Modelling of particle flows in a scrape-off layer with limiter biasing

The superconducting tokamak Tore Supra will be equipped with an actively co oled 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 pl asma density which is essential for long plasma discharges. The present des ign work of the CIEL includes a biasing system in order to enhance the part icle pumping. A fluid model, based on the classical fluid equation, is used to estimate the effects of the electric field on the particle hows 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 discu ssed as a function of the bias voltage. The model clearly illustrates the d ifferent resulting effects on particle pumping for a divertor and a limiter configuration which are designed respectively for poloidal or parallel par ticle 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 parall el Mach number are reproduced as a function of the applied voltage. The e-f olding 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, i n accordance with the model. Finally, the model is used to extrapolate the TEXTOR-94 results to CIEL suggesting that pumping speed enhancement of 25 t o 30% may be obtained.