Thermal analysis of a refined APT power coupler and thermal shield and the effect on the cryoplant

The present APT cryomodule shield cooling circuit employs helium gas up to 18 bars in order to operate at 4.6 K inlet temperatures without flow instabilities. The high pressure adds complexity to the cryoplant and cryogen distribution system. The low supply temperature, needed because a series flow circuit is used, increases the cryoplant refrigeration power. This paper describes a lower pressure cooling circuit operating at a high enough supply temperature that flow instabilities do not occur. The shield flow circuit is changed from series to parallel; and modifications are made to the designs of the power coupler outer conductor, thermal intercepts, thermal shield, and end beam tubes. This redesign results in a small decrease in cryoplant compressor power.