DEVELOPMENT AND PERFORMANCE OF BAKEABLE CRYOPUMPS FOR EXTREME HIGH-VACUUM

A new type of bakeable cryopump with modified Gifford-McMahon refrigerator system, which is separated spatially from the cryopump housing, has been developed. The cryopanels are cooled with helium gas circulating from the refrigerator system. Therefore, the cryopump housing and cryopanels can be baked simultaneously at temperatures of 150-200-degrees-C during regeneration. With such a bakeout process this cryopump can repeatedly attain the ultimate pressure of extreme high vacuum. By the use of the new cryopump with two cryopanels at temperatures of 20 and 80 K level, a test dome was pumped down to the pressure less than 1 x 10(-10) Pa in a few days after bakeout. With the new cryopump with three cryopanels of 3, 20 and 80 K level, the pumping time down to the pressure of 1 x 10(-10) Pa was only about 11 h. The pumping characteristics of these cryopumps for hydrogen in extreme high vacuum were measured with a conductance modulation method.