Ring warmup and helium recovery for the RHIC accelerator: Thermal analysis of the warmup of a superconducting magnet string

RHIC design criteria require that a faulty string be warmed rapidly to a serviceable temperature. Integral electric heaters warm the superconducting magnets. The magnets are very sensitive to temperature peaks and local gradients. Helium gas is circulated during warmup so that all points are warmed at nearly equal rates. An optimized string warmup procedure gives a continuous, uniform temperature profile, that is flat and rises steadily to room temperature at a rate which allows manageable recovery of the liquid helium content of the magnets. The ideal is not easily obtained. Various warmup schemes are studied. Limitations on the process, including those of the RHIC helium recovery system are discussed. A finite difference code (Fortran) is used to model a magnet string with flowing helium and electric heat generation. Temperature dependent properties of metal and gas including variable density are accounted for. Predictions are compared to physical test data.