Helium Transport and Exhaust in Tokamaks

The fourth in a series of workshops on issues of helium transport and exhaust in tokamaks, held under the auspices of the International Energy Agency, and sponsored by Oak Ridge National Laboratory and by Forschungszentrum KFA/Juelich, was held from 18 to 21 October 1999 in Jekyll Island, Georgia. Representatives from major fusion research laboratories attended, and 28 talks were given. The talks dealt with the transport and exhaust of helium, as well as to closely related issues involving similar recycling impurities (e.g., neon, argon) which are currently being investigated for use in producing strongly radiating operating modes. The status of experiments on these topics was described for JET, JT-60U, DIII-D, ASDEX Upgrade, Alcator C-Mod, Tore Supra, TEXTOR-94 and NSTX. Reports were also presented on results from the first operation of the Large Helical Device, and on recent analyses of the final campaign of TFTR. The workshop, as is by now traditional, also devoted attention to the interaction between modeling and experiments: the status of the b2-Eirene multi-process code was reviewed and results were presented comparing modeling with experiments using the b2-Eirene, The fundamental importance of helium transport and exhaust to the future production of burning plasmas has been underscored in the years since the Charleston workshop by the extensive series of experiments with D-T plasmas conducted by JET. While the level of fusion power production in these experiments was not yet sufficient to yield direct evidence of fusion-produced helium in the JET exhaust stream, the level inferred from core data was tantalizingly close to the level of detectability, and provides a strong impetus for further work along these lines. Still lacking this best (direct) data on helium transport and exhaust properties, the workshop tackled the major physics issues governing the overall helium (and recycling impurity) balance. Results presented at the previous (Charleston) workshop demonstrated that helium could be removed from ELMy H-mode plasmas with adequate efficiency. In part as a result, the ELMy H-mode now represents the conventional scenario for a future burning plasma. However, such conventional burning plasma scenarios are also subject to the deleterious properties of ELMy H-mode, most notably the intolerable heat flux levels associated with 'giant', or 'Type I' ELMs. The proposed amelioration of these heat fluxes, through the use of higher density or by using vertical target designs, has raised questions about whether there would be too low a level of helium concentration in the …