Calculation of Forces on the Aluminum Heat Shield Produced by Eddy Currents during Discharge of a Superconducting Cyclotron Magnet

The superconducting magnet in our 500 MeV cyclotron contains a (liquid nitrogen cooled) aluminum heat shield within the cryostat, and when, after more than two years of operation, the cryostat was disassembled, it was discovered that the outer surface of the heat shield had collapsed inward, thereby compressing the underlying super-insulation tightly against the can containing the liquid helium and coil. Consideration of the resultant geometry of the heat shield indicated that the collapsing force was produced by eddy currents generated in the aluminum either during discharge of the magnet or else when one of the radial support links happened to break. These forces have been calculated under both conditions, and the procedures used for these calculations are presented here together with the results. We conclude that the collapse of the heat shield was most probably caused when, on one occasion, the dump resistor accidentally burned out thereby producing an exceptionally fast discharge of the magnet.