Pumping of gaseous helium using argon frosted liquid helium cryocondensation pumps

It has been demonstrated in the Neutral Injection Test Bed that the JET (Joint European Torus) neutral injectors are capable of producing 10-s He beams repetitively with a rate of 7 min per shot without a prohibitive pressure rise. This can be achieved without modifications to the JET liquid-helium (LHe) cryocondensation pumps. The He is pumped by cryosorption on Ar, which is frosted onto the LHe temperature panels before each pulse. The measured effective pumping speed is about 0.25 times the value for H₂. The measurements involved three cryopumps, two of the JET open structure type and one with a conventional type of liquid-nitrogen-cooled baffle. The tests show that the Ar/He coverage ratio and the thickness of the Ar layer seem to be important. The sticking coefficients for He on Ar frost derived for the three pumps agree with each other within the errors