CYCLOTRON BEAM EXTRACTION SYSTEM
United States Patent 3582700
A method and apparatus for extracting a high-quality beam from an isochronous cyclotron by electrostatic deflection of a beam of charged particles from orbit and subsequent exposure of the beam within the evacuated region of the cyclotron to a reverse gradient in the normally declining fringe magnetic field.
Application Number:
04/775027
Publication Date:
06/01/1971
Filing Date:
11/12/1968
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Export Citation:
Assignee:
Cyclotron, Beam Ertraction System (Berkeley, CA)
Primary Class:
Other Classes:
315/502, 315/507
International Classes:
H05H7/10; H05H7/00; H05H13/08
Field of Search:
313/62 328/234
Primary Examiner:
Hossfeld, Raymond F.
Claims:
I claim
1. A method for extracting a beam of charged particles orbiting in an isochronous cyclotron within a guiding magnetic field and evacuated region comprising
2. The method the method of claim 1 including the further step of initially
3. Apparatus for extracting a beam of charged particles orbiting in an isochronous cyclotron within a guiding magnetic field and evacuated region comprising
4. Apparatus according to claim 3 further comprising
5. The apparatus of claim 3 wherein the magnetic channel means comprises
1. A method for extracting a beam of charged particles orbiting in an isochronous cyclotron within a guiding magnetic field and evacuated region comprising
2. The method the method of claim 1 including the further step of initially
3. Apparatus for extracting a beam of charged particles orbiting in an isochronous cyclotron within a guiding magnetic field and evacuated region comprising
4. Apparatus according to claim 3 further comprising
5. The apparatus of claim 3 wherein the magnetic channel means comprises
Description:
This invention relates generally to beam extraction from isochronous cyclotrons and more particularly to a method and means for extracting a high-quality radially focused beam from such cyclotrons.
One object of this invention is to provide a high-quality beam from an isochronous cyclotron.
Another object of this invention is, within the vacuum tank of an isochronous cyclotron, to radially focus the extracted beam which normally defocuses in the fringe magnetic field of the cyclotron.
Still another object of this invention is to provide method and means within the fringe field of an isochronous cyclotron to develop a reverse gradient in that field to radially focus an extracted beam of charged particles.
Other objects and advantages of this invention will become apparent from a consideration of the following description in connection with the drawings wherein
FIG. 1 is an overall perspective view of a small compact isochronous cyclotron which employs the extraction system of this invention;
FIG. 2 is a partially schematic sectional view of the cyclotron of FIG. 1 at its median plane;
FIG. 3 is a detailed plan view of the electrostatic deflection means of the cyclotron of FIG. 1;
FIG. 4 is a sectional view of the electrostatic deflection means taken along line 4-4 of FIG. 3;
FIG. 5 is a rear elevational view of the electrostatic deflection means of FIG. 3;
FIG. 6 is a top view of one embodiment of radial focusing means useful in the described system;
FIG. 7 is a cross-sectional view of the radial focusing means taken along line 7-7 of FIG. 6; and
FIG. 8 plots fringe magnetic field strength versus orbital radius to show the effect of the system of this invention on the magnetic field of the cyclotron.
FIG. 1 illustrates the general layout and components of a small diameter, azimuthally varying field, isochronous cyclotron with the top part of the magnet assembly raised for clarity of illustration. The described machine includes a main DC electromagnet including the upper iron yoke slab 1, a lower iron yoke slab 2, two interconnecting iron legs 3 and two cylindrical iron pole bases 4, one on each of the upper and lower yoke slabs respectively, and one of which is shown on FIG. 2. These iron components are doweled and assembled into a unitary magnetic core.
An upper water-cooled DC magnet coil 5a embraces the upper pole base and a lower magnet coil 5b embraces the lower pole base 4 within legs 3. The pole tips are warp plates 6a, 6b bolted to the pole bases which, with sidewalls 7, form the vacuum tank 8 within which particles are accelerated in the machine.
Three shaped hill pieces 9, mount on each of the warp plates in corresponding locations to produce the azimuthally varying field necessary for isochronous operation. FIG. 2 illustrates one set of the shaped hill pieces 9 on the lower warp plate 6b. A corresponding set bolts to the warp plate 6a over the upper pole base 4.
A pair of hollow 120° dees 10 within the vacuum tank provides a radiofrequency accelerating field. The dee stems 11 connect via conductor means to resonator tank 12. Oscillator 13 supplies radiofrequency energy to the dees. An ion source 20 supplies ions for acceleration in the central region between the two dees 10. Ion source 20 in the described embodiment may use ion-heated cathodes to generate ions in the central region as described in "A Small Cold-cathode High-intensity Cyclotron Ion Source" by D. K. Wells in IEEE Transactions on Nuclear Science, June 1967, pages 70--71. Vacuum means as at 21 evacuates the interior of vacuum tank 8.
The DC magnet provides a field within the machine which increases with increasing radius. Cyclic acceleration of the ions emitted from ion source 20 in the central region is achieved by the hollow dee electrodes 10 within the magnetic field between the pole bases of the magnet. The high-voltage radiofrequency energy supplied to the dees 10 alternately reverses the field across the gaps between them each time the emitted ions revolve 180°. Accordingly, at each gap crossing the ions accelerate through the existing potential difference between the two dees. Each ion coasts at constant speed within the interior region of each dee free from the electric field while the uniform magnetic field bends its path into a semicircle. At each acceleration step the radius of the orbit for each ion increases so that the particle ultimately spirals out to the boundary of the magnetic field. That field guides the particle radially many times through the radiofrequency electric accelerating field and, thus, its final energy is the sum of the individual energies gained at each crossing of the dee gaps.
Vertical focusing of the orbiting ions in the described cyclotron is provided by the three hill sectors 9 of increasing curvature which develop sectors of stronger field alternating with intermediate sectors of weaker field. The hill and valley regions at their boundaries develop a component of magnetic field which directs orbiting ions that may be off the median plane back toward the median plane as has been described by L. H. Thomas in Physics Review (1938) Volume 54, page 580.
All of the foregoing is generally known in the cyclotron art. In the described embodiment the extraction system comprises electrostatic deflection means 30, located between the dees 10 and a magnetic channel 31 which receives and radially focuses a beam of ions deflected by the electrostatic deflection means. High-voltage DC power supply 32 supplies a constant high-voltage potential to electrostatic deflection means 30. The potential is negative for positively charged ions and vice versa.
An embodiment with a negative potential is shown in FIGS. 3--5. The electrostatic deflector assembly mounts at the median plane of machine between the dees 10 and overlying a hill piece 9 of the main magnet. The deflection means 30 comprises a curved tungsten septum 35 maintained at ground potential and a curved deflector electrode 36 which is held at a high negative potential (for positively charged accelerated particles) by power supply 32. The septum 35 and deflector electrode 36 define between them a shaped electrostatic channel 37 with a high electric field gradient which, when traversed by a beam of orbiting particles, forces the ions to move to a larger radius where they no longer are held to a circular path by the main magnet.
The septum and deflector electrode are carefully shaped and located so that the extracted beam remains centered in the channel as it moves to greater radius. Both mount upon a water-cooled nonmagnetic baseplate 38 pivotally mounted on one of the hill pieces 9. The septum 35 is clamped to the baseplate 38 and an upper cooling plate 39 by means of heat-conductive clamp bars 40a, 40b, respectively. Coolant admitted at inlet 41 circulates through conduit 42 imbedded in baseplate 38, through tube 43 into upper cooling plate 39 in series and then leaves through return tube 44 and outlet 45.
A pair of alumina insulators 46 cantilevered from brackets 47 support the hollow copper deflector electrode 36 from baseplate 38. The electrode is also cooled by coolant supplied to its interior through hollow electrical conductor 48a and returned through hollow electrical conductor 48b. These same conductors 48a, 48b provide the high electrical potential to the electrode from power supply 32.
The baseplate 38 of the deflector means can be pivoted by remote means, not shown, operable outside the vacuum tank to locate the entrance to the electrostatic channel 37 for maximum beam extraction efficiency.
Normally as ions accelerate from ion source 20 to the extraction radius, each oscillates in both the radial and vertical directions. The magnitude of these oscillations is kept small by the magnetic field to limit particle losses. The period of the oscillations is determined by the magnetic field but phase is generally random. These incoherent oscillations give height and width to the cross section of any single turn of beam current. High extraction efficiency in the described cyclotron is achieved by making the magnitude of the peak to peak amplitude of this incoherent radial oscillation similar to the spacing of septum 35 and deflector electrode 36 which define the electrostatic channel 37.
In addition to the described oscillation, harmonic coils excite an oscillation where all ions are in phase by creating a "bump" at one azimuth in the main magnetic field. The magnetic "bump" separates adjacent turns of beam current at the peak amplitude of this coherent oscillation. Separation is progressively larger as the amplitude of oscillation increases as a function of the number of times the beam traverses the "bump." If the oscillation were allowed to grow excessively, the beam would be lost. But the oscillation is allowed to grow only to the extent that the turn spacing is of sufficient magnitude to jump the septum. The beam, as a consequence, is very effectively steered into the electrostatic channel.
Three pairs of harmonic coils 48 are located on the pole bases of the magnet at 120° intervals inside the vacuum tank, one on each pair on opposite sides of the median plane. On of each pair appears schematically in FIG. 2. The coils are Y-connected electrically with a delta configuration formed by three resistors interconnecting adjacent ones of the coils on the same side of the median plane. The current through the coil pairs is then a function of, and the azimuthal position of the magnetic "bump" is determined by, where the DC coil supply connects at an 180° spacing to the delta. The magnitude of the "bump" is a function of the magnitude of the coil current.
After leaving the electrostatic channel 37 the extracted beam of ions follows a path of increasing radius. The precise radius of curvature is a function of the decreasing strength of the fringe field. As indicated on FIG. 8 the decreasing gradient results in good vertical focusing, but the extracted beam normally nonlinearly defocuses radially with this same declining gradient.
In the described embodiment the magnetic channel 31 illustrated in detail in FIGS. 6 and 7 receives the beam of ions deflected by electrostatic deflection means 30 and focuses the beam radially. FIG. 8 shows that the fringe magnetic field strength, B, normally decreases with radius. Although the beam is focused vertically as a result of this decreasing gradient, the beam defocuses radially. Magnetic channel 31, however, provides a reversal in the magnetic field gradient at that portion of the curve designated radial focusing in FIG. 8. The increasing magnetic field developed by iron in magnetic channel 31 refocuses the beam radially.
The magnetic channel in the illustrated embodiment comprises a series of aluminum brackets 51 which hold a pair of iron bars on opposite sides of the median plane of the machine. These iron bars concentrate the lines of force in the fringe magnetic field and thus increase the field strength at the channel location. Each bracket comprises a baseplate 52 bolted to the bottom warp plate 6b by a nonmagnetic capscrew 53 and nonmagnetic spacer 54. Nonmagnetic bolt 55 and nut 56 secure a pair of nonmagnetic jaws 57a, 57b to baseplate 52. The jaws are spread by nonmagnetic lever bar 58 at one end and channel spacer 59 at their middle. The open ends of each of jaws 57a, 57b carry iron bars 60 which are mounted in the jaws upon nonmagnetic shims 61.
The specific magnetic channel disclosed is for illustrative purposes only. The describe reversal in gradient of the fringe magnetic field can be accomplished by other means such as similarly placed electromagnet coils.
It will also be apparent to those skilled in the art that other modifications may be practiced and equivalents substituted for those specific elements described above which are within the scope of the invention defined by the appended claims.
One object of this invention is to provide a high-quality beam from an isochronous cyclotron.
Another object of this invention is, within the vacuum tank of an isochronous cyclotron, to radially focus the extracted beam which normally defocuses in the fringe magnetic field of the cyclotron.
Still another object of this invention is to provide method and means within the fringe field of an isochronous cyclotron to develop a reverse gradient in that field to radially focus an extracted beam of charged particles.
Other objects and advantages of this invention will become apparent from a consideration of the following description in connection with the drawings wherein
FIG. 1 is an overall perspective view of a small compact isochronous cyclotron which employs the extraction system of this invention;
FIG. 2 is a partially schematic sectional view of the cyclotron of FIG. 1 at its median plane;
FIG. 3 is a detailed plan view of the electrostatic deflection means of the cyclotron of FIG. 1;
FIG. 4 is a sectional view of the electrostatic deflection means taken along line 4-4 of FIG. 3;
FIG. 5 is a rear elevational view of the electrostatic deflection means of FIG. 3;
FIG. 6 is a top view of one embodiment of radial focusing means useful in the described system;
FIG. 7 is a cross-sectional view of the radial focusing means taken along line 7-7 of FIG. 6; and
FIG. 8 plots fringe magnetic field strength versus orbital radius to show the effect of the system of this invention on the magnetic field of the cyclotron.
FIG. 1 illustrates the general layout and components of a small diameter, azimuthally varying field, isochronous cyclotron with the top part of the magnet assembly raised for clarity of illustration. The described machine includes a main DC electromagnet including the upper iron yoke slab 1, a lower iron yoke slab 2, two interconnecting iron legs 3 and two cylindrical iron pole bases 4, one on each of the upper and lower yoke slabs respectively, and one of which is shown on FIG. 2. These iron components are doweled and assembled into a unitary magnetic core.
An upper water-cooled DC magnet coil 5a embraces the upper pole base and a lower magnet coil 5b embraces the lower pole base 4 within legs 3. The pole tips are warp plates 6a, 6b bolted to the pole bases which, with sidewalls 7, form the vacuum tank 8 within which particles are accelerated in the machine.
Three shaped hill pieces 9, mount on each of the warp plates in corresponding locations to produce the azimuthally varying field necessary for isochronous operation. FIG. 2 illustrates one set of the shaped hill pieces 9 on the lower warp plate 6b. A corresponding set bolts to the warp plate 6a over the upper pole base 4.
A pair of hollow 120° dees 10 within the vacuum tank provides a radiofrequency accelerating field. The dee stems 11 connect via conductor means to resonator tank 12. Oscillator 13 supplies radiofrequency energy to the dees. An ion source 20 supplies ions for acceleration in the central region between the two dees 10. Ion source 20 in the described embodiment may use ion-heated cathodes to generate ions in the central region as described in "A Small Cold-cathode High-intensity Cyclotron Ion Source" by D. K. Wells in IEEE Transactions on Nuclear Science, June 1967, pages 70--71. Vacuum means as at 21 evacuates the interior of vacuum tank 8.
The DC magnet provides a field within the machine which increases with increasing radius. Cyclic acceleration of the ions emitted from ion source 20 in the central region is achieved by the hollow dee electrodes 10 within the magnetic field between the pole bases of the magnet. The high-voltage radiofrequency energy supplied to the dees 10 alternately reverses the field across the gaps between them each time the emitted ions revolve 180°. Accordingly, at each gap crossing the ions accelerate through the existing potential difference between the two dees. Each ion coasts at constant speed within the interior region of each dee free from the electric field while the uniform magnetic field bends its path into a semicircle. At each acceleration step the radius of the orbit for each ion increases so that the particle ultimately spirals out to the boundary of the magnetic field. That field guides the particle radially many times through the radiofrequency electric accelerating field and, thus, its final energy is the sum of the individual energies gained at each crossing of the dee gaps.
Vertical focusing of the orbiting ions in the described cyclotron is provided by the three hill sectors 9 of increasing curvature which develop sectors of stronger field alternating with intermediate sectors of weaker field. The hill and valley regions at their boundaries develop a component of magnetic field which directs orbiting ions that may be off the median plane back toward the median plane as has been described by L. H. Thomas in Physics Review (1938) Volume 54, page 580.
All of the foregoing is generally known in the cyclotron art. In the described embodiment the extraction system comprises electrostatic deflection means 30, located between the dees 10 and a magnetic channel 31 which receives and radially focuses a beam of ions deflected by the electrostatic deflection means. High-voltage DC power supply 32 supplies a constant high-voltage potential to electrostatic deflection means 30. The potential is negative for positively charged ions and vice versa.
An embodiment with a negative potential is shown in FIGS. 3--5. The electrostatic deflector assembly mounts at the median plane of machine between the dees 10 and overlying a hill piece 9 of the main magnet. The deflection means 30 comprises a curved tungsten septum 35 maintained at ground potential and a curved deflector electrode 36 which is held at a high negative potential (for positively charged accelerated particles) by power supply 32. The septum 35 and deflector electrode 36 define between them a shaped electrostatic channel 37 with a high electric field gradient which, when traversed by a beam of orbiting particles, forces the ions to move to a larger radius where they no longer are held to a circular path by the main magnet.
The septum and deflector electrode are carefully shaped and located so that the extracted beam remains centered in the channel as it moves to greater radius. Both mount upon a water-cooled nonmagnetic baseplate 38 pivotally mounted on one of the hill pieces 9. The septum 35 is clamped to the baseplate 38 and an upper cooling plate 39 by means of heat-conductive clamp bars 40a, 40b, respectively. Coolant admitted at inlet 41 circulates through conduit 42 imbedded in baseplate 38, through tube 43 into upper cooling plate 39 in series and then leaves through return tube 44 and outlet 45.
A pair of alumina insulators 46 cantilevered from brackets 47 support the hollow copper deflector electrode 36 from baseplate 38. The electrode is also cooled by coolant supplied to its interior through hollow electrical conductor 48a and returned through hollow electrical conductor 48b. These same conductors 48a, 48b provide the high electrical potential to the electrode from power supply 32.
The baseplate 38 of the deflector means can be pivoted by remote means, not shown, operable outside the vacuum tank to locate the entrance to the electrostatic channel 37 for maximum beam extraction efficiency.
Normally as ions accelerate from ion source 20 to the extraction radius, each oscillates in both the radial and vertical directions. The magnitude of these oscillations is kept small by the magnetic field to limit particle losses. The period of the oscillations is determined by the magnetic field but phase is generally random. These incoherent oscillations give height and width to the cross section of any single turn of beam current. High extraction efficiency in the described cyclotron is achieved by making the magnitude of the peak to peak amplitude of this incoherent radial oscillation similar to the spacing of septum 35 and deflector electrode 36 which define the electrostatic channel 37.
In addition to the described oscillation, harmonic coils excite an oscillation where all ions are in phase by creating a "bump" at one azimuth in the main magnetic field. The magnetic "bump" separates adjacent turns of beam current at the peak amplitude of this coherent oscillation. Separation is progressively larger as the amplitude of oscillation increases as a function of the number of times the beam traverses the "bump." If the oscillation were allowed to grow excessively, the beam would be lost. But the oscillation is allowed to grow only to the extent that the turn spacing is of sufficient magnitude to jump the septum. The beam, as a consequence, is very effectively steered into the electrostatic channel.
Three pairs of harmonic coils 48 are located on the pole bases of the magnet at 120° intervals inside the vacuum tank, one on each pair on opposite sides of the median plane. On of each pair appears schematically in FIG. 2. The coils are Y-connected electrically with a delta configuration formed by three resistors interconnecting adjacent ones of the coils on the same side of the median plane. The current through the coil pairs is then a function of, and the azimuthal position of the magnetic "bump" is determined by, where the DC coil supply connects at an 180° spacing to the delta. The magnitude of the "bump" is a function of the magnitude of the coil current.
After leaving the electrostatic channel 37 the extracted beam of ions follows a path of increasing radius. The precise radius of curvature is a function of the decreasing strength of the fringe field. As indicated on FIG. 8 the decreasing gradient results in good vertical focusing, but the extracted beam normally nonlinearly defocuses radially with this same declining gradient.
In the described embodiment the magnetic channel 31 illustrated in detail in FIGS. 6 and 7 receives the beam of ions deflected by electrostatic deflection means 30 and focuses the beam radially. FIG. 8 shows that the fringe magnetic field strength, B, normally decreases with radius. Although the beam is focused vertically as a result of this decreasing gradient, the beam defocuses radially. Magnetic channel 31, however, provides a reversal in the magnetic field gradient at that portion of the curve designated radial focusing in FIG. 8. The increasing magnetic field developed by iron in magnetic channel 31 refocuses the beam radially.
The magnetic channel in the illustrated embodiment comprises a series of aluminum brackets 51 which hold a pair of iron bars on opposite sides of the median plane of the machine. These iron bars concentrate the lines of force in the fringe magnetic field and thus increase the field strength at the channel location. Each bracket comprises a baseplate 52 bolted to the bottom warp plate 6b by a nonmagnetic capscrew 53 and nonmagnetic spacer 54. Nonmagnetic bolt 55 and nut 56 secure a pair of nonmagnetic jaws 57a, 57b to baseplate 52. The jaws are spread by nonmagnetic lever bar 58 at one end and channel spacer 59 at their middle. The open ends of each of jaws 57a, 57b carry iron bars 60 which are mounted in the jaws upon nonmagnetic shims 61.
The specific magnetic channel disclosed is for illustrative purposes only. The describe reversal in gradient of the fringe magnetic field can be accomplished by other means such as similarly placed electromagnet coils.
It will also be apparent to those skilled in the art that other modifications may be practiced and equivalents substituted for those specific elements described above which are within the scope of the invention defined by the appended claims.