Magnet structure for particle acceleration
A magnet structure for particle acceleration includes at least two coils that include a continuous path of superconducting material [e.g., niobium tin (Nb 3 Sn) having an A15-type crystal structure] for electric current flow there through. The coils can be mounted in a bobbin, which together with the coils form a cold-mass structure. The coils are cooled to their superconducting temperatures via cryocoolers. Radial-tension members are coupled with the cold-mass structure to keep it centered, such that it remains substantially symmetrical about a central axis and is not pulled out of alignment by magnetic forces acting thereon. A wire can be wrapped around the coils, and a voltage can be applied thereto to quench the coils to prevent their operation of the coils in a partially superconducting condition, which may otherwise cause damage thereto. A magnetic yoke surrounds the cold-mass structure and includes a pair of poles that, in part, define an acceleration chamber there between. The inner surfaces of the poles have tapered profiles that establish a correct weak focusing requirement for ion and that reduce pole diameter by increasing energy gain versus radius. An integral magnetic shield is positioned about the yoke to contain magnetic fields emanating there from and can have a tortuous configuration to contain magnetic fields having a variety of orientations. The magnet structure can be very compact and can produce particularly high magnetic fields.
1. A magnet structure for use in an ion accelerator comprising:
a cold-mass structure including:
at least two coils that comprise a material that is superconducting at a nominal temperature of 4.5 K and that radially circumscribe an acceleration chamber and a segment of a central axis extending across the acceleration chamber, wherein a median acceleration plane extends orthogonally from the central axis across the acceleration chamber; and
a bobbin in which the coils are mounted, the bobbin including a pair of outer wings to support the coils in an outward radial direction and an inner wing between the coils and intersecting the median acceleration plane to support the coils in an inward axial direction;
a cryostat enclosing the cold-mass structure;
a dry cryocooler coupled with the cold-mass structure to cool the coils;
radial-tension links coupled with the bobbin and applying outward radial tension on the bobbin at a plurality of positions; and
a magnetic yoke wrapped around the cold-mass structure, circumscribing the segment of the central axis, and including a pair of poles having tapered inner surfaces that define a pole gap between the poles and across the acceleration chamber.
2. The magnet structure of claim 1 , wherein the superconducting material is Nb 3 Sn.
3. The magnet structure of claim 2 , wherein the superconducting material is an A15 type-II superconductor.
4. The magnet structure of claim 2 , wherein the coil comprises a composite conductor in the form of windings of:
a wound Nb 3 Sn strand; and
a copper channel containing the wound Nb 3 Sn strand, wherein the coil has a cross-section including a plurality of sections for each winding of the wound composite conductor.
5. The magnet structure of claim 4 , further comprising a fiber/epoxy composite filler surrounding each section and between each section of the wound composite conductor.
6. The magnet structure of claim 1 , wherein the pole gap defined by the tapered inner surfaces of the poles expands over an inner stage as the distance from the central axis increases and decreases over an outer stage as the distance from the central axis further increases.
7. The magnet structure of claim 1 , wherein the coils have a radius no greater than 20 inches.
8. The magnet structure of claim 1 , wherein the cryocooler is a Gifford-McMahon cryocooler or a pulse-tube cryocooler.
9. The magnet structure of claim 1 , wherein the bobbin is in the form of a substantially solid block.
10. The magnet structure of claim 9 , wherein the bobbin defines a radial passage in and through the bobbin.
11. The magnet structure of claim 1 , wherein the wings of the bobbin are at least as thick as the coil, measured radially.
12. The magnet structure of claim 1 , further comprising a pressurized bladder positioned between each coil and the bobbin to apply radial inward force on the coil.
13. The magnet structure of claim 1 , wherein the radial tension links comprise elastic tension bands.
14. The magnet structure of claim 13 , wherein the elastic tension bands comprise spiral-wound glass or carbon tape impregnated with epoxy.
15. A magnet structure for use in a synchrocyclotron comprising:
a cold-mass structure including at least two superconducting coils, wherein the cold-mass structure is contained in a cryostat that circumscribes an acceleration chamber;
a magnetic yoke wrapped around the cold-mass structure and including a pair of poles that define a pole gap between the poles and across the acceleration chamber, wherein the superconducting coils and the poles are structured to produce a radially decreasing combined magnetic field reaching at least 8 Tesla for synchrocyclotron acceleration in the acceleration chamber; and
an integral magnetic shield surrounding the yoke in substantially all directions and positioned outside the contour of a 1,000 gauss magnetic flux density that can be generated by the magnet structure outside the yoke when a voltage is applied to the superconducting coils to generate the combined magnetic field of at least 8 Tesla inside the acceleration chamber.
16. The magnet structure of claim 15 , wherein the integral magnetic shield has a tortuous shape configured such that most magnetic field lines extending from the magnetic yoke will intersect the integral magnetic shield at a plurality of locations and at a plurality of angles.
17. The magnet structure of claim 15 , wherein the integral magnetic shield comprises iron.
18. The magnet structure of claim 15 , further comprising a cryocooler coupled with the cold-mass structure to cool the coils.
19. The magnet structure of claim 18 , wherein the cryocooler includes a head that is positioned outside the boundary of the integral magnetic shield.
20. A method for generating a magnetic field for ion acceleration comprising:
providing a cold-mass structure in a cryostat that circumscribes an acceleration chamber, the cold-mass structure including at least two superconducting coils centered about a central axis, a cryocooler coupled with the cold-mass structure; a magnetic yoke positioned about the cold-mass structure and including a pair of poles that define a tapered pole gap there between and across the acceleration chamber;
cooling the superconducting coils to or below the critical temperature of the superconductor and applying a voltage to the cold-mass structure to generate a magnetic field of at least 8 Tesla within the pole gap; and
injecting an ion into the acceleration chamber and accelerating the ion in an outward spiral in the acceleration chamber, wherein the ion is subjected to a radially decreasing magnetic field in the acceleration chamber as it is accelerated outward, and wherein a weak-focusing field index parameter, n, is in the range from 0 to 1 across the entire span of the spiral traversed by the ion, where n=−(r/B)(dB/dr), and where B is the magnetic field, and r is the radius from the central axis.
21. The method of claim 20 , wherein the superconducting coils comprise Nb 3 Sn.
22. The method of claim 21 , wherein a magnetic field of at least 9.9 Tesla is generated in the pole gap.
23. The method of claim 20 , wherein radial-tension links are coupled with the cold-mass structure, the method further comprising applying an outward radial force on the cold-mass structure to maintain the positioning of the cold-mass structure.
24. The method of claim 23 , wherein an integral magnetic shield is provided about the yoke at a distance outside the contour of a 1,000 gauss magnetic flux density generated by the cold-mass structure and by the magnetic yoke.
25. The method of claim 20 , wherein the pole gap increases 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.
26. The method of claim 20 , wherein the cold-mass structure and yoke generate a magnetic field of at least about 9 Tesla within the acceleration chamber.
27. The method of claim 20 , wherein the coils are maintained in a dry state in the cold-mass structure when the magnetic field is generated.