US2860279A - High current linear ion accelerator - Google Patents

Description

Nov. 11, 1958 R. E. HESTER ETAL 2,860,279

HIGH CURRENT LINEAR ION ACCELERATOR Filed April 18, 1955 2 Sheets-Sheet 1 POWER AMPLIFIER PHASE SH! FT ER ION SOURCE TARGET OSClLLATOR l8 INVENTORS ROSS E. HESTER L) BY DEAN O. KIPPENHAN WILLIAM A. S. LAMB ATTORNEY Nov. 11, 1958 R. E. HESTER ET AL 2,860,279

HIGH CURRENT LINEAR ION ACCELERATOR Filed April 18, 1955 2 Sheets-Sheet 2 OSCILLATOR 0 O OSCILLATOR o SHIFTER AMPLIFIER ION TARGET I SOURCE VACUUM VACUUM PUMP PUMP O O O O O O D. c. D. c. D. 0. D. 0. POWER POWER POWER POWER SUPPLY SUPPLY sUPPLY sUPPLY lol 101 101 101 072?/ INVENTORS.

R055 E. HESTER y DEAN O. K/PPENHAN WILLIAM A, 5. LAMB ATTORNEY United States Patent HIGH CURRENT LINEAR ION ACCELERATOR Ross E. Hester and Dean 0. Kippenhan, Hayward, and William A. S. Lamb, Livermore, Califi, assignors, by mesne assignments, to the United States of America as represented by the United States Atomic Energy Commission I Application April 18, 1955, Serial No. 502,251

8 Claims. (Cl. SIS-5.42)

The present invention relates to apparatus for the production of high energy charged particle beams and, more particularly, to a linear accelerator of the class characterized by the periodic exposure of charged particles to an alternating electric field.

In general, apparatus for the acceleration of ions may be divided into two major classes, magnetic machines such as the cyclotron or bevatron where ions are caused to follow a curvilinear trajectory, and linear accelerators in which particles are accelerated along a straight axis.

r In the latter class of machines, the most usual design comprises apparatus for injecting particles into an alternating electric field, means being provided to shield the particles from the field at such times as the field is opposed to the forward motion of the particles. The present invention embodies a novel structure for establishing the necessary electric fields and for shielding the ions during adverse portions of the radiofrequency cycle, the device being characterized by a minimum of stored energy, relatively low voltage between adjacent electrodes, an efficient accelerating action, and a higher beam current than has heretofore been feasible in accelerators of corresponding size.

In particular, the invention makes use of one or more cells, each adapted to provide an increment of velocity to an ion beam. Each cell is comprised of a coaxial transmission line which is excited at its resonant frequency by suitable oscillator means. An ion beam passage transpierces each of the coaxial lines, the passage being normal to the axis of the line and being located near a point of maximum voltage. Each of the cells is made gas-tight and is evacuated by suitable pumping apparatus. Provided the parameters of the apparatus are determined in a manner to be hereinafter described, the electric field within the coaxial line produces an accelerating effect on an ion beam, the ions being injected axially into the transverse passage.

As Will be seen, ions which are injected during a favorable interval of the radiofrequency cycle are accelerated across the gap between the outer and inner conductor of the line, shielded within the inner conductor as the electric field reverses, and again accelerated across the remaining gap between the two conductors. By providing a series of such cells with their beam passages aligned on a common axis, intense ion beams may be accelerated to any desired energy.

It is thus an object of the invention to provide improved apparatus for the acceleration of charged particles.

It is a further object of the invention to provide a linear accelerator capable of the production. of relatively intense ion beams.

It is an object of the invention to provide a linear ion accelerator characterized by a relatively low stored energy factor, conservative gap voltages, and efficient energy pick-up.

Still another object of the invention is to provide ap paratus utilizing the electric field between coaxial conductors to achieve the acceleration of charged particles.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood by reference to the following specification taken in conjunction with the accompanying drawing, in which Figure 1 is a diagrammatic view of the basic ion accelerating cell of the invention;

Figure 2 is a longitudinal elevation view of one embodiment of the device, in partial section;

Figure 3 is a cross-sectional view of a portion of the apparatus of Fig. 2 taken along line 33 thereof; and

Figure 4 is a plan view of the apparatus of Figure 2.

Referring now to the drawing and more particularly to Fig. 1 thereof, there is shown a schematic view of a one quarter wave length coaxial transmission line 11 comprising an outer conductor 12 and an inner conductor 13. If the conductors 12 and 13 are shorted at one extremity 14 and a power oscillator 16 and coupling loop 17 are caused to excite the line 11 at the resonant frequency thereof, an alternating sinusoidal voltage may be made to appear between the unshorted extremities of the two conductors. If a bore 18 is transpierced through the high voltage extremity 19 of the line 111, the bore being normal to the axis of the line, acceleration of ions across the system may be achieved. An ion injected into the bore 18 at a favorable instant in the radiofrequency cycle will be accelerated across the first gap 21 between theory, the apparatus may be considered a machine, where:

v=velocity of particle c=velocity of light A=Wavelength of radiofrequency excitation then denotes the repeat length or the distance: g+d, where:

g=length of acceleration gap 21 d=drift distance within center conductor 12.

Assuming the electric field across the gap 21 to be uniform, and assuming the presence of foils or grids across the entrances of bore 18 to give a purely time vary ing field in the gap, the gap length g may be calculated as follows:

where:

In the usual case of linear acceleration operation, ions are caused to cross the accelerating gaps within one quarter of each radiofrequency cycle of electrical excitation, in order to achieve phase stability of the ion beam with reference to the accelerating field. To bring about this condition will normally be fixed at zero, and 1; to the value corresponding to one quarter cycle. Thus the above equation simplifies to:

This quadratic equation is easily solved for g:

In order to have the ion enter the second gap 21" when sinwt=0, it must drift for 4n+1 quarter cycles, where n is zero or any whole integer. In the normal case of a drift time of one quarter cycle, the drift length or diameter of the center conductor 13 may be determined as follows:

q 1 gmw 4f The length of the second gap 21 may be calculated in an analogous manner, taking into account the altered values resulting from acceleration of the ions through the first gap 21.

It has been found desirable to construct the line 11 in a paddle-shaped configuration in order to approximate the uniform field assumed in the above equations, the line thus being of less width in the direction of the bore 18 than in a direction transverse to the bore. In practice a decrease in the calculated gap lengths of the order of twenty-five per cent, with the extra length added to the drift space, will be found desirable in order to compensate for field penetration of the drift space.

It will be observed that considerations of beam stability in the present invention are similar to those of conventional linear accelerators; that is, phase stability and radial stability of the ion beam are mutually exclusive insofar as the inherent effect of the R. F. field on the beam is concerned. As with standard machines, it is most feasible to operate on the phase stable portion of the radiofrequency cycle, and to provide auxiliary means for radial focussing, for example, solenoid magnetic lenses, or alternating gradient focussing means.

Referring now to Fig. 2, there is shown a practical embodiment of the invention employing two accelerating cells and adapted for the acceleration of a 100 ma. proton beam to an energy approximately 0.5 mev. The embodiment herein described is particularly adapted to serve as an ion injector for a larger high current ion accelerator.

A first accelerating cell 22 is comprised of a hollow rectangular vacuum tank 23 mounted on suitable supports 24. The lower extremity of a vertical quarter wave n m ssion l n i disposed t n th ank transpiercing the top wall thereof through an aperture 27.

The line 26 is formed of a hollow elongated outer conductor 28 and an inner conductor 29 disposed coaxially therein. The inner conductor 29 is electrically separated from the outer conductor 28 at all points except the uppermost extremity where a suspension assembly 31 supports the inner conductor and a vacuum tight shorting plate 32 makes electrical connection between the two members.

The suspension assembly 31 comprises a circular top plate 33 coaxially disposed on the upper extremity of the outer conductor 28 and having a central aperture 34 through which the upper extremity of the inner conductor 29 passes. A flanged cylindrical sleeve 36 secured to the upper end of the inner conductor 28 by a cross-pin 37 supports the conductor upon the top plate 33. The shorting plate 32 is an annular conducting diaphragm coaxially and sealingly disposed between the two conductors 28 and 29.

An unloaded quarter-wave line fitted to the parameters of the present embodiment would be approximately twenty feet in length. In order to shorten the line to increase rigidity and render the apparatus more compact, it has been found desirable to compound the line 26 by making the portion of the line above tank 23 of greater diameter than that portion Within the tank. By this means, well understood within the art, the line 26 in the present embodiment has been shortened to approximately twelve feet.

A beam passage bore 38 transpierces the apparatus, including tank 23 and conductors 2t; and 29, along a horizontal axis proximal to the lower extremity of the line 26. To properly shape the electric field in the region of the bore 38 to a suitable configuration for the acceleration of ions, the portion of the line 26 within the tank 23 is flattened transversely to the bore as shown in Fig. 3. The flat paddle-shaped geometry produces a more parallel field between the conductors 28 and 29 and Contributes to the radial stability of an ion beam passing through the bore 38. To provide for tuning of the line 26 the lower extremity of the outer conductor 28 is equipped with an adjustable closure assembly 39.

The closure assembly 39 comprises a dished conductor 41 slideably disposed within the outer conductor 28 in such a manner as to form an electrical short across the lower extremity thereof. A control rod 42 is swivelly attached to the undersurface of the conductor 41 and projects downward through a threaded aperture 43 in the bottom member 44 of the tank 23. The control rod 42 is threadedly engaged with the aperture 43 and is provided with a suitable handle 46 so that the rod may be manually rotated with consequent raising or lowering of the conductor 41. In order to maintain the vacuum within the tank 23 an annular seal 47 is disposed in the tank on the upper surface of bottom member 44 coaxial with the control rod 42.

In order that the line 26 may be evacuated of atmosphere by pumps operating on the tank 23, longitudinal slots 43 are cut into that portion of the outer conductor 28 which is within the tank. To prevent excessive energy loss by radiation the rims 49 of the slots 43 are projected outward. For similar reasons the rims 51 of the beam passage bore 38 at the outer conductor 23 are outwardly flared. To remove heat generated within the line 26 by the electrical resistance thereof, suitable coolant circulating tubes 52 may be Wound about the external surface of the outer conductor 28.

A second accelerating cell 53 is disposed proximal to the first cell 22, a beam passage bore $4 of the second cell being aligned on a common axis with the corresponding bore 38 of the first cell. is generally similar in structure-to the first cell 22 and includes a second rectangular vacuum tank 56 mounted on supports 57. A second vertical transmission line 58 comprising an outer conductor 59, inner conductor 61 and suspension assembly 62, transpierces the top plate The second cell 53 n of the second tank 56. A second adjustable closure assembly 64, similar in structure to closure assembly 39, is disposed within the second tank 56 at the lower extremity of the second outer conductor 59. The second beam passage bore 54 transpierces the outer conductor 59 and inner conductor 61 near the lower extremity thereof, with rims 66 of the bore being projected outward with respect to the surface of the outer conductor. Longitudinal pump-out slots 67 with outwardly flared rims are cut into the surface of the outer conductor 59 within the tank 56. Coolant tubes 68 are wrapped around the upper portion of the outer conductor 59.

Inasmuch as the ion velocity through the second cell 53 is greater than that through the first cell 22 owing to the acceleration of ions across the first cell, gap and drift distances must be correspondingly greater. Substitution of the altered values in the previously stated equations will indicate an increased spacing between the inner and outer conductors 61 and S9, and an increased thickness for the inner conductor. Increasing the diameter of the second line 58 over that of the first line requires that the second line be shortened in order to maintain the resonant frequencies of the two lines at a common value. Physical connection of the two cells 22 and 53 is achieved by a hollow vacuum cylinder 69 disposed between the two tanks 23 and 56 coaxial with the beam passage bores 38 and 54 and coaxial with a beam aperture 71 in the adjacent faces of the two tanks.

An ion source 72 which may be of any suitable design is disposed proximal to the first accelerating cell 22 and is caused to emit a beam of ions along the axis of beam passage bores 38 and 54, the beam being directed towards the first rectangular vacuum tank 23. The source in the present embodiment is a type emitting an eighty kilovolt proton beam. It will be apparent, however, that diverse ions may be accelerated and various injection energies employed provided the electrical parameters of the accelerating cells are adjusted accordingly. A beam buncher 73 is disposed adjacent the source 72 and is adapted to receive the ion beam therefrom, the buncher being connected with the first vacuum tank 23 by a beam conduit 74. Inasmuch as the ion accelerating components of the invention are operative for only one-quarter of each radiofrequency cycle, whereas the source develops maximum intensity by emitting a continuous beam, the use of the buncher will increase the output current of the accelerator and restrict undesirable heating and space charge effects from un-utilized beam.

In the present embodiment the buncher 73 comprises a hollow cylindrical drift tube 76 mounted coaxial with the ion beam within a conducting cylinder 77. The tube 76 is supported by an upwardly projecting stem 78 which is insulated from the cylinder 77. The stem 78 and the cylinder 77 are connected with the inner conductor 79 and outer conductor 81, respectively, of a transmission line 82. The tube 76 and cylinder 77 thus form an electrically open end to the transmission line 82. Thus if excitation is supplied to the line 82, an alternating field will be set up between the tube 76 and cylinder 77, the field being generally horizontal in the region of the ion beam.

By means to be hereinafter described, the line 82 is excited at a frequency similar to that of the accelerating cells 22 and 53, and at a particular phase relationship thereto, with the result that ions emerging from source 72 and traversing the buncher 73 will be given a varying acceleration or deceleration as determined by their time of arrival at the buncher. Thus the ion beam is velocity modulated and will tend to coalesce into spaced volumes of high ion density and low ion density as it drifts through the conduit tube 74. By suitable adjustment of the operating parameters. principally by adjustment of the phase relationship between the buncher 73 and first cell 22, the high ion density portions of the beam may be timed to arrive at the first cell at the most favorable interval of 6 the radiofrequency cycle therein. The general theory of the beam bunching action, as well as alternative structures, are well understood within the art and may be obtained from any standard text on Klystron vacuum tubes.

In the present invention, as in prior forms of linear ion accelerator, radial focussing of the ion beam and phase focussing of the beam are mutually exclusive. Thus if ions are injected into the machine during the portion of the raidofrequency cycle which produces a radially confining effect on the ion beam, phase instability will be present. Conversely, ions injected during the phase stable portion of the cycle will experience disruptive electric forces at right angles to the beam axis. An analysis of this effect may be had by reference to The Physical Review 80, 493 (1950) by E. M. McMillan. To overcome this apparent limitation in the operation of linear ion accelerators, it is customary to operate the accelerators on the phase stable portion of the radiofrequency cycle and to provide auxiliary means to counteract the resultant radial instability and prevent divergence of the beam beyond tolerable limits. Various auxiliary focussing means may be adapted to this purpose, for example, grids placed transverse to the beam at the entrance to each accelerating gap as disclosed in U. S. Patent No. 2,545,595 for Linear Accelerator issued to L. W. Alvarez, March 20, 1951, or one of various classes of magnetic lens systems.

In the present embodiment radial focussing of the ion beam is achieved by solenoidal magnetic lenses 83, 84, 86, and S7, lenses 83 and 84 being coaxially mounted on conduit 74, lens 86 being similarly disposed on vacuum cylinder 69, and lens 87 being disposed coaxially about a beam delivery tube 88 which projects from the wall of second vacuum tank 56 coaxial withthe beam passage bore 54. As may be seen from an analysis of the forces acting on charged particles projected along the axis of a solenoid, a radially focussing effect on the ion beam will be produced.

Referring now to Fig. 4, there are shown additional components of the accelerator including two diffusion vacuum pumps 89 adapted to exhaust the system through manifolds 91 connecting with vacuum tanks 23 and 56 of the two accelerating cells 22 and 53.

Each of the accelerating cells 22 and 53 is energized by a radiofrequency oscillator 92, which may be of any suitable design, the oscillators having an operating frequency equal to the value used in the previously given cell dimension calculations (12.25 mc. in the present embodiment). It will be observed that the phasing of the second accelerating cell relative to the first cell must be such that ions enter each cell at a similar point in the radiofrequency cycle. Such condition may be achieved by adjustably keying one oscillator to the other until eflective operations results. Alternately, the two oscillators may be operated at a common phase provided the separation of the two cells, as determined by the length of the connecting vacuum cylinder 69, is set in accordance with the previously given drift space equation. Physical coupling of the oscillators 92 to the cells 22 and 53 is achieved by transmission lines 93 which transpierce the outer conductors 28 and 59 and terminate in coupling loops 94 therein.

Excitation of the beam buncher 73 is keyed to a signal from the first cell 22, the signal being obtained from a pick-up loop 96 which is mounted within outer conductor 28 and linked with the magnetic field therein. A coaxial line 97 connected with the loop 96 delivers the signal to a conventional variable phase shifting device 98 which is necessary inasmuch as the output current of the accelerator will be found to be critically dependent upon the phase relationship between the buncher 73 and the accelerating cells 22 and 53. The signal from phase shifter 98 is delivered to a power amplifier 99 which excites transmission line 82 and thus excites the beam buncher 73. Further components include conventional power supplies 101 adapted to furnish direct current excitation to the solenoidal focussing coils 83, 34, 86, and 87.

In operation excitation from the oscillators Q2 sets up alternating radially oriented electric fields within the two transmission lines 26 and 5% as previously described. An ion beam is emitted from the source 72 and modulated into spaced volumes of high ion density by passage through the field within the buncher 73. By adjustment of the phase shifter 98 the volumes of high ion density are caused to pass into the first outer conductor 23 at the desired interval of the radiofrequency cycle so that acceleration across the gap between the outer and inner conductors will take place. The ions then drift through the field free region within the bore 33 of the inner conductor 29 as the electric field between the twoconductors reverses. Thus the ions emerge into the remaining gap between the two conductors at an interval of the radiofrequency cycle favorable to further acceleration.

The ions then drift through the vacuum cylinder 69 and receive further acceleration crossing the two gaps between the inner and outer conductor of the second cell 53. In traversing the described apparatus the ions experience a radially focussing force through the effects of the magnetic fields within the solenoid coils 83, 84, 86, and $7. In this manner the defocussing effects of the electric field in the two accelerating cells, as well as coulomb repulsion forces, are satisfactorily overcome. The high energy ion beam emerging from the second cell 53 is delivered to a desired target 102 which may be a substance to be bombarded or further acceleration means.

While the described embodiment of the invention uses two accelerating cells, it will be apparent that a single cell could be employed where a lesser output energy is required and that a greater number of cells could be used where higher ion energies are desired. The important requirement in the provision of additional cells is that they be spaced or phased in accordance with the previously given relationships so that ions arrive at each cell at substantially similar portions of the radiofrequency cycle. It will further be observed that diverse classes of coaxial transmission line may be used as a basis for the cells, the open-ended quarter wave lines employed in the described embodiment being preferable on the basis of economy in space, weight, and materials. As an example, half wave lines closed at each extremity might be utilized provided the beam passage bores were substantially at the midpoint of the lines.

Thus 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 it is not intended to limit the invention except as defined in the following claims.

What is claimed is:

1. A linear ion accelerator comprising, in combination, a source and buncher unit emitting a pulsed ion beam, a one-quarter Wave coaxial transmission line positioned in the path of said beam at right angles thereto, said transmission line transpierced by a beam bore at the line of trans'version with said ion beam, said transmission line having a hollow outer conductor and an inner conductor electrically joined at the exrtemity most removed from said beam bore, said outer conductor having a width whereby said ions pass thereacross in an interval corresponding to substantially three-quarter cycle at the resonant frequency of said line, said inner conductor having a width whereby ions are shielded for an interval corresponding to substantially one-quarter cycle at the resonant frequency of said line, oscillator means coupled to supply energization to said line, vacuum conduit means disposed around said ion beam and connecting said source and buncher unit and said transmission line, and vacuum pumping means connected to evacuate said outer conductor and said vacuum conduit, whereby said pulsed ion beam is accelerated by passage through the electric field 8 between said inner conductor and said outer conductor.

2. In a linear ion accelerator, the combination comprising an ion source emitting a beam of ions along an axis, a plurality of coaxial transmission lines spaced along said axis at right angles thereto, each of said coaxial transmission lines having a hollow outer conductor and an inner conductor, each of said coaxial transmission lines having a transverse beam bore co-linear with said axis, said coaxial transmission lines joined at said bore by hollow vacuum tubulations disposed coaxial with said axis, vacuum pumping means for evacuating said coaxial transmission lines and said vacuum tubulations, and radiofrequency oscillator means electrically coupled with said coaxial transmission ilnes to establish radial electric fields, whereby ions traversing the gaps between said outer conductors and said inner conductors are accelerated.

3. A linear ion accelerator as desuribed in claim 2 wherein said coaxial transmission lines are one-quarter wave length at the operating frequency of said oscillator means, said coaxial transmission lines having an electrical short at the extremity most removed from said beam bore.

4. A linear ion accelerator as described in claim 2 wherein the width of said outer conductor along said axis is substantially equal to the distance traversed by said ions in an interval corresponding to three-quarter cycles of the operating frequency of said oscillator means, and the width of said inner conductor along said axis is substantially equal to the distance traversed by said ions in an interval corresponding to one-quarter cycle of the operating frequency of said oscillator means.

5. A linear ion accelerator as described in claim 2 wherein said transmission lines are keyed in controlled phase relationship with respect to each other.

6. A linear ion accelerator comprising, in combination, an ion source emitting an ion beam along an axis, a plurality of one-quarter wave coaxial resonators spaced along said axis at right angles thereto, said resonators asymmetrically positioned with respect to said axis and shorted at the extremities most removed from said axis, said resonators having a transverse ion beam bore co-linear with said axis and having a greater width transverse to said axis than parallel with said axis, oscillator means exciting said resonators at a common frequency, an electrode system disposed between said ion source and said coaxial resonators and establishing an alternating electric field parallel with said axis, said electrode system being keyed in controlled phase relationship with said resonators, means providing radial focussing of said ion beam with respect to said axis, and means for evacuating said resonators and said electrode system of atmosphere, whereby said ion beam will be bunched and accelerated.

7. In a linear ion accelerator, the combination comprising an ion source emitting an ion beam, a coaxial resonator having a hollow outer conductor and an inner conductor and having an electrical length equal to an integral number of quarter wave lengths, said coaxial resonator being further characterized by a transverse bore receiving said ion beam from said source, said resonator having a diameter along said bore substantially equal to the distance traveled by said ion beam in an interval corresponding to three fourths of an electrical cycle at the resonant frequency of said resonator, an electrical oscillator exciting said resonator at the resonant frequency thereof, and vacuum pumping means for evacuating said resonator.

8. In a linear ion accelerator, the combination comprising an ion source emitting an ion beam, a coaxial resonator disposed transversely with respect to said ion beam and being transpierced by a passage through which said ion beam passes, said resonator having an electrical length substantially equal to an integral number of quarter wave lengths, said resonator having a hollow outer conductor which outer conductor has a diameter References Cited in the file of this patent UNITED STATES PATENTS Cage Feb. 13, 1940 McArthur May 14, 1940 Llewellyn Jan. 23, 1945 Ryan Aug. 21, 1945 Anderson Oct. 22, 1946 Webster Feb. 27, 1951 Alvarez Mar. 20, 1951 Willshaw Jan. 8, 1952

Images