Gas recycling studies on the TFR tokamak

Abstract

Gas recycling has been studied in a large number of tokamaks for several years. On TFR, a previous experiment with an inconel limiter has shown that, after many discharges in hydrogen, when we change the working gas to deuterium the wall adsorbed isotope accounts for less than 5% after a few shots. In order to improve our better knowledge of the gas recycling we have performed similar experiments with two different limiters — an inconel limiter and a graphite limiter — at room temperature and with hot walls (250 ° C < T< 300 ° C). With the graphite limiter, when the filling gas is changed from 100% H 2 to 100% D 2, the isotopic ration H (H + D) is between 25 and 30% after five shots and reaches the limit of ~12% after ten shots. This ratio decreases more rapidly with hot walls but the lower limit is higher: 10 and 20% with inconel and graphite limiters respectively, instead of 5% and 12% at room temperature. This recycling coefficient is always higher with hot walls; this consequently should reduce the plasma edge temperature. The sputtering and the thermal load on the wall and the limiters might then also decrease.