Plasma discharge fuelling by reactive gases

Abstract

In the framework of developing on-line in situ wall conditioning methods for long pulse tokamaks, the injection of reactive gases into the scrape-off layer during a discharge was investigated. Transport in the plasma scrape-off layer is used to distribute the gas atoms around the torus and condition the walls by deposition of appropriate material. Based on the experience from boronization, we use trimethylboron (TMB) as a first test gas. It is shown that significant amounts of TMB can be puffed during the flat top phase of a discharge without affecting its further evolution negatively. It is further demonstrated that the plasma can be fuelled from the very beginning, including the gas prefill, and maintained exclusively with TMB. Feed back density control is easily achieved. TMB fuelled (puffed) and D2 fuelled discharges show very similar values of Zeff, radiated power fraction, and other global parameters. The effective confinement of B and C atoms from TMB is evaluated using spectroscopic signals.