US2790902A - Ion accelerator beam extractor - Google Patents

Description

April 0, 1957 B. T. WRIGHT 2,790,902

ION ACCELERATOR BEAM EXTRACTOR Filed March 5. 1954 ELECT R0 DE POWER SUPPLY 5 22 F. M. RADIO POWER FREQUENCY OSCILLATOR SUPPLY ION INJECTOR POWER SUPPLY \39 Window 42% 2 v v n m w 4 J, 600 I \43 O l 590" INVENTOR. Mogne138/ BYRON T. WRIGHT 3 i i i BY Turget34 6|0" n ANGULAR POSITION ATTORNEY.

-efliciencyraccelerator beam deflector which relates to improvements in ion acceleraticularly to means for extracti x 7 cle beam from a high energy accelera- ..s hesyn tqt qa as .--..H t te.-Pa ic e a c er es f .t' esi q type have largely been used for the acceleration oif electrons. However, radiation losses eflie'ctively limit the ener y hic mayb rsa b h sm y. pertain otherconsiderations, well understood within the a t. lim th att na le rti e nsr ii h ic l p rincipal,types of magnetic accelerators: the cyclotron, t c .syu hrdyc qt an h tio -r A c di y, it,,appears that the most feasible means of obtaining higher ,energy, particles consists of utilizing the synfchrotron principle. for the acceleration of ions of substantially greater mass than the electron, 7 p exampleof a synchrotron classaccelerator which -is adapted to the acceleration of relatively heavy, particles is the proton synchrotron, hereinafter referred as the beyatron Thedetailedconstruction and operation of the .device'is disclosedin the abandoned application of W. M. Brobeck et al., Serial No. 196,048, filed November 16, 1950, anabstract of which was published ]anuary 30, 119.51 in the Oflicial ,Gazettepof the United States Patent .Oflice 642 0. CL. 1880,. Additional disclosure, may be foundinU. S. Patent 2,658,929,, issued to-G. M. Farly for. :Bevatron Acceleration Regulation on November 10,

The.. stated..accelerator was initiallydesiggned to utilize an internaLtarget which might be the materialto be bombarded or aconvertor. nieansiforfproducing an external. neutron. beam. .Thepossibilitypf extracting I an external ion beam..was.not {given extended consideration inasmuch as: such-anoperation, using known techniques, wasnecessarily difiicult in this form of accelerator. It was recognized however,...that such a beamwouldhave .valuable research applications, and accordingly the presentinvention contemplates a novel and practical means for. achieving this result. ,It. is therefore an object of this invention to provide .means forextracting a charged particle beam from a magnetic accelerator. I, A M

It is afurther object to ,provideimeans whereby a magnetic beam "deflector maybe disposed in a particle accelerator at a suflicient distance ,from the accelerating cirbi t a's not to -appreciably affect the particles during the interval of acceleration. l v in It .is an object "of L-the :present invention to provide m gns whereby particles .circul'atinginthe accelerating orbit of a synchrotron may be-caused, ,toenter the full new o'f a deflector niagnetina single revolutionwithout h aviri'g prviously been subjected to an appreciable influence from thefield.-. p ,1 It is another object of this invention tq'provide a high is not criticalet parameters. Theinventiomboth astp its organig'ation arid niethpd of operation, together with furtherohje'ctsand advan- 'ice e'r f, wil best be u iidei'stood by reference to the i peeiflc ion takes in conjunction with the accsmpsfiyifi drawing, in which:

Figure 1 is a partially broken-out plan view' of a 5 Bavarian with auxiliary equipment and showing the present invention;

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

l' ig i;1 r e is a cross-sectional view of a segment of the bevatron taken along line .3 3 of Fig. 1, and showih g additional components of the beam extractor; and

jg l cfl is a diagram showing the radial position of the ion beam as it describesits final revolution under the action ot the beam extractor. A

7 Referencewill first be rnade to such of the structure and operation of the beva-tron as is necessary to an understanding ofthe present invention, additional description of this accelerator being available in the previously mentioned patent, patent application, and abstract therefrom. I 7 V As shownin Fig 1, the bevatron comprises a tubular vacuum tank having four arcuate 90 sections 11 joined by four straight jsections 12., 13, 14, and 16. Suitable vacuum pumps, in this instance oil diffusion pnmps 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 electromagn'et 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 portionsof the vacuum tank, which is normal to the plane of Fig. 1'.

An io'n injector 23 .is disposed adjacent one of the straight sections 12 of the vacuum tank and comprises an ion source and means for imparting a relatively small initialacceleration to the ions. In the present embodiment, the injector 23 is a small linear accelerator of conventi'onal 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 direction of the channel therein, an arcuate tubulation 24 isdispos'ed with one 45 extremity tangent to straight section Hand connects the interior of the tank with the injector 23. To constrain 'the'io'ns trominjectdr '23 to follow the arcuate configuration of the tabulation 2'4, a curved elongated deflector 'elec't'ro'de26 is insulatingly'mounted within the tubulation, similarly shaped grounded electrode 27, and ined at an elevated'pbtential of such magnitude 'asto "cause ions from injector 23 to follow an arcuate path and enter section 1'2fg'e'rie'1ally tangent to the center line ni'rest. As will hereinafter bedescrihed, electro- 55 magnets 19 may be'ei' er'g'iie'd to produce a fie'l'dof s ufiic ient magnitude 'that the i'ons will be deflected within arc't are s c ons I1 and will renew aclosed orbit assene r'a lIy Cb-extensive with the center-line of "the vacuum tank.

60 Atu'bu l'ar accelerating "electiode'i'w is disposedinthe straight section 16 of the vacuum tank which is threequarters of a revolution "in advance of the 'section'lz'into which the ions are infected. The electrode is spaced apart from the wall of the vacuum tankand is mounted 65 coaxialiy therein, by means of suitable insulators 31, 'An lectri al'c cilla cr $2,,Q Va ia le ir su cy. is sacred w t asn t blerpows su p yfi a dap dt impress an alternating potentialon the electrode 29. For a detailed disclosure of a suitable oscillator nit-tans,

70 as well as a more hxhaustive'description of a suitable pows ss ni .2, i92 eise sn te CW 'herriadeito'the-previouslycitd l atent No.' 2,'658,999.

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, insuch 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 eifect 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 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 sufliciently great that electromagnetic deflecting means may be utilized to deflect the decelerated ions without the field of the deflecting means having an appreciable .efiect on ions circulating in the primary equilibrium orbit 28.

As shown in Figs. 1 and 3, the deflector means may be an electromagnet 38 connectedwith 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 de celerated 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 the accelerator will be more readily understood by reference 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 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 etficiency of approximately one percent per square inch of detector may be had, with the detector located outside the shielding of the accelerator.

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 inventio except as defined in the following claims.

What is claimed is:

1. In a. charged particle accelerator having a stable closed curvilinear particle orbit defined by a magnetic field, the combination comprising a particle decelerating substance disposed radially outward from said orbit,

charged particle deflecting means disposed radially inward from said orbit, and means decreasing the magnetic field strength in relation to the energy of said particles to expand said orbit and impinge the particles therein on said particle decelerating substance whereby said particles will lose energy traversing said particle decelerating substance and oscillate into the influence of said deflector means.

2. In a charged particle accelerator characterized by a magnetically established closed particle orbit, the combination comprising a target element disposed adjacent said orbit radially outward therefrom, said target element having a substantial thickness of dense material to decrease the energy of charged particles caused to traverse it, a charged particle deflecting element disposed radially inward from said orbit, and electromagnetic means expanding the diameter of said orbit whereby particles circulating therearound impinge upon said target element and undergo deceleration with consequent osciilation into the influence of said deflecting element.

3. In a charged particle accelerator having a closed stable particle orbit established by an electromagnet, a

earn extractor comprising an energy absorbant target element positioned adjacent the outer boundary of said orbit, means for decreasing the field of said electromagnet to enlarge said orbit and direct high energy particles through said energy absorbant target element thus causing said particles to assume an oscillatory trajectory in which said particles oscillate radially with respect to said stable orbit, and a deflector magnet disposed proximal to the inner boundary of said orbit and having a field transecting the oscillatory trajectory of particles emerging from said target element whereby said particles are diverged from said orbit.

4. In a bevatron having a pulsed electromagnet adapted to establish a closed ion orbit, a beam extractor comprising a target member disposed within the field of said electromagnet radially outward from said orbit, said target member having a substantial thickness of ion energy attritive material to induce radial oscillation of ions traversed therethrough by the expansion of said orbit by the decreasing field of said pulsed electromagnet, and ion deflecting means disposed radially inward from said orbit, said deflecting means having an azimuthal distance from said target member equal to the azimuthal distance travelled by said ions in an odd multiple of half cycles at the frequency of said radial oscillation.

5. In a bevatron, a beam extractor substantially as described in claim 4 wherein said ion deflecting means comprises a magnet having a field transverse to said orbit to deflect ions from said orbit.

6. In a bevatron having a closed ion orbit established by an electromagnet, said electromagnet having variable field strength whereby said orbit is expandable, a beam extractor comprising a target element positioned at the maximum expansion of said orbit and formed of ion energy attritive material to cause said beam to seek a new orbit of substantially reduced diameter, and a deflector magnet disposed at the point of maximum inward movement of said ions, said deflector magnet having a magnetic field normal to the plane of said orbit to impel said ions away therefrom.

7. In an ion accelerator of the synchrotron class having an ion orbit defined by pulsed electromagnet means, an ion extractor comprising a block of substantially dense target material disposed adjacent the perimeter of said orbit to induce radial oscillation of ions traversed therethrough, means decreasing the field of said electromagnet to expand said orbit and impinge the ions therein on said block of target material, and an electromagnetic deflector having a magnetic field perpendicular to the plane of said orbit to deflect ions passing through said field radially outward with respect to said orbit, said electromagnetic deflector having an orbital distance from said target material equal to the orbital distance traversed by said ions in one-half cycle of said radial oscillation.

References Cited in the file of this patent UNITED STATES PATENTS 2,615,129 McMillan Oct. 21, 1952 2,640,923 Pollock June 2, 1953 2,721,949 Gund et al Oct. 25, 1955

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