High-Field Superconducting Synchrocyclotron

• US 20070171015A1
• Filed: 01/19/2007
• Published: 07/26/2007
• Est. Priority Date: 01/19/2006
• Status: Active Grant

• Current Assignee: Massachusetts Institute of Technology
• Original Assignee: Massachusetts Institute of Technology
• Inventors: Antaya, Timothy
• Granted Patent: US 7,541,905 B2
• Time in Patent Office: Days
• US Class Current: 335/216
• CPC Class Codes: H05H 13/00 Magnetic resonance accelerators; Cyclotrons strophotrons, turbine tubes H01J25/62
• CPC Class Codes: H05H 13/02 Synchrocyclotrons, i.e. frequency modulated cyclotrons
• CPC Class Codes: H05H 7/04 Magnet systems , e.g. undulators, wigglers free-electron laser H01S3/0903; Energisation thereof
• CPC Class Codes: Y10S 505/806 Niobium base, Nb
• CPC Class Codes: Y10S 505/924 Making superconductive magnet or coil
• CPC Class Codes: Y10T 29/49014 Superconductor

Abstract

The magnetic field in an acceleration chamber defined by a magnet structure is shaped by shaping the poles of a magnetic yoke and/or by providing additional magnetic coils to produce a magnetic field in the median acceleration plane that decreases with increasing radial distance from a central axis. The magnet structure is thereby rendered suitable for the acceleration of charged particles in a synchrocyclotron. The magnetic field in the median acceleration plane is “coil-dominated,” meaning that a strong majority of the magnetic field in the median acceleration plane is directly generated by a pair of primary magnetic coils (e.g., superconducting coils) positioned about the acceleration chamber, and the magnet structure is structured to provide both weak focusing and phase stability in the acceleration chamber. The magnet structure can be very compact and can produce particularly high magnetic fields.

Claims

1. A magnet structure including a magnetic yoke comprising a pair of poles that define an acceleration chamber with a median acceleration plane, wherein the poles are joined at a perimeter and separated to form a pole gap in a central region, wherein each pole is structured to shape a magnetic field in the median acceleration plane so that the magnetic field decreases with increasing radius from a central axis to the perimeter when the magnetic yoke is fully magnetized by a pair of magnetic coils positioned about the acceleration chamber and when a central magnetic field of at least 5 Tesla is directly generated in the median acceleration plane by the magnetic coils.

2. The magnet structure of claim 1, wherein the magnetic yoke has an outer radius, measured from the central axis parallel to the median acceleration plane, of no more than about 114 cm.

3. The magnet structure of claim 1, wherein the magnetic yoke has an outer radius, measured from the central axis parallel to the median acceleration plane, of no more than about 89 cm.

4. The magnet structure of claim 1, wherein the separation between the poles across and throughout the acceleration chamber is at least 6 cm.

5. The magnet structure of claim 1, wherein the separation between the poles across and throughout the acceleration chamber is at least 3.8 cm.

6. The magnet structure of claim 1, wherein the magnetic yoke contains a resonator structure including electrodes between the poles for generating a particle-acceleration voltage in the acceleration chamber.

7. The magnet structure of claim 6, wherein the resonator structure is coupled with a radiofrequency voltage source.

8. The magnet structure of claim 1, wherein the peak gap between the poles is at least about 37 cm.

9. The magnet structure of claim 8, wherein each of the poles includes a pole wing that converges beyond the peak gap to produce a gap between the pole wings that is less than one-third the peak gap.

10. The magnet structure of claim 8, wherein each of the poles includes a pole wing that converges beyond the peak gap to produce a gap between the pole wings that is less than 20% of the peak gap.

11. The magnet structure of claim 10, wherein the pole wings have inner surfaces that slope toward the median acceleration plane at an angle of between 80 and 90°

with the median acceleration plane.

12. The magnet structure of claim 10, wherein the pole wings have inner surfaces that slope toward the median acceleration plane at an angle of about 85°

with the median acceleration plane.

13. The magnet structure of claim 9, wherein the magnetic yoke defines passages for the magnetic coils.

14. The magnet structure of claim 13, further comprising magnetic coils in the passages defined in the magnetic yoke.

15. The magnet structure of claim 14, wherein the pole wings shield the acceleration chamber from the magnetic field generated by the magnetic coils.

16. The magnet structure of claim 9, wherein the magnetic yoke further comprises localized magnetic tips positioned circumferentially on the upper and lower pole wings.

17. The magnet structure of claim 16, wherein the localized magnetic tips are discontinuous.

18. The magnet structure of claim 1, wherein the poles are tapered to shape a magnetic field in the median acceleration plane that decreases with increasing radius from a central magnetic field of at least 8.9 Tesla.

19. The magnet structure of claim 1, wherein the poles are tapered to shape a magnetic field in the median acceleration plane that decreases with increasing radius from a central magnetic field of at least 9.5 Tesla.

20. The magnet structure of claim 1, wherein the poles are tapered to shape a magnetic field in the median acceleration plane that decreases with increasing radius from a central magnetic field of at least 10 Tesla.

21. The magnet structure of claim 1, wherein the poles are tapered to shape a magnetic field in the median acceleration plane that decreases with increasing radius from a central magnetic field between 7 and 13 Tesla.

22. The magnet structure of claim 1, wherein the poles are tapered to shape a magnetic field in the median acceleration plane that decreases with increasing radius from a central magnetic field of at least 13 Tesla.

23. The magnet structure of claim 1, wherein the magnetic yoke is structured to contribute about 2 Tesla of additional magnetic field in the median acceleration plane when the magnetic yoke is fully magnetized.

24. The magnet structure of claim 1, wherein the magnetic yoke is structured to contribute no more than about 3 Tesla to the median acceleration plane when the magnetic yoke is fully magnetized.

25. The magnet structure of claim 1, wherein the magnetic yoke comprises gadolinium.

26. The magnet structure of claim 1, wherein a weak-focusing field index parameter, n, is in the range from 0 to 1 across substantially all of the median acceleration plane, wherein n=−

(r/B)(dB/dr), and wherein dB/dr<

0, where B is the magnetic field and r is the radius from the central axis.

27. The magnet structure of claim 1, wherein the pole gap expands over an inner stage as the distance from the central axis increases, and wherein the pole gap decreases over an outer stage as the distance from the central axis further increases.

28. The magnet structure of claim 1, wherein the magnetic yoke has a height, measured orthogonal to the median acceleration plane, less than about 100 cm.

29. The magnet structure of claim 1, wherein the magnetic yoke has a mass less than about 23,000 kg.

30. The magnet structure of claim 1, further comprising a pair of primary magnetic coils positioned about the acceleration chamber.

31. The magnet structure of claim 1, further comprising additional magnetic coils for shaping the field generated by the primary magnetic coils.

32. The magnet structure of claim 31, wherein the primary magnetic coils and the additional magnetic coils are coupled with at least one voltage source.

33. The magnet structure of claim 32, wherein at least one of the additional magnetic coils is coupled with the voltage source to conduct electrical current in a first direction through the additional magnetic coil, wherein the primary magnetic coils are coupled with the voltage source to conduct electrical current in a second direction through the primary magnetic coils, and wherein the second direction is opposite to the first direction such that the magnetic field generated by the additional magnetic coil will at least partially cancel the magnetic field generated by the primary magnetic coils over a region of the median acceleration plane.

34. The magnet structure of claim 32, wherein the additional magnetic coils comprise a material that is superconducting at a temperature of at least 4.5 K.

35. The magnet structure of claim 30, wherein the primary magnetic coils comprise a material that is superconducting at a temperature of at least 4.5 K.

36. The magnet structure of claim 35, wherein the primary magnetic coils comprise Nb₃Sn or NbTi.

37. The magnet structure of claim 1, wherein the magnetic yoke defines a passage along the central axis for injection of ions into the acceleration chamber.

38. The magnet structure of claim 1, wherein each pole is tapered along its inner surface to produce a magnetic field in the median acceleration plane that decreases with increasing radial distance from the central axis.

39. The magnet structure of claim 1, wherein the poles include changes in composition, wherein the compositions have different magnetic properties, to shape the magnetic field in the median acceleration plane.

40. A magnet structure including a magnetic yoke comprising a pair of poles that define an acceleration chamber with a median acceleration plane, wherein the poles are joined at a perimeter and separated to form a pole gap in a central region, wherein each pole has an inner surface tapered to shape a magnetic field in the median acceleration plane so that the magnetic field decreases with increasing radius from a central axis to the perimeter when the magnetic yoke is fully magnetized by a pair of magnetic coils surrounding the acceleration chamber and when a magnetic field of at least 5 Tesla is directly generated in the median acceleration plane by the magnetic coils.

41. A magnet structure including:

a magnetic yoke comprising a pair of poles that define an acceleration chamber with a median acceleration plane, wherein the poles are joined at a perimeter and separated to form a pole gap in a central region, the magnetic yoke also defining passages for primary magnetic coils about the acceleration chamber; and

at least one additional magnetic coil contained in the acceleration chamber for counterbalancing the magnetic field generated by the primary magnetic coils, wherein the additional magnetic coil is electrically insulated from the magnetic yoke.

42. The magnet structure of claim 41, wherein the additional magnetic coil is configured and positioned to generate a magnetic field component that counterbalances the magnetic field generated by the primary coils so that the magnetic field in the median acceleration plane decreases with increasing radius from a central axis to the perimeter when the magnetic yoke is fully magnetized by the primary coils.

43. A method for shaping a magnetic field for ion acceleration comprising:

providing a magnet structure including a magnetic yoke comprising a pair of poles that define an acceleration chamber with a median acceleration plane, wherein the poles are joined at a perimeter and separated to form a pole gap in a central region, wherein each pole is structured to shape a magnetic field in the median acceleration plane that decreases with increasing radius from about 7 Tesla at a central axis to the perimeter; and

injecting an ion into the central region and accelerating the ion in an outward spiral trajectory through the acceleration chamber.

44. The method of claim 43, wherein the magnet structure provides a restoring force to keep the ion oscillating with stability in the acceleration chamber.

45. The method of claim 43, further comprising providing a resonator structure in the magnet structure, wherein the resonator structure imparts the ion with an energy of at least 250 MeV.

46. The method of claim 43, wherein the magnetic yoke generates a magnetic field of no more than about 2 Tesla in the acceleration chamber.

47. The method of claim 43, wherein each pole has a taper along its inner surface that shapes the magnetic field.

48. The method of claim 47, wherein the magnet structure includes a pair of pole wings at the perimeter of the acceleration chamber that sharpen the magnetic field for extraction of the accelerated ion from the acceleration chamber.

49. The method of claim 43, further comprising displacing the magnet structure between uses.

50. The method of claim 43, wherein the ion includes more than one proton.

51. The method of claim 43, further comprising passing electrical current through magnetic coils positioned about the acceleration chamber to directly generate at least about 5 Tesla of the magnetic field at the central axis in the acceleration chamber.

52. A method for shaping a magnetic field for ion acceleration comprising:

providing a pair of primary magnetic coils about an acceleration chamber including a central axis and a median acceleration plane;

providing a plurality of additional magnetic coils nested closer than the primary magnetic coils to the central axis;

passing electrical current through the primary magnetic coils to directly produce a magnetic field of at least about 5 Tesla at the central axis in the median acceleration plane; and

passing electrical current through the additional magnetic coils to shape the magnetic field so that it decreases across the median acceleration plane at increasing radial distances from the central axis, wherein electric current is passed through at least one of the additional magnetic coils in a direction opposite to the direction in which electrical current is passed through the primary magnetic coils to generate a magnetic field in opposition to the magnetic field generated by the primary magnetic coils.