Within the framework of the European Fusion Technology Programme a cryopump for the primary vacuum system of ITER is being developed at Karlsruhe Research Centre (FZK). The on-going activities are currently culminating in the design and construction of a prototypical model cryopump. The torus high vacuum pumping system shall be used both during the various plasma operations to pump the torus exhaust gas and during all other phases of operation. Consequently, the composition of the gases to be pumped can vary within a very wide range.

For validation of the proposed cryopump concept, a test campaign has been started to determine experimentally the pumping speed as a function of mixture composition. One of the key objectives was to investigate the influence of the inert gases N2, He, Ne and Ar on the mixture pumping speed and to work out the most critical composition conditions. The tests were performed in the test facility TITAN, which has already been used in former test programmes. The investigated cryopanel was similar in size and structure as the one which has been recommended for ITER and will be used in the model pump. The pumping speed at the panel can be derived by coupling the measured pumping speed upstream with Monte Carlo simulation results for the conductance of the facility.

The tests have shown that inert gas contents according to the ITER-specification do not pose a problem to achieve the required pumping speed of about 1 l/(s-cm²) (related to the active cryopumping surface). Pumping of H2 based mixtures depends stronger on the concentration of noble gases, than it is the case with D2 based mixtures. Thus, already small percentages of He in the H2 based mixture reduce the pumping speed to almost the pure He level, which is the lowest of all gas species. This is due to competitive sorption of He and the hydrogens at the cryopanel charcoal.

To study this effect in more detail, a high resolution quadrupole mass spectrometer has been applied, which is able to scan all the masses of interest and on top of that reaches a base-line separation of helium and deuterium. The mass spectrometer has been calibrated carefully to yield quantitative composition data. It was found that within mixture pumping there are distinctive composition changes due to the enrichment of those gas species, which can be pumped worse, like helium. However, the total pressure during pumping stays in a range which is appropriate for ITER.