Compact, cold, superconducting isochronous cyclotron

A method for ion acceleration comprising:

• employing an isochronous cyclotron comprising:
• a) at least two superconducting coils that are substantially symmetric about a central axis, wherein the coils are on opposite sides of a median acceleration plane, and wherein the coils have (a) outer surfaces remote from the central axis and (b) opposed median-acceleration-plane-facing surfaces;
• b) a magnetic yoke having an outer radius measured from the central axis that is no greater than 36 cm surrounding the coils and in physical contact with the coils across the outer surface of each coil and across the median-acceleration-plane-facing surface of each coil to substantially reduce or eliminate strain on the coils due to decentering forces and without an intervening cryostat between the magnetic yoke and the coils, wherein the magnetic yoke contains at least a portion of a beam chamber, wherein the median acceleration plane extends through the beam chamber, wherein the magnetic yoke includes a plurality of sector pole tips that form hills on each side of the median acceleration plane and valleys between the hills, where the hills and valleys are positioned within a constant sector periodicity that produces an azimuthal veriation in the magnetic field generated in the median acceleration plane, and wherein the hills are radially separated across the median acceleration plane by a gap that is narrower than a gap that separates the valleys across the median acceleration plane;
• c) a cryogenic refrigerator physically and thermally coupled with the superconducting coils and with the magnetic yoke;
• d) an electrode coupled with a radiofrequency voltage source and mounted in the beam chamber;
• and
• f) a cryostat mounted outside the magnetic yoke and containing the coils and the magnetic yoke;
• introducing an ion into the median acceleration plane at an inner radius;
• providing electric current from the radiofrequency voltage source to the electrode to accelerate the ion at a fixed frequency in an expanding orbit across the median acceleration plane;
• cooling the superconducting coils and the magnetic yoke with the cryogenic refrigerator, wherein the superconducting coils are cooled to a temperature no greater than their superconducting transition temperature, and wherein the magnetic yoke is cooled to a temperature no greater than 100 K;
• providing a voltage to the cooled superconducting coils to generate a superconducting current in the superconducting coils that produces a radially increasing magnetic field that is at least 6 Tesla at the inner radius where the ion is introduced and that is at least 7 Tesla at an outer radius for ion extraction in the median acceleration plane from the superconducting coils and from the yoke, wherein the azimuthal variation in the magnetic field produced by the hills and valleys provides a restoring force orthogonal to the median acceleration plane that counters an inherent instability in the accelerated ion due to the radial increase in the magnetic field;
• and
• extracting the accelerated ion from beam chamber at the outer radius.

A compact, cold, superconducting isochronous cyclotron comprising:

• at least two superconducting coils that are substantially symmetric about a central axis, wherein the coils are on opposite sides of a median acceleration plane, and wherein the coils have (a) outer surfaces remote from the central axis and (b) opposed median-acceleration-plane-facing surfaces;
• a magnetic yoke having an outer radius measured from the central axis that is no greater than 36 cm surrounding the coils and in physical contact with the coils across the outer surface of each coil and across the median-acceleration-plane-facing surface of each coil to substantially reduce or eliminate strain on the coils due to decentering forces and without an intervening cryostat between the magnetic yoke and the coils, wherein the magnetic yoke contains a beam chamber, wherein the median acceleration plane extends through the beam chamber, wherein the magnetic yoke includes a plurality of sector tips that are separated from the rest of the of the magnetic yoke by non-magnetic material and that form hills on each side of the median acceleration plane and valleys between the hills, where the hills and valleys are positioned with a constant sector periodicity that produces an azimuthal variation in the magnetic field generated in the median acceleration plane, wherein the hills are radially separated across the median acceleration plane by a gap that is narrower than a gap that separates the valleys across the median acceleration plane, and wherein the superconducting coils and the physically coupled magnetic yoke are configured to generate a radially increasing magnetic field that is at least 6 Tesla at an inner radius for ion introduction and that is at least 7 Tesla at an outer radius for ion extraction when the superconducting coils and the magnetic yoke are cooled to a temperature no greater than 50K and when electric current is passed through the superconducting coils at the coils' critical current capacity, and wherein the azimuthal variation in the magnetic field produced by the hills and valleys provides a restoring force orthogonal to the median acceleration plane to counter an inherent instability of an ion accelerated by the radially increasing magnetic field;
• a cryogenic refrigerator physically and thermally coupled with the superconducting coils and with the magnetic yoke;
• and
• a cryostat mounted outside the magnetic yoke and containing the coils and the magnetic yoke inside a thermally insulated volume in which the coils and the magnetic yoke can be maintained at cryogenic temperatures by the cryogenic refrigerator.