THE CROSSOVER OF REFRIGERATOR-COOLED CRYOPUMPS

According to the PNEUROP-acceptance rules for cryopumps the crossover is defined as the maximum amount of nitrogen (mbar I) which can be pum ped by the cryopump within a short interval, whereby the temperature o f the cryopanels must not exceed 20 K. Measurements carried out with a standard refrigerator-cooled cryopump (intake flange DN 200) have sho wn that the present definition is not sufficient. The temperature rise caused by the gas flow mainly depends on the preloading of the pumpin g surfaces. The report proposes a new, more complete and practicable d efinition of the crossover.