Effective hydrogen gas temperature in a cryopumped beam line vessel

Thermal accommodation of hydrogen gas molecules to cold surfaces modifies the effective gas temperature in cryogenically pumped vacuum vessels. In Doublet III neutral beam lines, gas pumping is achieved by liquid-helium-cooled panels which are surrounded by liquid-nitrogen-cooled thermal shields. By measuring the gas pressure during the injection of a known amount of gas while cooling the thermal shields by liquid nitrogen, we have determined the effective gas temperature according to T/T0=(Pi/Pio)2, where Pi is the pressure monitored by an ionization gauge and the subscript o refers the case of room temperature. The effective gas temperature was determined to be about 150 K when only the thermal shields were cooled down to the liquid nitrogen temperature. Implications of the reduced gas temperature on pressure measurement, pumping speed measurement, and neutral beam reionization loss calculation are discussed.