RECENT PROGRESS OF THE SUPERCONDUCTING ACCELERATOR MODULES FOR THE JAERI FEL

The new cooling system of the JAERI superconducting accelerator module named ''multi-refrigerators system'' consists of a liquid He reconden sor and a shield cooler utilizing a compact closed-loop He gas refrige rator, and their integration into each accelerator module cryostat. Be cause of restricted cooling power of the refrigerator, we designed 80 K and 40 K heat shields to minimize the static heat load to a liquid H e container of the cryostat. When thermal anchors are installed in opt imized positions, the heat load to the liquid He container is calculat ed to be 1 W, and estimated to be less than 3 W by a simple 1-dimensio nal calculation. The cooling power of the 4 K refrigerator was about 1 1 W or more at 4.5 K, which is three times larger than required by the thermal design. The vibrational amplitude of the cryostat due to the refrigerators' motion was measured to be less than 0.2 mum (p-p) on a test cryostat surface, and it was well within our requirement. The mul ti-refrigerators system has been operated very reliably for several mo nths.