HELIUM PUMPING AT 4.2-K BY MOLECULAR SIEVE-5A

Helium pumping speeds measured for the Excalibur model CVR 1106 cryosorption pump fall into two regimes depending on the feed rate. At feed rates below 6. 2 multiplied by 10** minus **6 Torrls** minus **1cm** minus **2, pumping speeds decrease with loading from 2. 2 ls** minus **1cm** minus **2 to 0. 3 ls** minus **1cm** minus **2. At feed rates higher than 6. 2 multiplied by 10** minus **6 Torrls** minus **1cm** minus **2, the speed varies periodically with loading. A typical cycle begins with pumping speeds near 1. 7 ls** minus **1cm** minus **2; speeds decline from this value to about 0. 6 Torrls** minus **1cm** minus **2 at which time the pump goes through a rapid transition that reverts it back to its original high pumping speed. During the transition, the system reaches presures greater than 10** minus **2 Torr, and rapid boiling occurs in the liquid helium reservoir. At the end of the transition, the system pressure decreases rapidly to the initial low operating pressure, liquid helium boil-off declines, and the pump operates normally for a definite period. The length of each cycle depends on the feed rate.