Plasma-wall impurity experiments in ISX-A

The ISX-A was a tokamak designed for studying plasma-wall interactions and plasma impurities. It fulfilled this role quite well, producing reliable and reproducible plasmas which had currents up to 175 kA and energy containment times up to 30 msec. With discharge precleaning, Z/sub eff/ was as low as 1.6; with titanium evaporation, Z/sub eff/ approached 1.0. Values of Z/sub eff/ greater than or equal to 2.0 were found to be proportional to residual impurity gases in the vacuum system immediately following a discharge. However, there was no clear dependence of Z/sub eff/ on base pressure. Stainless steel limiters were used in most of the ISX-A experiments. When carbon limiters were introduced into the vacuum system, Z/sub eff/ increased to 5.6. After twelve days of cleanup with tokamak discharges, during which time Z/sub eff/ steadily decreased, the carbon limiters tended to give slightly higher values of Z/sub eff/ than stainless steel limiters. Injection of less than 10/sup 16/ atoms of tungsten into discharges caused the power incident on the wall to double and the electron temperature profile to become hollow

Materials experience with tokamak plasmas

The primary effects of using various wall and limiter materials have been in the amount and kind of impurities they introduce into the plasma. The materials employed to date include gold, stainless steel, inconel, glass, alumina, and titanium for first walls and carbon, stainless steel, inconel, alumina, silicon carbide, boron carbide, molybdenum, tantalum, titanium, and tungsten for limiters and divertors. Limiter surfaces bear the brunt of the plasma bombardment and so typically introduce impurities far out of proportion to their relative size. The ratio of the bombarded limiter area to that of the first wall is of order 1 to 10/sup 3/

Overview of results from the ATF torsatron

This viewgraph paper covers the following topics: plasma improvements due to improved impurity control; confinement scaling; edge fluctuations; and bootstrap currents

Materials experience with tokamak plasmas

The primary effects of using various wall and limiter materials have been in the amount and kind of impurities they introduce into the plasma. Only a limited number of materials have been employed to date. These include gold, stainless steel, inconel, glass, alumina, and titanium for first walls and carbon, stainless steel, inconel, alumina, silicon carbide, boron carbide, molybdenum, tantalum, titanium, and tungsten for limiters and divertors. Limiter surfaces bear the brunt of the plasma bombardment and so typically introduce impurities far out of proportion to their relative size. The ratio of the bombarded limiter area to that of the first wall is of order 1 to 10/sup 3/

ISX: a tokamak for surface and impurities studies

The ISX (Impurity Study Experiment) is a moderate size tokamak slightly larger than the ORMAK tokamak. ISX is being built explicitly for the study of impurities and plasma-wall interactions. It is scheduled to begin experiments in the spring of 1977. Several features have been deliberately designed into the ISX which make it particularly adaptable to surface studies. The first is a welded stainless steel vacuum system, bakeable to 400$sup 0$C, with a projected base pressure greater than or equal to 2 x 10$sup -9$ torr. Another feature is that of ''easy'' demountability of the vacuum system. Replacement of the entire vacuum system should take about two weeks. A third feature is diagnostic access to the edges of the plasma. The initial surface physics question to be answered is how best to keep surfaces clean: by baking, by direct or indirect wall bombardment discharges, or by gettering. Later experiments will involve using wall materials other than stainless steel to determine their effects on the plasma. (auth