Application of solenoid focusing in a superconducting heavy-ion linear accelerator

Abstract

When the resonators of a heavy-ion linear accelerator are operated in the phase-stable region, their inherent radial-defocusing property makes external radial focusing elements necessary for preventing beam blow-up. Although quadrupoles are generally chosen for room-temperature ion linacs for reasons of space and power efficiency, solenoid focusing is superior in superconducting heavy-ion linacs in the energy range of present interest. Since power and cooling requirements are low, for equal beam-optical properties, superconducting solenoids are cheaper and simpler focusing elements than the corresponding quadrupoles. Calculations made for a superconducting-helix linac for heavy ions show that with resonators containing five 1 2 λ sections, the solenoids required are small and use magnetic fields readily attainable in commercial construction. Calculations made with the computer code TRIM show that a relatively small iron shield can be built which reduces the leakage field to negligible values in a relatively short space.