent accelerators, referred to as resonant linear accelerators, have used drift tubes placed in a large resonant cavity excited in such mode that the electric field is parallel to the axis of the tubes.

In so-called waveguide accelerators, electric fields are produced parallel to the axis of the guide which may be standing or traveling electromagnetic waves, the waves having a characteristic phase velocity. An electron traveling at the same velocity as the wave is continuously accelerated by the plurality of axially spaced loading or barrier plates adapted to:

reduce the phase velocity (which otherwise might be of the order of magnitude of the speed of light) to a small fraction of this original velocity. The radiofrequency power arriving at the target end of the accelerator may be fed back to the input end for economy of power consumption.

In all of the aforementioned conventional direct or resonant linear accelerators, the electron or ion beams (hereinafter referred to as particle beams, charged-particle beams or beams of charged particles) are longitudinally accelerated by electrostatic or radiofrequency fields and have a filiform shape substantially lying along the axis of the accelerator. It has been found that the intensity of such beams is limited by accelerator structure and certain inherent attributes thereof which may be associated with the defocussing effects of the space charges. Attempts to increase the intensity of accelerators operating with flliform beams have not proved noticeably successful.

It is, therefore, the principal object of the present invention to provide a method of increasing the beam intensity of a linear particle accelerator as well as an improved accelerator structure.

A further object of this invention is to provide a direct or resonant linear acceleraton adapted to produce a beam of high intensity with minimum defocusing thereof.

I have found that these dbjects and others which will become apparent hereinafter are attainable by dispensing with the filiform shape of the beam and utilizing instead one or more tubular beams of the same or different cross section and accelerating these beams with annular particle-accelerator means along an elongated evacuated accelerator chamber terminating inrotationally symmetrical deflector means adapted to direct the tubular beam inwardly toward the axis.

The present invention is applicable to both directions and resonant linear accelerators of charged particles operating with ion or electron beams. The basic system thus comprises an elongated evacuated accelerator chamber having an axis along which the particle-accelerator means is distributed; means is provided at one end of this chamber injecting into the latter a tubular beam of charged particles while a collimator or prefocusing means is disposed upstream of the particle-accelerator means. Downstream of the particle-accelerator means, usually at the target end or output end of the accelerator, I provide the deflector means which has rotationally symmetrical electrostatic deflector electrodes. The particle-accelerator means intermediate the electrostatic annular deflector means and the prefocusing lens may be'of any conventional type, i.e. may include drift tubes or tubular electrodes sustaining uniform or nonuniform electrostatic fields across 1 the gaps between them or resonant arrangements designed to sustain a traveling or standing electromagnetic wave capable of propagating the particles at substantially the phase velocity of the wave. Each of the electrodes means of an electrostatic or direct accelerator and each of the diaphragm of a resonant or waveguide accelerator preferably is of disc shape and lies in a respective plane perpendicular to the axis of the accelerator.

The above and other objects, features and advantages of the present invention will become more readily apparent from the following description, reference being made to the accompanying drawing in which:

FIG. 1' is a diagrammatic axial cross-sectional view through a direct accelerator making use of a tubular beam in accordance with the present invention and of nonuniform electrostatic fields;

FIG. 1A is a cross-sectional view taken along the line IA-IA of FIG. 1;

FIG. 2 is a partial longitudinal view, also'in diagrammatic form, of an accelerator of the direct type using a uniform electrostatic field in accordance with the principles of the present invention;

FIG. 2A is a cross-sectional view taken along line 2A 2A of FIG. 2;

FIG. 2B'is a fragmentary view similar to FIG. 2A but illustrating a modification of the present invention;

FIG. 2C is a view generally similar to FIG. 2 showing another embodiment of this invention;

FIG. 3 is a view similar to FIG. 1 of awaveguide accelerator embodying this invention;

FIGS. 4 and 5 are cross sections in diagrammatic form representing drift tubes and interdigital waveguide particle-accelerator structures; and" LII FIGS. 6A and 6B are diagrammatically longitudinal sections showing resonant cavities forming accelerator structures in accordance with the principles of the present invention.

In FIGS. 1 and 1A, 1 show a direct linear accelerator making use of a tubular ion or electron beams and a uniform electrostatic field. In accordance with this invention, the accelerator comprises an elongated accelerator chamber or envelope 7 which is evacuated by a pump and is provided, at one end, with a source S of an annular ion beam represented at F. A suitable annular electron source is described in the Journal of Applied Physics, Vol. 30, Page 826 (1959), while a suitable annular ion source is described in the Journal de Physique et Radium, Vol. 12, Page 563(1951 Ahead the source S in the evacuated chamber 7 of the accelerator, there is provided a rotationally symmetrical annular lens 1 or an annular immersion lens, which prefocuses the beam F. The intermediate lens 1 shown in FIG. 1 comprises three equidistant metallic disc members 1a, lb and formed with annular slits la, lb and 1c through which the beam F passes. The electrostatic potential is delivered to these electrodes from the usual source Id. The electrodes la, 1 b and 10 may each include an outer member'as represented for example at la" and an inner member 1a" electrically connected together. Theinner members are mounted upon a support rod 30 while the outer members are mounted upon the housing 7. As is well known, the axial terminal electrode 1c is of different potential, thereby focusing the annular beam at a location L just ahead of the particle-accelerator means. Other electrostatic lens assemblies may, of course, be used. In each case, the intermediate lens assembly 1 will focus the particle beam radially and can be considered the optical equivalent of a condensing lens whose focus L is a circle centered on the axis A of the accelerator.

Downstream of the lens assembly 1, I provide particle-accelerator means as represented by the electrode assemblies 2,

Y 3, 4 and 5 axially spaced along the chamber 7" and separated by accelerator gaps 2' 3 and 4 across which the electrostatic potential is applied by the source EF of conventional design. As indicated, each of the electrode assemblies also comprises an inner electrode (e.g. represented at 212 and an outer electrode 2b of coaxial tubular construction mounted respectively upon the support rod 30 and the housing 7. The source EF may provide the required potentials at the inner and outer electrode members, which are aligned along planes perpendicular to the axis A, so as to form further annular lenses radially focusing the beam F. The electrostatic field may be continuous or pulsed. The accelerated beam B, emerging y from the particle accelerator means 2 5, passes into an annular deflector means represented at 6 and rotationally symmetrical with respect to the axis A and designed to focus the beam at a point L at the output side of the accelerator. The deflector here comprises an inner electrode 6a curved forwardly and inwardly and receiving different potentials from the deflectro-field source DF of conventional type. The inner electrode 6a is held in the central support 30 while the outer :electrode is anchored in the outer wall of housing 7. A further enhancement of the intensity of the beam focused at the point L may be obtained if several cylindrical beams and a filiform beam are accelerated through the system (cf. FIG. 2B).

In FIGS. 2 and 2A, I have shown a partial longitudinal section of the particle-accelerator means of a direct accelerator provided with a source S, an intermediate lens] and an annular deflecting means 6, as described in connection with FIG. I.

The particle-accelerator means is received within the evaluated envelopes l2 and comprises a plurality of axially spaced electrode disc means connected to the source AF. Each of the disc means 8-l.1, which serve to accelerate the beam and radially focus the latter, comprises a pair of coplanar electrodes as shown, for example, at 8a and 8b, respectively anchored to the envelope 12 and a support 31. The coplanar disc members define circular slits (e.g. St) between them to permit the passage of the electron or ion beam.

' 2B. In this case, a central support is omitted and a further axial passage 34 formed along the entire length of the accelerator to accommodate a filiform beam.

'FIG. 3 shows a waveguide type of linear accelerators, (see NEW SCIENTIST, 14 Dec. 1967, page 663 ff.) which comprises a tubular waveguide 14 disposed within the evacuated envelope 12 and receiving an annular electron beam from an injector 13 of the character previously described. A standing or traveling wave is established in the resonant cavities 15a between the loading discs 15 which are here shown to extend inwardly in coplanar arrays from the inner and outer peripheries of the tubular waveguide. The loading discs or diaphragms 15 may also be disposed interdigitally (FIG. 5) or may be provided on only one of the walls of the tubular waveguide. Wave power may be introduced in the usual manner by the oscillator 0 into the resonant cavities. Downstream of the particle- accelerator waveguide 14,1 provide the annulardeflector arrangement 16 previously described. In each" of the resonant cavities, a traveling or standing Eewave is maintained with a phase velocity with which the beam vtravels, the crests of the E- wave. An electrode static focusing leans may be provided between the annular electron source 13.,a rid the waveguide 14. Radiofrequenciesare preferred for the accelerating traveling or standing waves.

Two other particle-accelerator arrangements suitable for use with the genera] combination described in connection with FIG. 1, are represented in FIGS. 4 and 5. In the former, the particle-accelerator means between the annular lens and the annular deflector accelerates the ion beam between a plurality of drift-tube assemblies 18, each comprising an inner member 18a and an outer member 18b spaced apart along the annular slit 180. The particle-accelerator means illustrated in FIG. 5 comprises interdigital loading discs 19 provided with substantially circular slits 19a axially aligned along the chamber and extending alternately inwardly from the inner and outer peripheries of the chamber. Here again, a radiofrequency field can be injected via a generator (FIG. 2) into the particle-accelerator means which extends from the upstream lens to the downstream annular deflector. In the systems of FIGS. 4 and 5, a pump arrangement such as that described in connection with FIG. 1, evacuates the accelerated chambers. In the system of FIGS. 6A and 63, acceleration takes place in a resonant cavity into which the tubular ion or electron beam is introduced from an annular source represented at 20. In the system of FIG. 6A, acceleration occurs in a cylindrical cavity whereas in the system of FIG. 6B, a coaxial cavity is employed. In both case, a vacuum pump evacuates the device.

I claim:

1. An ion accelerator comprising an elongated evacuated accelerator chamber having an axis; means at one end of said chamber for injecting generally axially into said chamber an annular ion beam substantially centered on said axis; and annular ion-accelerator means including a plurality of annular electrodes spaced along said chamber for accelerating said annular ion beam in the direction of the other end thereof.

2. An ion accelerator comprising an elongated evacuated accelerator chamber having an axis; means at one end of said chamber for injecting generally axially into said chamber an annular ion beam substantially centered on said axis; annular ion-accelerator means along said chamber for accelerating said annular ion beam in the direction of the other end thereof; and annular deflector means at said other end of said chamber rotationally symmetrical about said axis for focusing the annular ion beam accelerated by said particle-accelerato means toward said axis.

3. An ion accelerator asdefined in claim 2 wherein said ionaccelerator means includes a multiplicity of spaced-apart annular accelerator electrodes adapted to sustain ion-acceleration fields between the successive annular electrodes, each of said electrodes including an annular outer member and an annular inner member defining with the corresponding outer member an annular window traversed by said beam.

4. A charged-particle accelerator comprising: an elongated evacuated accelerator chamber having an axis; means at one end of said chamber for injecting into said chamber a tubular beam 'of said particles substantially centered on said axis; particle-accelerator means along said chamber for accelerating said tubular beam in the direction of the other end thereof; and annular deflector means at said other end of said chamber rotationally symmetrical about said axis for focusing the tubular particle beam accelerated by said particle-accelerator means toward said axis, said particle-accelerator means including a pluralityof spaced-apart annular accelerator electrodes adapted to sustain particle-acceleration fieldsbetween the successive annular electrodes, each of said accelerating electrodes including an inner electrode member surrounded by said beam and an outer electrode member coaxial with said inner electrode member and said beam while surrounding the latter.

5. A charged-particle accelerator as defined in claim 4 wherein said electrode members of each of said accelerator electrodes are discs lying in a common plane perpendicular to said axis.

6. A charged-particle accelerator as defined in claim 4 wherein said inner and outer members of each of said accelerator electrodes are coaxial cylinders.

7. An ion accelerator as defined in claim 3 wherein said accelerator electrodes are discs lying in respective axially spaced planes perpendicular to said axis and'formed with circular slots axially aligned along said chamber.

8. An ion accelerator as defined in claim 2 wherein said ion accelerator means includes annular waveguide means in said chamber between said injecting means and said deflector means for sustaining a radiofrequency field accelerating said beam.

9. An ion accelerator as defined in claim 8 wherein said waveguide means includes a plurality of loading-disc means axially spaced apart along said chamber and lying in planes perpendicular to said axis.

10. A charged-particle accelerator comprising: i

an elongated evacuated accelerator chamber having an axis;

means at one end of said chamber for injecting into said chamber a tubular beam of said particles substantially centered on said axis;

particle-accelerator means along said chamber for accelerating said tubular beam in the direction of the other end thereof; and

annular deflector means at said other end of said chamber rotationally symmetrical about said axis for focusing the tubular particle beam accelerated by said particle-accelerator means toward said axis, said particle-accelerator means including annular waveguide means in said chamber between said injecting means and said deflector means for sustaining a radiofrequency field accelerating said beam, said waveguide em means including a plurality of loading-disk means axially spaced apart along said chamber and lying in planes perpendicular to said axis, each of said loading-disc means including an inner disc member defining the inner boundary of said beam and surrounded thereby, and an outer disc member coplanar with said inner disc member and defining the outer boundary of said beam.

11. A charged-particle accelerator comprising:

an elongated evacuated accelerator chamber having an axis;

means at one end of said chamber for injecting into said chamber a tubular beam of said particles substantially centered on said axis;

particle-accelerator means along said chamber for accelerating said tubular beam in the direction of the other end thereof; and

annular deflector means at said other end of said chamber rotationally symmetrical about said axis for focusing the tubular particle beam accelerated by said particle-accelerator means toward said axis, said particle-accelerator means including a plurality of disc members formed with annular slits axially aligned along said chamber and axially spaced apart therein, said discs extending alternately inwardly into the path of said beam from within and without and constituting an interdigital accelerator structure.

12. A charged-particle accelerator as defined in claim 4,

further comprising an intermediate lens for radially focusing the tubular beam, said lens being disposed upstream of said particle accelerator means.

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