Independent current control in the coupled superconducting coils of a heavy-ion cyclotron magnet

A heavy ion energy booster cyclotron for a Tandem Van de Graaff is currently under construction at the Chalk River Nuclear Laboratories. The magnet, consisting of a 170 tonne iron yoke, 1.4 m diameter iron poles, and a pair of 3 MA-turn superconducting coils has been built and tested to an average midplane induction of 5 T. The coils, fitted around the poles above and below the midplane, are split electrically into an inner and outer pair. Independent current settings in each pair provides coarse adjustment of the radial profile of the field to allow an energy and ion acceleration range from 50 MeV/u lithium to 10 MeV/u uranium. The coils, which represent a variable inductive load, (inductances decrease by roughly a factor 20 as the iron saturates) are energized by a dual 8 V, 2500 A power supply with automatic voltage/current crossover. A special feedback circuit prevents induced reverse currents that would produce undesirable reverse radial stresses in the coils. Long term current and field stabilities of better than 1 × 10^-5/h have been achieved.