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A Model for Diffusion in the Tokamak Edge and Application to the Modular Pump Limiters on Tore Supra

Robert T. McGrath, C. Christopher Klepper, Taner Uckan, Peter K. Mioduszewski

Fusion Science and Technology

Volume 14 / Number 2P1 / September 1988 / Pages 339-353


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The relative positioning of the limiter modules on Tore Supra is investigated with the objective of optimizing the overall performance of the system and the operational flexibility of the experiment. Limiter system performance is optimized by simultaneously maximizing the power handling and particle exhaust capabilities. This must be accomplished for the entire range of edge q values anticipated on Tore Supra. In addition, it is desirable to independently maximize power handling, to allow operation at very high levels of plasma auxiliary heating, or particle exhaust, to allow operation at high pellet injection fueling rates. The relative merit of one configuration with respect to another is determined using a diffusion model for charged-particle radial transport coupled with a detailed three-dimensional mapping of the magnetic field structure in the edge plasma region. To implement the model, assumptions must be made about the edge plasma conditions including the rate of charged-particle diffusion. These assumptions affect the absolute values of the power handling and particle exhaust capabilities of the system but do not affect the merit of one configuration relative to another. Working within the constraints imposed by the availability of ports on Tore Supra, the best limiter configuration for a system of seven modular limiters is identified. The performance to be expected for this optimized configuration for various modes of Tore Supra operation is reported. Very long flux tubes must be avoided if the limiter system is to operate near its full design capacity of 8.0 MW. For the assumed edge conditions, Böhm diffusion with a plasma temperature of 150 eV at the last closed flux surf ace, the configuration identified can exhaust between 17 and 21 Torr · ℓ/s while removing 5 to 8 MW of power incident on the limiter surfaces. Operational modes that pump as much as 26 Torr · ℓ/s are also possible if incident power levels are reduced to 4.0 MW. Operation with large amounts of auxiliary heating, in excess of 15 MW, is also possible by power sharing with the actively cooled inner bumper limiter. In this situation, particle pumping rates may be as low as 9 Torr · ℓ/s.

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