Cryosorption of helium on argon frost in Tokamak Fusion Test Reactor neutral beamlines

Helium pumping on argon frost has been investigated on Tokamak Fusion Test Reactor (TFTR) neutral beam injectors and shown to be viable for limited helium beam operation. Maximum pumping speeds are ∼25% less than those measured for pumping of deuterium. Helium pumping efficiency is low, >20 argon atoms are required to pump each helium atom. Adsorption isotherms are exponential and exhibit a twofold increase in adsorption capacity as the cryopanel temperature is reduced from 4.3 K to 3.7 K. Pumping speed was found to be independent of cryopanel temperature over the temperature range studied. After pumping a total of 2000 Torr ℓ of helium, the beamline base pressure rose to 2×10−5 Torr from an initial value of 10−8 Torr. Accompanying this three order of magnitude increase in pressure was a modest 40% decrease in pumping speed. The introduction of 168 Torr ℓ of deuterium prior to helium injection reduced the pumping speed by a factor of two with no decrease in adsorption capacity.