The effect of vacuum gas pressures and species on internal heat leak in the SSCL magnet design

This work describes a procedure for calculating the heat transfer into the cryogens inside a 50-mm Collider Dipole Magnet (CDM) for the Superconducting Super Collider (SSC) during its operation in a Collider Ring Half Cell vacuum. Figure 1 is a schematic cross-section of the CDM showing various major components and their temperatures. Described here is the use of a species gas model for assessing the heat transfer due to conduction through the residual gases in the cryostat insulating vacuum. Residual gas heating is a major source of heat to the helium-cooled regimes in the magnet. It is a function of several variables that include the gas(es) involved, and it is found to decrease linearly with vacuum pressure and exponentially with gas temperature. Thermal radiation, in contrast, varies with temperature and diminishes as temperatures decrease on the T4 curve. The gas model used here is based on results from a Residual Gas Analyzer (RGA) measurement of the vacuum in the Fermi National Accelerator Laboratory (FNAL) ER 40-mm Dipole Magnet String Test (ST). At present, no vacuum tests have been conducted on 50-mm CDMs. The work was done to analyze the effect on residual gas heat conduction of presumed gas species in the vacuum total pressure. It is seen to be a significant improvement over using a ''single gas'' model. Results would indicate that unless the species gas model is used, the residual gas heating can be significantly overestimated depending on the vacuum and gases involved