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The ITER fusion machine to be built at Cadarache, France requires the development of novel vacuum technologies for the cryopumping systems for the torus, neutral beam injectors and cryostat, mechanical rough pumping systems, and for leak detection and localization. The presence - actual or potential - of tritiated gas species exerts a strong influence on the technologies which can be deployed in ITER vacuum systems. The outgassing rates from plasma facing components and the gas species evolved are strongly dependent on the selection of plasma facing materials; the present design of ITER includes beryllium, tungsten and carbon fibre composites. The cryopump programme, ongoing at FZK for the last 20 years, covers all aspects of the development, from investigation of candidate pumping concepts through characterization of small scale cryopanel coupons to testing of a full scale ITER prototype Torus Exhaust Cryopump, which is presently being fabricated. Designs of the cryopumps for the applications noted above have been standardized to the greatest extent practical. A focussed programme to develop a robust, tritium-compatible cryosorption panel concept has been carried out and a technical facility for coating panels with calibrated thin layers of activated charcoal sorbent has been built and used in the production of cryopanels for ITER and other fusion cryopumping applications.
Mechanical roughing is by a set of pump trains which include specially adapted roots pumps with enhanced internal and external leak tightness. The latter is essential to preclude cross-contamination between the tritiated process gases and the lubrication oil of the timing gears. A reduced scale pump equipped with ferrofluidic shaft seals between the process gas and gear compartments has been successfully tested. Scale-up studies indicate that the concept can be used at ITER-relevant capacities, but experimental validation remains to be done.
Early leak detection and precise leak localization are essential to maximization of machine availability. Localization of water leaks to the level of individual in-vessel components is challenging due to the large number of first wall modules and divertor cassettes. The detection and localization of air leaks, which will in many cases be associated with vacuum vessel ports or penetrations for plasma heating and diagnostic devices is complicated by the diversity of these systems. A sequence of procedures for leak localization is foreseen. Following detection of an in-vessel water leak, it is proposed to add tracers to cooling water circuits and then individual cooling water loops. The tracers will be exhausted from the machine by the Torus Exhaust Cryopumps and the regeneration stream will be analysed for their presence. The low concentration of tracers expected from typical leaks complicates their detection. This step will be followed by the deployment of an in-vessel remotely operated detection device to traverse the plasma-facing components and pinpoint the location of the leak; the details of this procedure and the associated hardware development remain to be done.
The development of the ITER-specific vacuum technologies and the resulting system designs will be described in the paper and critical issues still to be resolved in the ongoing R&D programme identified.
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