Magnetic shims to alter magnetic fields

1. A particle accelerator comprising:

a coil to provide a magnetic field to a cavity;

a cryostat comprising a chamber for holding the coil, the coil being in the chamber to define an interior region of the coil and an exterior region of the coil;

magnetic structures adjacent to the cryostat, the magnetic structures having one or more slots at least part-way therethrough; and

one or more magnetic shims in one or more corresponding slots, the one or more magnetic shims being movable to change a magnetic field produced via the magnetic structures and thereby adjust a position of the coil.

2. The particle accelerator of claim 1 , wherein the one or more magnetic shims are controllable to move into or out of corresponding slots.

3. The particle accelerator of claim 2 , wherein the one or more magnetic shims are computer controlled.

4. The particle accelerator of claim 2 , wherein the one or more magnetic shims are controllable in response to rotation of the particle accelerator.

5. The particle accelerator of claim 4 , wherein the one or more magnetic shims are controllable to adjust the position of the coil to compensate for motion of the coil caused by rotation of the particle accelerator.

6. The particle accelerator of claim 1 , wherein at least one of the magnetic shims is within the interior region.

7. The particle accelerator of claim 6 , wherein all of the magnetic shims are within the interior region.

8. The particle accelerator of claim 1 , wherein the one or more magnetic shims comprise ferromagnetic material.

9. A proton therapy system comprising:

the particle accelerator of claim 1 ; and

a gantry on which the particle accelerator is mounted, the gantry being rotatable relative to a patient position;

wherein the particle accelerator is configured to output protons to the patient position.

10. The proton therapy system of claim 9 , wherein the particle accelerator is rotatable relative to a fixed position; and

wherein the proton therapy system further comprises a control system to control movement of the one or more magnetic shims based on a rotational position of the gantry.

11. An electromagnet comprising:

a superconducting coil in a substantially circular configuration to generate a magnetic field;

magnetic structures adjacent to the superconducting coil, the magnetic structures having one or more slots at least part-way therethrough; and

one or more magnetic shims in one or more corresponding slots, the one or more magnetic shims being movable to change a magnetic field produced via the magnetic structures and thereby adjust a position of the superconducting coil.

12. The electromagnet of claim 11 , wherein the superconducting coil is configured to generate a magnetic field that is stronger in an interior of the circular configuration than in an exterior of the circular configuration; and

wherein the one or more magnetic shims are closer to the interior of the circular configuration than to the exterior of the circular configuration.

13. The electromagnet of claim 12 , wherein at least part of the one or more magnetic shims are within the interior of the circular configuration.

14. The electromagnet of claim 11 , wherein all of the one or more magnetic shims are within the interior of the circular configuration.

15. The electromagnet of claim 11 , wherein the one or more magnetic shims are controllable to move into or out of corresponding slots.

16. The electromagnet of claim 15 , wherein the one or more magnetic shims are computer controlled.

17. The electromagnet of claim 15 , wherein the one or more magnetic shims are controllable in response to rotation of the electromagnet.

18. The electromagnet of claim 17 , wherein the one or more magnetic shims are controllable to adjust the position of the coil to compensate for motion of the coil caused by rotation of the electromagnet.

19. The particle accelerator of claim 11 , wherein the one or more magnetic shims comprise one or more magnetic shims comprised of ferromagnetic material.

20. A proton therapy system comprising:

a particle accelerator comprising the electromagnet of claim 11 for outputting particles comprising protons; and

a gantry on which the particle accelerator is mounted, the gantry being rotatable relative to a patient position;

wherein the particle accelerator is configured to output protons to the patient position.

21. The proton therapy system of claim 20 , wherein the particle accelerator is rotatable relative to a fixed position; and

wherein the proton therapy system further comprises a control system to control movement of the one or more magnetic shims based on a rotational position of the gantry.

22. A method of adjusting a position of a superconducting coil in an electromagnet, the method comprising:

determining a rotational position associated with the electromagnet; and

based on the rotational position, controlling a position of one or more magnetic shims in a magnetic structure proximate to the superconducting coil to change a magnetic field produced via the magnetic structure and thereby adjust a position of the superconducting coil.

23. The method of claim 22 , wherein the electromagnet is part of a particle accelerator that is mounted on a gantry that is rotatable; and

wherein the rotational position is a position of the gantry.

24. The method of claim 22 , wherein the superconducting coil is in a substantially circular configuration to generate a magnetic field;

wherein the magnetic field is stronger in an interior of the circular configuration than in an exterior of the circular configuration; and

wherein the one or more magnetic shims are closer to the interior of the circular configuration than to the exterior of the circular configuration.

25. The method of claim 24 , wherein at least part of the one or more magnetic shims are within the interior of the circular configuration.

26. The method of claim 24 , wherein all of the one or more magnetic shims are within the interior of the circular configuration.

27. The method of claim 22 , wherein the one or more magnetic shims are controllable to move into or out of corresponding slots in the magnetic structure.

28. The method of claim 22 , wherein the position of the one or more magnetic shims are controllable using a computer.

29. The method of claim 22 , wherein the one or more magnetic shims are controllable to adjust the position of the superconducting coil to compensate for motion of the superconducting coil caused by rotation of the electromagnet.

30. The method of claim 22 , wherein the one or more magnetic shims comprise one or more magnetic shims comprised of ferromagnetic material.

31. A particle accelerator comprising:

a coil to provide a magnetic field to a cavity in which particles are accelerated;

a cryostat comprising a chamber for holding the coil;

magnetic structures adjacent to the cryostat, the magnetic structures having one or more slots at least part-way therethrough; and

magnetic shims in corresponding slots, the magnetic shims being movable to adjust a magnetic field in the cavity in order to affect a particle orbit within the cavity and a position of the coil.

32. The particle accelerator of claim 31 , further comprising:

an extraction channel, through which the particles pass for output from the particle accelerator, the extraction channel having an entrance at the cavity;

wherein one or more magnetic shims are movable in one or more corresponding slots so that an energy of particles entering the extraction channel from the cavity matches an energy required for the particles to traverse the extraction channel.

33. The particle accelerator of claim 32 , wherein the entrance to the extraction channel does not include an energy absorbing structure through which a particle beam passes prior to entry to the extraction channel.

34. The particle accelerator of claim 31 , wherein the magnetic structures comprise pole pieces and wherein the coil is a first coil; and

wherein the particle accelerator further comprises a second coil to operate as an active return for magnetic field resulting from current through the first coil.

35. A proton therapy system comprising:

the particle accelerator of claim 31 ; and

a gantry on which the particle accelerator is mounted, the gantry being rotatable relative to a patient position;

wherein the particle accelerator is configured to output protons to the patient position.