The ITER fore-pumping system fulfils the task of reducing the pressure to a roughing level (10 Pa) for different reactor subsystems; the later are equipped with the cryogenic pumps as primary UHV pumps. Thus, the fore-pumping system must provide both the initial pumpdown of the vessel itself and the regular pump-out of the batch regenerating cryopumps. Torus and NBI cryopumps are regenerated in a cycled mode, therefore the evacuation of the desorbed gases must be gained within a certain interval: the torus cryopump (8 m3) - within 2.5 min, NBI cryopump (200 m3) - within 8 min. The pumping speed of a single fore-pumping train is defined for the heaviest duty as 1.5 m3/s at 10 Pa inlet pressure. This capacity is far larger than the capacity of commercially available tritium vacuum pumps such as membrane, wobbling piston or metal bellows pumps. The work reported in this paper is dealing with the mechanical pumps of large capacity: roots, screw and normetex (scroll) pumps. Compressing of the light gases in these pumps is radically affected by the gap sealing between their rotors and between the rotor and the walls.
During the nominal ITER operation, the pumped gas will be deuterium-tritium mixture (~90%). At the start ITER will be operated with hydrogen plasmas, nevertheless already during the hydrogen campagnes the vacuum system shall be tritium compatible. Usually to pump hydrogen a ballast gas (typically nitrogen) is added to the process gas in order to enhance the pumping performance and a purge gas (typically nitrogen) is added to the shaft sealing system in order to avoid penetration of the hydrogen into the lubricant chamber and to prevent explosive conditions. In ITER (as well as in future fusion machines) the outlet will be directed to the tritium processing plant, herewith no additional gases are allowed to be introduced in the pumping system. The performance of different mechanical pumps and pump trains with deuterium and hydrogen has been analysed. The results are presented in this paper. As well, the paper briefly summarises the main features of a tritium compatible Roots pump.
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