EXPERIMENTS ON STEADY-STATE PARTICLE CONTROL IN TORE-SUPRA AND DIII-D

Particle control is playing an increasingly important role in tokamak plasma performance. The present paper discusses particle control of hy dogen/deuterium by wall pumping on graphite or carbonized surfaces, as well as by external exhaust with pumped limiters and pumped diverters . Wall pumping is ultimately a transient effect and by itself not suit able for steady state particle exhaust. Therefore, external exhaust te chniques with pumped diverters and limiters are being developed. How w all pumping phenomena interact and correlate with these inherently ste ady state, external exhaust techniques, is not well known to date. In the present paper, the processes involved in wall pumping and in exter nal pumping are investigated in an attempt to evaluate the effect of e xternal exhaust on wail pumping. Some of the key elements of this anal ysis are: (1) charge-exchange fluxes to the wall play a crucial role i n the core-wall particle dynamics, (2) the recycling fluxes of thermal molecules have a high probability of ionization in the scrape-off lay er, (3) thermal particles originating from the wall, which are ionized within the scrape-off layer, can be directly exhausted, thus providin g a direct path between wall and exhaust which can be used to control the wall inventory. This way, the wall can be kept in a continuous pum ping state in the sense that it continuously absorbs energetic particl es and releases thermal molecules which are then removed by the extern al exhaust mechanism. While most of the ingredients of this analysis h ave been observed individually before, the present evaluation is an at tempt to correlate effects of wall recycling and external exhaust.