US3641446A

US3641446A - Polyergic cyclotron

Info

Publication number: US3641446A
Application number: US886237A
Authority: US
Inventors: Hayden S Gordon
Original assignee: US Air Force
Current assignee: US Air Force
Priority date: 1969-12-18
Filing date: 1969-12-18
Publication date: 1972-02-08
Anticipated expiration: 1989-02-08

Abstract

A device for the production of a multiple energy proton beam comprising a negative hydrogen ion cyclotron with stripping foils placed at different radial and azimuthal positions. The negatively charged hydrogen ions at the various energy levels represented by the radial positions of the foils are stripped of their electrons thereby becoming protons which reverse their direction of curvature because of the reversed polarity and exit from the cyclotron along paths that, with proper azimuthal positioning of the foils, meet at a single point outside the cyclotron. A circular combining magnet centered on this point bends the proton trajectories as required to continue along a common path thus creating the multiple energy or polyergic beam.

Description

United States Patent [151 3,641,446

Gordon 5] Feb. 8, 1972 [54] POLYERGIC CYCLOTRON Primary Examiner-Raymond F. Hossfeld [72] Inventor. Hayden S. Gordon, Berkeley, Calif. Atmmey Ha"y A. Herbert, JL and Robert K Duncan [73] Assignee: The United States of America as represented by the Secretary of the Air BSTRACT A device for the production of a multiple energy proton beam [22] Filed; I); 18, 1969 comprising a negative hydrogen ion cyclotron with stripping foils placed at different radial and azimuthal positions. The

[21] App!" 886,237 negatively charged hydrogen ions at the various energy levels represented by the radia! positions of the foils are stripped of 52 us. CL ..328/228 313/62 328/234 their e'ecmns hereby bewming WhiCh reverse [5]] Int. Cl H6511 13/00 direction of curvature because of the reversed polarity and [58 1 mm olSearch ..328/228, 229, 230, 234; mm the Paths 313/62 azimuthal positioning of the foils, meet at a single point outside the cyclotron. A circular combining magnet centered on I 56] References Cited this point bends the proton trajectories as required to continue along a common path thus creating the multiple energy or UNITED STATES PATENTS p y s beam- 2,872,574 2/1959 McMillan et al ..328/234 3 Claims, 6 Drawing Figures VICIVJN Talk POLYERGIC CYCLOTRON BACKGROUND OF THE INVENTION 1. Field of the Invention Energetic particle generators, particularly proton generating cyclotrons.

2. Description of the Prior Art Facilities for simulating the high-energy penetrating radiations of space require apparatus for generating beams of protons, one of the principal constituents of space radiations. Since proton radiation in space has a wide energy range and is for the most part isotropic, the proton beams generated for the test facility should be polyergic and should irradiate the sample from different directions in order to simulate space radiations as closely as practicable. Present proton beam generators produce monoergic beams making it necessary to provide a separate generator for each energy level represented in the simulating radiation. This greatly increases the cost and complexity of the test facility.

SUMMARY OF THE INVENTION The purpose of the invention is to provide a single apparatus capable of producing a polyergic beam of protons. Briefly, the apparatus comprises a negative hydrogen ion (H') cyclotron in which a plurality of stripping foils are placed at different radial and azimuthal positions. The H ions at various energy levels corresponding to the radial positions of the foils are stripped of their electrons by the foils, leaving hydrogen nuclei or protons which, being positively charged, follow a path of reversed curvature out of the cyclotron. With proper azimuthal positioning of the stripping foils the trajectories of the exiting protons at the various energy levels may be made to converge on and intersect at a point outside the cyclotron. A combining magnet of constant field strength located at this point is designed to turn each of the component proton trajectories through the proper angle to have all trajectories continue along a common path, thus forming a polyergic beam.

As a further feature of the invention, the cyclotron may be designed with the dee structure, or particle accelerating electrode structure occupying roughly one-third of the main magnet space and with a set of stripping foils in each of the two remaining thirds for producing, with the aid of external combining magnets, a pair of polyergic beams in the manner described above. In providing a radiation test facility, it is desirable that the test chamber contain a number of virtual radiation sources at different locations for radiating the test sample from different directions in order to simulate the isotropic nature of space radiation. The use of such dual-beam cyclotrons to feed the virtual sources (through suitable beam transport systems) reduces the polyergic cyclotron requirement to half the number of virtual sources.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic sectional view taken along the median plane of a polyergic cyclotron in accordance with the invention;

FIG. 2 is a diagram showing the geometrical relationships at the combining magnet;

FIG. 3 is a graph relating the product of combining magnet flux density and radius required to produce a collimated beam to the turning angles at the various energy levels and to the angular separation of the incident proton beams of highest and lowest energy level;

FIG. 4 is a large-scale illustration of a stripping foil in the form of a paddle inserted into the ion beam envelope; and

FIGS. 5a and 5b are plan and elevation views respectively of a stripping foil in the form of a rotatable ribbon extending across a portion of the ion beam envelope.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1, which is a schematic sectional view taken along the median plane of a single dee, azimuthally varying field all (AVF) cyclotron, l is a pole face of the main magnet having the spiral hills and valleys characteristic of the most advanced type of AVF cyclotron and 2 is the outline of the main magnet coil. The construction shown provides for the generation of two polyergic beams as already mentioned and as will be explained more fully later. In this'design the dee structure, or particle-accelerating electrode structure, the outline of which is shown at 3, is restricted to roughly one-third of the angular space of the cyclotron in orderto provide sufficient angular space outside the dee structure for the generation and extraction of polyergic protons in accordance with the invention. As well understood, charged particles, in this case negative hydrogen ions, are introduced at the center of the cyclotron and follow generally circular paths about the center that increase in radius due to their gain in kinetic energy with each passage through the dee structure. The orbits of particles at several energy levels ranging from 20 mev. to mev. are labeled in FIG. I. Cyclotrons, including the AVF type, are well known and adequately covered in the literature, for example, in Principles of Cyclic Particle Accelerators by Livingood, D. Van Nostrand Co., Inc., 1961.

The invention is not in any way limited to use with AVF cyclotrons, but may be applied to any negative hydrogen ion accelerator in which the charged particles orbit with increasing radius as the particle velocity increases. This includes fixed-frequency cyclotrons, usually referred to simply as cyclotrons, and synchrocyclotrons. The most likely application is to cyclotrons because of their high duty factor compared to synchrocyclotrons. The invention may be applied to any type of cyclotron from the simple type with smooth magnet pole faces to the most advanced type having magnet faces with spiral hills and valleys to which the invention is shown applied in the drawing. The choice depends upon the maximum particle energy desired. The maximum energy obtainable in simple cyclotrons with smooth pole faces is limited by the relativistic increase in particle mass as the particle velocity increases. Since the orbital period t is directly related the particle mass in accordance with the equation where M is the particle mass, B is the magnetic field strength, and q is the charge on the particle, the increasing mass increases the period t until the particle becomes so far out of phase with the accelerating radio frequency field at the dee that no further acceleration occurs. To counter this effect B may be increased with increasing radius in such manner as to keep the ratio M/B constant. The increase in B may be accomplished by a radical decrease in the magnet air gap; however, the resulting bowing of the magnetic lines in the gap produces forces on the particles tending to drive them away from the median plane of the cyclotron or, in other words, tending to destroy the axial focus. This defocusing effect can be counteracted by alternately increasing and decreasing the air gap azimuthally, through providing alternate high and low sectors in the pole faces, in order to produce an alternating gradient of the field strength in azimuth. This produces net forces on the particles tending to hold them in the median plane and therefore to preserve the axial focus. This effect is enhanced if the boundaries of the high and low pole face sectors are spiral. Therefore, by the use of alternate spiral high and low sectors in the pole faces a radial decrease in air gap can be employed to counteract the effect of relativistic particle mass increase without destroying the axial focus of the cyclotron. This construction, including the spiral boundaries, is well known in the art and describe in the literature, for example, in Livingood cited above. U.S. Pat. No. 2,872,574 to McMillan et al. is another example of the use of an azimuthally varying field, but without the spiral modification, to offset the axial defocusing effect of a radially decreasing airgap.

The construction details of cyclotrons in general are well known in the art and are adequately described in the literature, Livingood and the patent to McMillan et al. again being cited as examples. Negative hydrogen ions, which are hydrogen atoms with one electron attached, may be introduced near the center of the cyclotron by a probe inserted through an axial passage in one of the magnet poles as shown in FIG. 6 of the McMillan et al. patent. The ion generator and the specific design of the probe are not a part of the invention, any suitable design capable of constantly delivering ions to the entrance gap of the dee structure near the center of the cyclotron is sufficient.

The only modification of a conventional cyclotron necessary for the addition of the invention is possibly a restriction of the angular extent of the dee structure in order to provide room for the insertion of the stripping foils and for the passage of the generated proton streams out of the cyclotron, as will be described later. The dee structure 3 employed in FIG. 1 is of the single-dee type. Dee structures in general including the single-dee structure are described in Livingood. Early cyclotrons employed a dee structure having two dees each of approximately 180 extent. The radiofrequency energization of the two dees was such that their voltages were of equal amplitude and opposite phase relative to ground. In the singledee construction the missing dee is replaced by a grounded electrode called the dummy dee'that mimics the openings of the missing dee. The radiofrequency energization in this case is applied between the dee and the grounded dummy dee. The single dee structure need not have an angular extent of 180 but may have lesser values provided the proper phase relationship is maintained between the orbiting particles and the radiofrequency fields at the entrance and exit of the dee structure. This requires that the angular distance inside the dee and the angular distance outside the dee each be equal to an odd multiple of the angle through which the orbiting particle travels during one-half period of the radiofrequency energization. This condition can always be attained if the angular distance traveled by an orbiting particle during one-half period is equal to 360 divided by an even integer. Examples of dee sizes satisfying the phase requirement are:

180(360/2Xl 90(360/4Xl 135(360/8X3); 108(360/ x3); 150(360/l2X5); 126(360/20X7);etc.

The angular velocity (u of an orbiting particle is proportional to the magnetic field strength B in accordance with the equation (2) a =qB/M. Therefore, by proper selection of the radio frequency and the value of B the angular distance traveled by the orbiting particle during a half period of the radio frequency can be made equal to 360 divided by an even integer as required.

A dee structure suitable for use as dee structure 3 in FIG. 1 is illustrated in FIGS. 6, 7, and 8 of the McMillan et al. patent. Any one of the three identical dee assemblies 89, 96, and 102 may be used. Each of these assemblies is of the single-dee type, discussed above, comprising, as best seen in FIG. 6, a dee 47 and a grounded dummy dee 52. Each of the three dee structures is a complete particle accelerator in itself operating independently of the other two. Three structures spaced 120 apart are used to provide three-phase operation giving six accelerations per rotation. Single-phase operation with a singledee structure giving two accelerations per rotation is employed by applicant and could be employed in the patent without affecting the cyclotron operation other than to reduce the number of accelerations per orbit from six to two and, as a result, to proportionately increase the number of orbits required for a particle to obtain a given velocity. As stated earlier, the dee structure 3 is limited to roughly one-third the angular space of the cyclotron in order to provide angular space for the generation of protons and their extraction from the cyclotron in accordance with the invention.

In order to simultaneously derive protons at various energy levels from the cyclotron of FIG. 1, small stripping foils (either metallic or nonmetallic) are placed in the cyclotron at radial positions corresponding to the desired energy levels, the energy level increasing with increasing radius. FIG. 1 illustrates foils positioned at points 4, 5, 6, 7, and 8 in the cyclotron for intercepting hydrogen ions at energy levels ranging from to I00 mev. A negative hydrogen ion is a hydrogen atom to which an electron has become attached and consequently consists of a positive nucleus and two electrons. Such ions can be produced by subjecting hydrogen gas to an electron rich plasma. The foils strip the electrons from the negative hydrogen ions leaving the positively charged nuclei or protons which pass through the foil. The number of ions converted depends upon the ion current in the cyclotron and the amount of foil area presented to the envelope of the ion current. Except for the foil at the highest energy location the amount of foil area presented to the envelope is only a fraction of the envelope cross-sectional area in order to pass sufficient negative ions to feed the stripping foils at higher energy locations in the cyclotron. This area may be controlled by controlling the extent to which the foil is inserted in the envelope or by turning a narrow ribbon of foil crossing the ion envelope to control the projected area of the foil exposed to the circulating ions. F IG. 4 illustrates the first method in which a paddlelike stripping foil 20 lying in a radial plane normal to the median plane of the cyclotron is inserted into the negative hydrogen ion beam envelope 2]. FIGS. 5a and 5b are plan and elevation views, respectively, of the second method in which a stripping foil 22 in the form of a ribbon extends across a portion of the negative hydrogen ion beam envelope 21. The foil may be rotated about an axis 23 normal to the median plane of the cyclotron, its effective area in this way being made directly related to sin 0. With respect to the ion beam envelope 21, as well known in the art and as described, for example, in Livingood, the orbiting ions in a cyclotron oscillate both radially and axially about an equilibrium orbit generating an envelope the size of which depends upon the amplitude of the oscillations.

Due to the positive charge of the protons, they follow paths of reversed curvature out of the cyclotron, as illustrated in FIG. 1. The reversal of the direction of curvature results from the fact that the polarity of the particle charge changes at the stripping foil from negative for the hydrogen ion to positive for the proton, whereas the direction of the magnetic field remains the same. By proper selection of the azimuthal positions of the stripping foils, the trajectories of the exiting protons may be given such directions that if allowed to continue they would intersect at a single arbitrary point 9 outside the cyclotron. A combining electromagnet 10, having a circular pole face centered on point 9 and a magnet coil 1], is designed to have the proper constant uniform flux and the proper pole face radius, depending upon the proton energies and exiting directions, to bend the trajectories along a common path 12, thus forming a polyergic proton beam. In the embodiment of FIG. 1, a second set of stripping foils, generally indicated by reference number 13, and a second combining electromagnet 14-15 are provided to produce a second polyergic proton beam 16 in the same manner as beam 12. As shown, the foils 13 occupy the same radial positions as foils 4-8 so that beam 16 has the same energy components as beam 12; however, different radial positions could be used if desired to give beam 16 a different energy distribution.

The design of the combining magnets will be discussed further with reference to FIGS. 2 and 3. A proton moving through a uniform magnetic field in a plane normal to the flux lines follows a curved path such that the produce of the flux density B and the radius of curvature R is a constant the value of which depends upon the kinetic energy of the proton. Tables are available relating the BR product to the kinetic energy of the proton in electron volts. Proton beams that converge on a point can be turned by a constant uniform magnetic field having a circular edge centered on the point so as to become collinear with a radial line extending from the point provided the flux density, the radius of the circular edge, the particle energies, and the angle between any two proton beams, for example, the beams of maximum and minimum energies, are properly related. This is illustrated in FIG. 2 where (as seen also in FIG. 1) 9 is the point on which the lowest energy proton beam 17 and the highest energy proton beam 18 converge, 10 is the circular pole face of the combining magnet centered on point 9, R is the radius of the pole face or circulat edge of the field, R is the radius of curvature of the lowenergy beam. R is the radius of curvature of the high-energy beam, 0, is the low-energy beam turning angle, 6 is the highenergy beam turning angle, and A is the angular separation of the highand low-energy beams. From the geometry of FIG. 2 the following relationships are seen to exist:

tan

6) x 2(tan" Therefore, for a given value of A and given maximum and minimum proton kinetic energies which determine the values of the constant (BR) and (BR) the correct value of the product B R is that which satisfies equation (6). Either B, or

R may be chosen and the other determined by dividing the chosen value into the value of the product. HO. 3 relates graphically the B R product to the values of 9, A, and proton energies.

In lieu of using a BR product table the values of R and R may be computed for a chosen value of B as follows:

where K, and K are the kinetic ehergies in electron volts of the lowest and highest energy protons (2O mev. and lOO mev. in the example given), m is the mass of a proton, and q is the proton charge (approx. l.6X l0 coulombs). Substituting the computed values of R and R together with the given value of )t in equation (3), the correct value of R is that which satisfies this equation.

lclaim:

1. Apparatus for producing a polyergic beam of protons comprising: a negative hydrogen ion cyclotron having a plurality of stripping foils positioned at different radial positions for converting negative hydrogen ions at different energies into protons at different energies which follow trajectories of reversed curvature out of the cyclotron, said foils having such azimuthal locations that the trajectories of the exiting protons converge toward a point outside said cyclotron; and means at said point for bending said converging trajectories into a common polyergic beam.

2. Apparatus as claimed in claim 1 in which said bending means comprises means producing a uniform constant magnetic field having a circular edge centered on said point, the product of the flux density of said field and the radius of said circular edge having such value, depending upon the angular separation of any two of said converging proton trajectories and the energies of the protons therein, that all of said converging trajectories are turned into collinearity with a radial line extending from said point.

3. Apparatus as claimed in claim 1 in which the angular space of the cyclotron has three sectors with the particle-accelerating electrode structure of the cyclotron in one of the sectors and said stripping foils in another; and a second set of stripping foils in the remaining sector having different radial and azimuthal positions as required to produce protons at various energies which exit the cyclotron along trajectories that converge on a second point outside the cyclotron in the same manner as the first-named foils, and means at said second point for bending the converging trajectories into a second common polyergic beam.