US2789221A - Method and apparatus for nuclear particle acceleration - Google Patents

Description

METHOD AND APPARATUS FOR NUCLEAR PARTICLE ACCELERATION Filed April 20, 1954" C. A. TOBIAS April 16, 1957 2 Sheets-Sheet 1 INVENTOR. CORNELIUS A. TOE/AS ATTORNEY.

C. A. TOBIAS April 16, 1957 METHOD AND APPARATUS FOR NUCLEAR PARTICLE ACCELERATION 2 Sheets-Sheet 2 Filed April 20, 1954 INVENTOR. CORNELIUS A. TOE/AS ATTORNEY.

United States Patent 2,782,221 --rtrnrnon nNnAPPARATus. FOR'NUCLEAR V PARTICLE ACCELERATION Coi'lielius Tobias, Walnut Creek, Califi, assignor to the United States of America as represented by the United States Atomic Energy Commission Application April 20, 1954, Serial No.424,534 11. (c1. 250 27 This invention relates to a. method and means I for accelerating ions to higher energies than has heretofore been feasible. .ln particular, the invention relates to a vrnethod and apparatus for accelerating'multiply charged atomic nuclei .in magnetic particle accelerators. The invention will be found applicable to charged particle accelerators of the class which employ magnetic :rneans to confine ions in arcuate orbits. Three types of acce1erators falling within this class are the cyclotron, :synchrocyclotron, and proton synchrotron or bevatron. Adetailed description of these devices may be had from thcfollowing patents and patent applications: cyclotron, Patent No. 1,948,384, Method and Apparatus for the Acceleration of Ions, issued to E. O.'Lawrence February .20, .1934; synchrocyclotron, Patent No. 2,615,129, issued to .M.,McMillan.October 21, 1952; and beyatron, abandoned U. -S. ..patent application Ser. No. 196,048, M. Brubeck, ;filed November 16, 1950, an abstract of which was-published January 30, 195-1, in the Oflicial Gazette of the U. S. Patent Oflice 642 O. G. 1880.

For .purposes of illustration, the invention will be described -as-appliedto a cyclotron and a-detailed description of 'thisembodiment will be given. .It'will be apparent .to those skilled in the art, however, that the invenltionisapplicable to ion accelerators ingeneral and particularly to the types recited above.

In the operation of ion accelerators, it is of course desirable to obtain beams of as high energy as is possible. The present invention contemplates achieving substantially higher energies, in accelerators :of existing sizes, by providing novel means for accelerating nuclei which are appreciably heavier than the usually employed protons, deuterons, and alpha particles. Such heavy nuclei transfer higher excitation energy to target nuclei and. exhibit a higher cross-section for interaction with fmafter. By accelerating heavier .ions, existing accelerators ma be adapted to produce ion beams of sufiicient "energ to reproduce a portion of the primary spectrum of cosmic radiation.

The degree of acceleration that an ion will experience in an accelerator is, in a general way, proportional to the square of its charge and inversely proportional to its j'r'n'ass. Thus, effective acceleration of heavy ions may be achieved if multiple charge can be placed and maintained on'them. It will be apparent, from a consideration of accelerator structure, that serious obstacles will be e'rlqountei'ed 'in producing and maintaining multiply charged ions within conventional devices. Considering, 'for example, the cyclotron, which comprises an ion source di'qiosed in a vacuum tank between two opposed magnet poles and between electrode structure which is adapted to create an alternating electric field at right angles to the magnetic field, several 'difiiculties will be "apparent. The ion source generally employed will not deliver "a satisfactory yield of multiply charged ions. "Assuming a suitable source could be found, the problem of adaptingit to tit-within the restricted space available at the .eenterof the cyclotron would be appreciable. 'It

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will be further observed that gas density is particularly high in the ion.source region of the cyclotron. This factor, in conjunction with the low velocity of ions immediately following emergence from the source, would act to promote charge exchange with the gas and complicate the problem of maintaining the ions. .in their highly charged state during the period of acceleration. An analysis will show that generally similar limitations apply to the acceleration ofheavy ionsin the other principal types of accelerator. I

Now, it has been found that these objections may be overcome, and thegeneralretficiency of the cyclotron improved, by utilizing a conventional ion source disposed outside the accelerator. The. source is adapted .to impart a-low charge to-atoms of relatively high atomic number. The ions thus produced are then accelerated by a suitable preaccelerator means, such as a-smalllinear accelerator, to a velocity comparable to the orbital velocity of their K electrons. The lightly charged, partially accelerated ions are directed into the cyclotron and caused to impinge on a thin metallic foil disposed near the center of the system. It will be appreciated that the charged stateof the ions will result-in their following an arcuate path in the magnetic field of the cyclotron. This curvature, which will be relatively slight owing to the low charge to mass ratio of the ions, must be taken into account in directing the ions towards the center of the accelerator.

in traversing the toilet or above the stated velocity, the ions will be stripped of their remaining orbital electrons. in this manner, completely ionized heavy nuclei can be made to appear in their natural orbit within the cyclotron and may be further accelerated by conventional operation of the device. It is to be observed that the multiply charged nuclei appear with an appreciable initial velocity and, owing to the absence of an internal ion source,.in an area relatively free from gaseous contaminants. These factors reduce the probability of ion loss by charge exchange and generally improve the efficiency of the accelerator.

In fixing the parameters for applying the invention to a given accelerator, certain conditions must be met. There is a relationship between the final ion energy E desired at the output of the cyclotron, the nuclear-charge Z, the injection energy E, and the injection charge Z1, produced by the ion source. If the injection energy-is too low, or the injection charge too high, the particles are deflected before they can penetrate to-the center of the magnetic field .where cyclotron acceleration "must begin. The condition to be fulfilled is:

In addition, the injection velocity must be at least as great as the orbital velocity of the K electrons of the nuclei in order to achieve effective stripping. Considering the 184 inch University of California frequency modulated cyclotron by way of example (see U. S. Patent 'No. 2,545,623, Frequency Modulation System, issued to K. R. MacKenzie March '20, 1951), Ne ions can'be accelerated to 1.8 billion electron volts if Ne++ ions are injected'at E1 l8 m. e. v.

It may be seen that certain differences in technique are necessary in applying the invention to the various types of ion accelerators. A bevatron, for example, comprises a tubular vacutun tank having four ar cuate f sections joined by four straight sections, an electromagnet being employed to deflect the ions around the curvedportio'ns. An'accel'e'rating electrode i's'disposed in one of the straight sections and means are employed to inject ions tangentially into another of the straight sections. Thus, the external ion source and preaccelerator means required to accomplish the present invention may be essentially similar to the means described with reference to the cyclotron. The electron stripping foil, however, will be preferably disposed near the point where the ions initially enter the vacuum tank. If the invention were thus applied to the University of California Bevatron, calculations indicate that ions could be'accelerated to an energy of 3 b. e. v. per nucleon, for example: A ions could be accelerated to 110 b. e. v.

It is therefore an object of the present invention to provide a method and apparatus for accelerating multiply charged, relatively heavy nuclei in a particle accelerator.

It is a further object to increase the utility of particle accelerators by adding to the variety of ions which may be accelerated therein.

It is an object to provide a method and means for producing ion beams having substantially higher energies than have heretofore been obtainable.

It is an object to improve the efliciency of magnetic particle accelerators by eliminating the need for an internal ion source.

Still another object of this invention is to provide means for decreasing ion loss by charge exchange in a particle accelerator.

Other objects and advantages of the invention will become evident by reference to the following detailed description of one embodiment in conjunction with the accompanying drawings, of which;

Figure 1 is a plan View of a cyclotron having suitable alterations to efiect the present invention;

Figure 2 is a cross-sectional view of the apparatus of Figure 1 taken along line 22 thereof; and

Figure 3 is a cross-section taken along line 3-3 of Figure 1 and showing the ion stripping means.

Referring now to the drawing, and more particularly to Figures 1 and 2 thereof, there is shown a cyclotron 11 comprising a fiat gas-tight tank 12 disposed in a gap between two opposed magnet pole pieces 13. The tank 12 is evacuated by diffusion pumps 14 through a manifold 16. The acceleration system includes a hollow generally semi-cylindrical dee electrode 17 mounted within tank 12 on insulators 18 and aligned with its open diametrical edge 19 offset a small distance from the axis of the magnets 13. Dee electrode 17 is joined to one extremity of a transmission line inner conductor 21 which projects outward from the body of tank 12 within an elongated rectangular extension 22 thereof. At the extremity most removed from dee electrode 17, inner conductor 21 is joined to the tank extension 22 which is adapted to serve as the outer conductor of a coaxial transmission line.

A radio-frequency oscillator 23, having a power supply 24, is adapted to excite the transmission line at the operating frequency of the cyclotron through a coupling loop 26. Inasmuch as the line is shorted at the extremity most removed from dee electrode 17, the maximum potential difference will appear between the open edge 19 of the dee electrode and the adjacent portions of the tank 12. In order to properly shape the resulting electric field for ion acceleration, a dummy electrode 27 is provided and comprises a conducting liner 28 secured within tank 12 and extension 22 and terminated by re-entrant edges 29 which are opposite and spaced apart from the upper end lower surfaces of the dee electrode edge 19. Thus, the potential difference induced by excitation of the transmission line structure will produce an alternating electric field in the gap 31 between the dee electrode 17 and dummy electrode edges 29, the electric field being generally perpendicular to the magnetic field as is required for cyclotron operation.

A preaccelerator assembly including, in this instance, an electrostatic ion generator 32 is disposed adjacent the cyclotron and adapted to produce lightly ionized heavy .nuclei. For example, generator 32 may be adapted to produce doubly charged neon ions; Inasmuch as the structure and operation of a suitable ion generator for 4 achieving this purpose is well understood within the art, this description will be limited to the basic elementsof generator 32, more detailed description being available by reference to U. S. Patent No. 2,578,908, Electrostatic Generator, issued to C. M. Turner December 18, 1951.

Ion generator 32 comprises a hollow prolate spheroidal electrode 33, open at its smaller extremity 34, and mounted by insulators 36 within a segmented outer shell 37. The insulators 36 are preferably disposed at a small angle with reference to the axis of electrode 33 and shell 37 in order to present a maximized charge leakage'path between the two members. Electrical charge from a power supply 38 is sprayed by corona points 39 onto a moving belt 41 which is so disposed as to carry the charge into the interior of the electrode 33. The charge may be transferred to the electrode-33 by a second set of discharge points and thus a potential of the order of several million volts may be placed thereon. A hollow tube 42 of dielectric material transpierces shell 37 along the axis thereof and is disposed with one extremity projected within the electrode 33 and terminated in an enclosed ion chamber 43. A filament 44 is mounted within the ion chamber 43 and is powered by an electrical generator 46 which is mounted within the electrode and driven by the charge carrier belt 41. A pressure vessel 47 is disposed within the electrode 33 and is adapted to feed the gas to be ionized into the chamber 43. The gas will be ionized by electron bombardment from filament 44 and the resultant ions will be accelerated through the tube 42 by the potential diiference between the electrode '33 and the shell 37.

The ion energy which may be produced'by an electrostatic device, such as generator 32, is limited by the potential difference which may be established, and in general, is less than the ion energy required to efiect the present invention. In order to further accelerate the ions prior to injection into the cyclotron, a linear accelerator 48 may be used.

The linear accelerator 48 may be of conventional design, for example the design disclosed in U. S. Patent No. 2,545,595, Linear Accelerator, issued to L. W. Alvarez March 30, 1951. The basic elements of such a device include an elongated cylindrical cavity resonator 49, and a radio-frequency oscillator 51, power supply 24, and coupling loop 53, adapted to excite the cavity resonator in such a manner as to establish an axial electric field. The cavity resonator 49 is evacuated by diffusion pumps 54 through a manifold 56, and is aligned such that ions from generator 32 enter a central aperture 57 in one ex tremity. A series of hollow conducting drift tubes 58 are disposed within the cavity resonator 49 and spaced apart along the axis thereof at intervals such that they shield the ions from the electric field during those periods of the radio-frequency cycle at which the field is opposed to the motion of the ions. Thus, the ions are exposed to the field only at such times as they will be accelerated thereby. Inasmuch as the ion velocity increases along the axis of the cavity resonator 49, successive drift tubes 58, and the intervening gaps, must increase in length to maintain the ions in phase with the field. In order to preserve the electrical characteristics of the cavity resonator 49, the drift tubes 58 must be decreased in diameter as they are increased in length.

The ion generator 32 and linear accelerator 48 are positioned such that the accelerated ions emerging from the accelerator may be directed through a suitable tubulation 59 into the vacuum tank 12 of the cyclotron 11. By fixing the alignment of the ion generator 32 and the linear accelerator 48, or through the use of beam steering means such as a magnet, the ions are caused to enter the tank 12 at an angle such that they will arrive at the open edge 19 of the dee electrode 17 tangent to the natural cyclotron orbit for completely ionized nuclei of similar mass and energy. In fixing this injection, angle, itwill be noted that the curvature of the ion trajectory 61 in the field of magnets 13 is" relatively s'lightov ing toithe lovv charge to mass ratio and high velocity of the ions.

Referring now to Figure 3; thereis shown a thin metallic foil 62 secured across a portion of the open edge IQ of the deep electrode l7 by cl ampingjmeans 63. The foi l 62' is displaced jrom the center of the dee elec trodee'dge 19 an amount suchthat it intercepts the ion injection trajectory 61, the amount of this displacement being generally equal to the radius of the cyclotron orbit foi'icompletelyionized' nuclei of the mass and energy of the i on's delivered by the linear accelerator 48. V A beam extractor assemblyi64 is disposed within the tank 12 at the maximuniion orbit comprises two concentric arcuate electrodes 66 and 67 secured to the wall of thetank insulators 68. The outermost electrode 66 is connected with a highv'oltage source 69 and the innermost electrode 67 islgrou'nd'ed by connectioniwith the tank 12.. The electrodes and 67 are positioned and curved in such amatti i asto intercept ionsfci'rculatin'gin the maximum orbiit'ofthecyclotron and deflect 'theniaway from th'e field of magnets 13; such that an ion beani'rnay be caused to emerge froma suitably placed thin window 71 in the wall ofthetank12..

In operation, lightly charged heavy nuclei are produced by generator 32 in .the manner. previously described. Owing to the potential difference between the electrode 33 andthe shell 37, a portion of these ions will be accelerate d along the tube 42 and enter the linear accelerator 48. Those ofthe ions which .arrive in cavity resonator 49 during the proper interval of the R. F. cycle will be further accelerated and will enterthe cyclotron with an appreciable velocity. As. has been stated, the operating parameters of the, linear accelerator 48 must be such as to produce ions having a velocity equal to or exceeding the orbital velocity of their K electrons. The ions enter tank 12 and strike the strippingfoil 62 while traveling. in a direction approximately normal thereto. It is desirable thatthe cyclotron oscillator 23 and the linear accelerator oscillator 51 be keyed together insuch a manner that the ions from the linear accelerator. arrive .at the center of the cyclotron in phase with the accelerating field thereof. v V v In traversing the foi1 62 the remaining orbital electrons will be stripped from the ions thus increasing their charge to a value equaltotheir atomic-number. The ions thus appear at the center of. the cyclotron, in a relatively gas free region, with a highcharge to mass ratio, and may-be accelerated by the alternating potential between dee electrode .17 anddummy electrode-29 in the conventional manner. During acceleration, themnltiply charged ions will spiral outward,'as denoted by. ion trajectory 72, and will eventually pass between extractorelectrodes 66 and 67 and be deflected outward through Windovl 71.

It. willbe appreciatedthat electrostatic generator 32 and linear accelerator 48 are but one example of a suitable preaccelerator means, and that other ion acceleration :devices may be employed. For example, a Cockcroft-Walton generator might replace generator 32, or a small separate cyclotron might be used to replace both generator 32, and linear accelerator 48. y The essential reqnirement' of the present invention is that external means beernployed to accelerate ions to an appreciable energy, and that the ion source of the principal accelerator be removed and means provided for the stripping of relatively low energy ions.

Thus, while the salient features of the invention have been described in detail with respect to one embodiment, it will of course be apparent that numerous modifications may be made Within the spirit and scope of the invention, and it is not intended to limit the invention to the exact details shown except insofar as they may be defined in the following claims.

What is claimed is:

l. A method of accelerating atomic nuclei to high energies comprising the steps: imparting a light charge to said g n i s 'd' '1ei.lt t fmediate energies, stripping'the orbital electrons from nuclei, and accelerating the resultant highly charged nuclei, toth'e desired high energies.

2 A method of acjceleratin' atomicnuclei to higlf ene'rgies'lcomprising the steps; ionizing .s aidnuclei by removal of a small proportion of the orbital electrons therefrom, accelerating said partially ionized nuclei to velocities at least as great as the orbital velocity of the K electrons thereof, stripping the remainder-of the grbital electrons from said nuclei, and further accelerating said nuclei to the required energy. a M

. 3[ In a methodof producing a high energybeam ef relatively heavy particles, the steps comprising' ionizing atomic nucleiof a neutral substencein such a manner as to impart a'r elativelylow charge to massra'tiothereto, accelerating" the resultant ions to a velocity at least great as the orbital velocity of theK electrons thereof by repeated exposure of said ioris to an electric fie ld, stripping the orbital electrons from said ions in such a manner as toimpart arelatively' high charge to massratio thereto, and further accelerating said ions by repeated exposure to a second electric field.

4. The method of producing a high energy beam of multiply charged atomic nuclei comprising the steps partinga low degree of ionization to said nuclei, accelerating said nuclei to a velocity at least as great as the orbital velocity of the K electrons thereof, injecting said accelerated ionized nuclei into anelectromagnetic charged particleaccelerator, stripping the remaining orbital electrons from said nuclei Within said charged particle accelerator, andfurther accelerating said nuclei by operation of said accelerator H v 5. The method of accelerating multiply charge d rela tively heavy atomic nuclei comprising the steps: ionizing nuclei of a neutral substance in such a m anner as to impart a low charge to massratio thereto, repeatedly exp osingjsaid nucleiito an electric field in such a manner as to accelerate said nuclei toga velocity at least as great as the maximum angular velocity of the orbital electrons thereof, directing said accelerated nuclei into an electromagnetic charged particle accelerator, stripping the orbital electrons from saidnuclei within said accelerator, andfurtheraccelerating said nuclei to high energies by operation of said accelerator. w

6. A method of produ'cin g'a high energy beam of multiply charged atomic nuclei comprising the steps: ionizing a gaseous substance by electron bombardment thereof, exposing the resultant ions to the action of charged particle acceleration means in such a manner as to acceleratesaid ions to a velocity at least as great as the orbital velocity of the K electrons thereof, stripping the orbital electrons from said-ions by passagethrougha metallic foil at the stated velocity and further accelerating said ions by exposure to the action of an electromagnetic charged particle accelerator. i I 4 W 7. Apparatus tor producing beamsof multiply charged atomic nucleicomprising in cornbination, a charged particle accelerator, an ion generator adapted to produce ionized atomic nuclei having a relatively low charge to mass ratio, means for directing said ionized nuclei into said charged particle accelerator, and means for stripping the orbital electrons from said nuclei, said means positioned in the path of said ionized nuclei, whereby said nuclei may be accelerated to high energies by the action of said charged particle accelerator.

8. Apparatus for the acceleration of multiply charged ions comprising, in combination, a charged particle accelerator, an ion generator adapted to produce ions having a relatively low charge to mass ratio, preaccelerator means adapted to accelerate said ions to intermediate velocities, said preaccelerator means being further adapted to inject said ions into said charged particle accelerator, and means for stripping the orbital electrons fromsaid ions, said stripping means disposed in the trajectory of said ions between i said preaccelerator means and the initial accelerating orbit be accelerated to high energies by said charged particle accelerator.

9. In apparatus for the production of high energy ion beams, the combination comprising an ion accelerator, an ion source adapted to impart a low degree of ionization to relatively heavy atomic nuclei, a preaccelerator element adapted to accelerate said ionized nuclei to a velocity at least as great as the orbital velocity of the K electrons thereof, said preaccelerator element being adapted to direct said ionized nuclei into said ion accelerator, and means for stripping the orbital electrons from said ions following preacceleration thereof, whereby said ions may be accelerated to high energies by operation of said ion accelerator.

' 10. In apparatus for the production of high energy multiply charged ion beams, the combination comprising a charged particle accelerator, an ion generator disposed apart from said charged particle accelerator, said ion generator being adapted to produce atomic nuclei having a low charge to mass ratio, a preaccelerator means adapted to accelerate said nuclei to a velocity at least as great as the angular velocity of the fastest orbital electrons thereof, said preaccelerator means being adapted to inject said nuclei into said charged particle accelerator, andmeans for stripping the orbital electrons from said nuclei, said stripping means being disposed in the trajectory along which said nuclei are' injected into said charged particle accelerator.

11. Apparatus for the production of high energy multiply charged ion beams substantially as described in claim 10, wherein said stripping means comprises a metallic foil disposed transversely with respect to the trajectory along which said nuclei are injected into said charged particle accelerator.

12. In a charged particle accelerator, apparatus for producing high energy beams of multiply charged ions comprising an ion source disposed apart from said charged particle accelerator and adapted to impart a low degree of ionization to electrically neutral nuclei, preaccelerator means adapted to accelerate said nuclei to a velocity at least as great as the orbital velocity of the K electrons thereof, said preaccelerator means being adapted to direct said nuclei into the accelerating field of said charged particle accelerator, and a metallic foil disposed in the path of said nuclei and adapted to strip the orbital electrons therefrom whereby said nuclei may be accelerated to high energies by operation of said charged particle accelerator.

13. In an ion accelerator characterized by an R. F. induced accelerating field, means for producing high energy beams of relatively heavy ions comprising an ion source disposed apart from said ion accelerator, said ion source being adapted to impart a low charge to atomic nuclei of a selected substance, pulsed preaccelerator means adapted to accelerate said nuclei in bunches to a velocity at least as great as the orbital velocity of the K electrons of said nuclei, said preaccelerator means being further adapted to inject said bunched nuclei into the field of said ion accelerator in phase with the acceleration cycle thereof, and metallic foil means disposed transverse to the path of said bunched nuclei in such a manner as to 8 strip the orbital electrons therefrom whereby said nuclei will be left with a relatively high charge and may be accelerated in the conventional manner. 7

14. In a charged particle accelerator, the combination comprising a vacuum vessel, an ion generator disposed apart from said vessel and adapted to produce ions having a relatively low charge to mass ratio, preaccelerator means adapted to accelerate said ions to a velocity at least as great as the angular velocity of the electrons occupying the K shell of said ions, said preaccelerator means being further adapted to inject said ions into said vessel, a metallic foil disposed in the path of said ions'and adapted to strip the orbital electrons therefrom in such a manner as to impart a relatively high charge thereto, a radio-frequency electrode system disposed within said vessel and adapted to establish an alternating electrical field, and magnet means adapted to establish a magnetic field within said vessel at right angles to said electric field whereby said ions will be caused to circulate within said vessel and periodically enter the influence of said electric field in such a manner as to be accelerated thereby.

15. In an ion accelerater of the class having two opposed generally circular magnet pole pieces and means disposed therebetween establishing an alternating electric field at right angles to the field of said magnet pole pieces, the combination comprising a source of accelerated partially ionized nuclei, said source disposed outside the field of said magnet pole pieces, said source directing said partially ionized'nuclei into the region between said magnet pole pieces, and means stripping the orbital electrons from said nuclei, said means disposed between said magnet pole pieces in the path of said nuclei.

16. In an ion accelerator having two spaced coaxial cylindrical magnet pole pieces and an electrode system disposed between said magnet pole pieces and adapted to establish an alternating electric field which is perpendicular to the field of said magnet pole pieces, said accelerator being of the class which is characterized by a spiral ion acceleration orbit, the combination comprising an ion source producing incompletely charged nuclei, said ion source disposed outside the field of said magnet pole pieces, preacceleration means accelerating said incompletely charged nuclei to a velocity at least as great as the velocity of the K electrons thereof, said preacceleration means aligned to direct the accelerated incompletely charged nuclei along a trajectory passing between said magnet pole pieces, and an ion strippingelement disposed between said magnet pole pieces on said trajectory, whereby said incompletely charged nuclei will be totally ionized and may be accelerated to high energies by operation of said ion accelerator.

17. An ion accelerator as described in claim 16 wherein said trajectory is tangent to the natural orbit of said nuclei in the field of said pole pieces, and said ion stripping clement comprises a metallic foil mounted transverse to said natural orbit.

References Cited in the file of this patent UNITED STATES PATENTS Farly Nov. 10,1953

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