High-field synchrocyclotron

A method for shaping a magnetic field for ion acceleration in a synchrocyclotron, the method comprising:

• providing a magnet structure including:
• a magnetic yoke comprising a pair of poles that define an acceleration chamber including a median acceleration plane and a central axis orthogonal to the median acceleration plane, wherein the magnetic yoke defines passages about the acceleration chamber and on opposite sides of the median acceleration plane, wherein the poles are joined at a perimeter and separated to form a pole gap across the acceleration chamber, and wherein each pole has an inner surface that is tapered to produce a perimeter pole wing distal from the central axis and proximate the passages for the coils, a central pole tip proximate the central axis and distal from the passages, and a peak pole gap at an intermediate distance between the central pole tips and the perimeter pole wings, the peak pole gap being at least triple the pole gap between the central pole tips and at least triple the pole gap between the perimeter pole wings;
• a pair of magnetic coils mounted in the passages defined by the magnetic yoke on opposite sides of the median acceleration plane;
• and
• at least one pair of radiofrequency accelerator electrodes mounted between the poles;
• passing electric current through the coils to generate a magnetic-field component in the median acceleration plane and to magnetize the magnetic yoke, the magnetized magnetic yoke generating another magnetic-field component in the median acceleration plane, wherein the coil-generated and magnetic-yoke-generated magnetic-field components jointly produce a combined magnetic field of at least 6.65 Tesla in the median acceleration plane, the combined magnetic field decreasing across the median acceleration plane with increasing radius and a weak-focusing field index parameter, n, being in the range from 0 to 1 from the central pole tips to the perimeter pole wings, where n=−(r/B)(dB/dr), and where B is the magnetic field, and r is the radius from the central axis;
• injecting an ion into the magnetic field in the acceleration chamber;
• generating an electric field with the radiofrequency accelerator electrodes to accelerate the ion in an outward spiral trajectory across the median acceleration plane and reducing the frequency of the electric field as the ion accelerates outward;
• and then
• extracting the accelerated ion from the acceleration chamber.