Particle pumping in TORE SUPRA

Abstract

The TORE SUPRA tokamak and its associated vacuum vessels are provided with routine clean high-vacuum conditions by turbomolecular pumping. During plasma discharges large quantities of very hot gases arriving at the plasma edge from the plasma scrape-off layer have to be controlled before striking the chamber wall, thereby provoking an increase in the impurity content and increasing the plasma density up to the disruptive level. In order to deal with these plasma-wall interactions, a Magnetic Ergodic Divertor (MED), consisting of six discrete coil structures, as well as one Horizontal Pump Limiter (HPL), and six Vertical Pumped Limiters (VPL), are installed in the vacuum chamber in order to cope with plasma-wall interactions and neutral gas recycling. Each structure has a thermal shield at the front surface (facing the plasma). These are made of polycrystalline and pyrolytic graphite bolted on stainless steel supports for MED and HPL, respectively. For the VPLs the shields are made of CFC Aerolor 05 brazed on hardened copper. The total heat removal capacity of these plasma-facing components is 12 MW. The design of particle collection openings and duct conductances allows a 10% exhaust efficiency. This means that, for TORE SUPRA, a flux of 3 × 10 21 particles s −1 has to be absorbed by the water-cooled titanium getter pumps, at the rear of these elements. All these facilities were installed for plasma operation at the beginning of the present year (1989) for a short time. Preliminary observations support theoretical predictions that the mechanisms involved in the scrape-off layer may provoke effects in the bulk plasma. The first results show that particle collection and heat removal is effective by MED, VPL and HPL, and that the plasma behaviour is not adversely affected by their presence. On the contrary, a tendency to improve the plasma performance is observed.