Confinement physics study in a small low aspect ratio helical device: CHS

Variation of the plasma position relative to the centre of the helical coil winding is a very effective means of controlling the MHD stability and the trapped particle confinement in heliotron/torsatron systems, but improving one of these two characteristics with this parameter simultaneously has a detrimental effect on the other. The inward shifted configuration is favour able for drift orbit optimization but is predicted to be unstable according to the Mercier criterion. Various physics problems, such as electric field structure, plasma rotation and MHD phenomena, have been studied in the Com pact Helical System (CHS) with a compromise intermediate position. With thi s standard configuration, CHS has yielded experimental results that contrib ute to the understanding of general toroidal, confinement physics and low a spect ratio helical systems. In the recent experiments; it was found that a wide range of inward shifted configurations give stable plasma discharges without any restriction to the special pressure profile. Such an enhanced r ange of operation made it possible to study experimentally the drift orbit optimized configuration in heliotron/torsatron systems. The effect of confi guration improvement was studied with plasmas in a low collisionality regim e.