Ion accelerator beam extractor

April 30, 1957

B. T. WRIGHT

2,790,902

ION ACCELERATOR BEAM EXTRACTOR

Filed March 3. 1954

3/·

ION

INJECTOR

POWER SUPPLY

ELECTRODE POWER SUPPLY

32-^ ^F- ^M· RADIO FREQUENCY OSCILLATOR

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2S &lt;23

INVENTOR.

BYRON T WRIGHT

POWER

RADIAL POSITION

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BY

0°

90°

180°

270°

360°

ANGULAR POSITION

Attorney.

United States Patent Office

2,790,902

Patented Apr. 30, 1957

2,790,902

ION ACcELEEXTOR BEAM EXTRACTOR

Byron T. Wright, Lois Angeles, Calif., assignor to the United States of America as represented by the United States Atomic Energy Commission

Application March 3,1954, Serial No. 413,973

Claims. (CI. 250—27) ,, This invention relates to improvements in ion accelerators anymore particularly to improved means for extracting a-charged particle beam frorri a high energy accelerator of the synchrotron class.

Heretofore, particle accelerators of the synchrotron type have largely been used for the acceleration of. electrpns. However, radiation losses effectively limit the energy which may be reached by this means. Similarly, certain other considerations, well understood within the art, limit the attainable particle., energy in the other principal types of magnetic accelerators: the cyclotron, the synchro-cyclotron, and the betatron. Accordingly, it appears that the most feasible means of obtaining higher energy particles consists of utilizing the synchrotron principle for the acceleration of ions of substantially greater mass than the electron.

An example of a synchrotron class accelerator which is adapted to the acceleration of relatively heavy, particles is the proton synchrotron, hereinafter referred as the bevatron. The detailed, construction and operation of the device is disclosed, in the abandoned application of W. M. Brobeck etal., Serial No. 196,048, filed November 16, 1950, an abstract of which was published January 30, 1951 in the Official Gazette , of the United States Patent Office 642 O. G. 1880. . Additional disclosure may be found .in U. S. Patent 2,658,999, issued to G. M, Farly for Bevatron Acceleration Regulation on November 10, 1953,. ..........

The., stated accelerator was initially ...designed to utilize an internal..target which might be the material to be bombarded or a.convertor means· for producing an external neutron, beam. The. possibility, of extracting an external ion beam was. not given, extended consideration inasmuch as· such, an operation, using known techniques, was. necessarily difficult in this form of accelerator. It was recognized,, however, ...that such a beam would have valuable research applications, and accordingly the present invention contemplates a novel and practical means for achieving this result. .

.It is therefore an object of this invention to provide means for extracting a charged particle beam from a magnetic accelerator, .

It is a further object to provide, means whereby a magnetic beam deflector may .be disposed in a particle accelerator at a sufficient distance .from the accelerating orbit as hot to appreciably affect the particles during the interval of acceleration.

It is an object of the. present, invention to provide mdans whereby particles circulating.in,the accelerating orbit of a synchrotron may be caused to enter the full field of a deflector magnet-in .a single revolution, without having previously -been subjected to an appreciable influence from the. field, J , ,.,

It is another object of this invention to provide a high efficiency accelerator-beam deflector which is not critically-dependent upon the accelerator magnet parameters.

The. invention, both as to its organization and method of operation, together with further objects and advan10 _ _t . . 2 tages thereof, will, best be understood by reference to the following specification taken in conjunction with the accompanying drawing, in which:

Figure 1 is a partially broken-out plan view of a bevafron with auxiliary equipment and showing the present invention;

Figure 2 is a cross-sectional view of a portion of the bevatron taken along line 2—2 of Fig. 1 and showing one element of the beam extraction apparatus;

Figure 3 is a cross-sectional view of a segment of the bevatron taken along line 3—3 of Fig. 1, and showing additional components of the beam extractor; and

Figure 4 is a diagram showing the radial position of the ion beam as it describes its final revolution under the action of the beam extractor.

Reference will first be made to such of the structure and operation of the bevatron as is necessary to an understanding of. the present invention, additional description of this accelerator being available in the previously mentioned patent, patent application, and abstract therefrom.

As shown in Fig. 1, the bevatron comprises a tubular vacuum tank having four arcuate 90° sections 11 joined by four straight sections 12, 13, 14, and 16. Suitable vacuum pumps, in this instance oil diffusion pumps 17, are adapted to evacuate the tank through openings 18 in the walls .of straight sections 12, 13 14, and 16. Disposed around, each of the arcuate vacuum tank sections 11 is a hollow arcuate 90° electromagnet segment 19. The electromagnet segments 19 are energized by windings 21 connected to a pulsed current source 22 and are adapted to establish a magnetic field, in the curved portions. of the vacuum tank, which is normal to the plane of Fig. 1.

An ioh injector 23 is disposed adjacent one of the straight sections 12 of the v'aciitim tank and comprises an ion source and means for imparting a relatively small initial. acceleration to the ions. In the present embodiment, the injector 23 is a small linear accelerator of conventional design which is adapted to inject protons at 10 m. e. v. In order to cause the ions to enter the vacuum tank generally parallel with the direction of the channel therein, an arcuate tubulation 24 is . disposed with one 45 extremity tangent to straight section 12 and connects the interior of the tank with the injector 23. To constrain the ions from injector 23 to follow the arcuate configuration of the tubulation 24, a curved elongated deflector electrode'2'6 is insulatingly mounted within the tubulation, 50 opposite a similarly Shaped grounded electrode 27, and is maintained at ah elevated potential of such magnitude as to cause ions from injector 23 to follow an arcuate path and enter section 12 generally tangent to the center line tliefeof. As will hereinafter be described, electro55 magnets 19 may be energized to produce a field of sufficient magnitude that the tons will be deflected within arcuate sections ΤΪ and will follow a closed Orbit 28 generally co-extensive with the center-liiie ’ of the vacuum tank.

Atubiilhr accelerating 'electrode '29 is disposed'in the straight section 16 of the vacuum tank which is threequarters Of a revolution in advance of the section 12 into which the ions fire injected. The electrode 29 is spaced apfirt from the wall Of the vaciium tank and is mounted 05 coaxially therein , by means of suitable insulators 31, An electricaTpscillator.32, of variable frequency, is connected with a, suitable'power supply 33 and is adapted to impress an aiternating_ potential on the electrode Ϊ9. For a detailed disclosure of a suitable o'scillator means, γθ as well as a more exhaustive' description of a suitable magnet .power supply 22, ion injector 23, and the controls and inter-connecttons 'tfierefietwBen, referehe’e ^!trSy be made to the previously cited Patent No. ' 2,658,999.

2,790,902

Considering now a brief description of the operation of the bevatron, as a preliminary to a disclosure of the present invention, it will be observed that a pulse of ions expelled from the injector 23 will be deflected by electrode 26 into the vacuum tank, and, under the influence of magnets 19, will circulate around the equilibrium orbit 28.

Oscillator 32 is operated at a frequency and phase such as to provide an attractive field as the ions approach the accelerating electrode 29, and a repulsive field as the ions leave the electrode. Thus the ions receive an energy increase during each revolution through the tank. It will be apparent that as the ions gain velocity, the frequency of oscillator 32 must be increased to maintain the proper phase relationship. Similarly, the field of magnets 19 must be continually increased, as the ions gain energy, to preserve the equilibrium orbit 28 in its initial position. A consideration of the properties of charged particles moving through magnetic fields will show that the diameter of the equilibrium orbit 28 is critically dependent upon the ratio of the particle energy to the field strength. Thus extremely sensitive co-ordination of the magnetic and electric fields is necessary for satisfactory operation of the accelerator. It will also be seen that the orbit may be expanded at the end of an interval of ion acceleration by increasing this ratio, for example: by decreasing the field of the magnets 19.

It is expected that the above-described bevatron will accelerate protons to energies in excess of 6 b. e. v. In this class of accelerator, and at this energy, the problem of extracting a usable external particle beam is appreciable. One means of achieving this result, well understood within the art, is to alter the magnet field strength causing the circulating ion beam to spiral to one wall of the vacuum tank and strike a suitably placed target. In this manner a beam of neutrons may be produced and, owing to the electrical neutrality of such particles, the beam will travel in a linear path and will readily pass through a suitably placed window in the wall of the tank.

However, it is desirable in some applications of the bevatron to obtain an external ion beam. Considering now the present invention, by which means circulating protons may be extracted from the accelerator, there is shown in Figs. 1 and 2, a target 34 disposed within straight section 14 of the vacuum tank. The target 34 is disposed radially outward from the equilibrium orbit 28, in such position that the circulating high energy ions may be caused to strike it by decreasing the field of the magnets 19 in the manner described above. In Fig. 1 the path of the circulating ions following the decrease in field strength is indicated by trajectory 36. The target 34 is preferably a rectangular block and is composed of such material that the ions will lose sufficient energy passing through it to establish a secondary equilibrium orbit having a diameter substantially less than that of the primary orbit 28. In the terminology used above, the effect of target 34 is to contract the orbit by decreasing the ratio of particle energy to field strength.

In the present embodiment, the primary equilibrium orbit 28 has a mean radius of 607 inches. The target 34 is a 3.4 inch thickness of beryllium which will decrease the energy of 6.197 b. e. v. protons by 29 m. e. v. establishing a new equilibrium orbit having a radius of 600 inches.

In accordance with bevatron theory, the decelerated ions undergo a sinusoidal radial oscillation in seeking the new equilibrium orbit, as indicated by ion path 37 in Fig. 1. As is well understood within the art, the magnitude of this oscillation, for ions of a given energy, is dependent upon the magnetic field index (n) of the accelerator, the index being defined as n=—(.r/H){dH/dr) where H is the magnetic field strength and r is the radius. In the present embodiment, n was made equal to 0.65 and it may be shown that the first maximum inward excursion of the decelerated ions, corresponding to completion of one-half cycle of radial oscillation, occurs threequarters of a revolution after emergence of the ions from the target 34. At the point of maximum inward excursion, in straight section 13, the radial position of the decelerated ions is 590 inches, corresponding to a mean displacement of 17 inches from the primary equilibrium orbit 28. This displacement is sufficiently great that electromagnetic deflecting means may be utilized to deflect the decelerated ions without the field of the deflecting means having an appreciable effect on ions circulating in the primary equilibrium orbit 28.

As shown in Figs. 1 and 3, the deflector means may be an electromagnet 38 connected with a power supply 39 and having poles 41 which project a short distance into straight section 13 in such a manner as to establish a vertical magnetic field at the point of maximum inward ion excursion. Magnet 38, in this embodiment, is of such strength as to impart a small deflection to the decelerated ions causing the beam to emerge from an ion transparent window 42 disposed in the outside wall of the following straight section 14, as shown by ion trajectory 43 in Fig. 1.

The action of the ions during the final revolution within ϊ the accelerator will be more readily understood by refer- &gt; ence to Fig. 4 wherein the angular position of the decelerated ion beam, measured in degrees of a revolution from the target 34, is plotted against radial position with respect to the center of the accelerator. The presence of straight sections 12, 13, 14, and 16 in the accelerator is disregarded in Fig. 4 inasmuch as they have no effect on the electromagnetic characteristics of the system. It will be appreciated that the numerical values in Fig. 4 are illustrative of one embodiment of the invention, and will vary according to the parameters of each particular accelerator.

In Fig. 4, the shaded area indicates the approximate width of the primary equilibrium orbit 28 which has a mean radius of 607 inches. The target 34 is preferably disposed adjacent the outer limit of this orbit. In this instance the decelerated ions require 270° of revolution to complete one-half cycle of radial oscillation, as shown by ion trajectory 37. The width of this oscillation, using the stated proton energy, target material, and the electromagnet characteristics of the present accelerator, is 20 inches. Thus the deflector magnet 38 may be disposed at a radius 20 inches less than that of the target 34. The distance, 14 inches in this instance, between the inner boundary of the primary equilibrium orbit 28 and the deflector magnet 38 is sufficient that the field of the magnet need not appreciably influence the movement of ions in the equilibrium orbit. It has been found satisfactory, in this embodiment, to maintain the field of the deflector magnet 38 at a strength sufficient to deflect the trajectory of the decelerated ions by 2.3 degrees, causing the ions to emerge from the window 42 after slightly less than 90° of further revolution. An analysis of the magnitude of the scattering induced by passing the beam through the target 34 will show that a relatively high proportion of the ions can be made to enter the effective field of the deflector magnet 38. An efficiency N of approximately one percent per square inch of detector may be had, with the detector located outside the shield- γ ing of the accelerator. J

While the invention has been disclosed with respect to a single preferred embodiment, it will be apparent to those skilled in the art that numerous variations and modifications may be made within the spirit and scope of the invention and thus it is not intended to limit the invention except as defined in the following claims.