Modelling of particle flows in a Scrape-Off Layer with limiter biasing

The superconducting tokamak Tore Supra will be equipped with an actively cooled toroidal pump limiter (TPL), in the framework of the CIEL (Composants Internes Et Limiteurs) project, dedicated to plasma facing component design for steady state operation. The TPL is equipped with throats, located only on the high field side, for particle collection allowing the control of plasma density which is essential for long plasma discharges. The present design work of the CIEL includes a biasing system in order to enhance the particle pumping. A fluid model, based on the classical fluid equation, is used to estimate the effects of the electric field on the particle flows in the Scrape-Off Layer (SOL). The modifications of the density, the particle flow (toroidal and poloidal) and the position of the stagnation point are discussed as a function of the bias voltage. The model clearly illustrates the different resulting effects on particle pumping for a divertor and a limiter configuration which are designed respectively for poloidal or parallel particle collection. The model is used to interpret the ALT-biasing experiments recently carried out on TEXTOR-94. The pumping capability is shown to be improved by about (15-20)% for positive biasing while the experimental measurements of parallel Mach number are reproduced as a function of the applied voltage. The e-folding length of the edge density in the SOL is also shown to increase from 1.5 to about 2.0 cm for a biased voltage of -400 to 400 V, respectively, in accordance with the model. Finally, the model is used to extrapolate the TEXTOR-94 results to CIEL suggesting that pumping speed enhancement of 25 to 30% may be obtained.