To investigate the effects of a light-element limiter on plasma parameters, aluminium rail limiters interchangeable with Mo rails were installed top, bottom, and outside directions in the ST tokamak. The inside limiter remained a fixed Mo rail. Compared with discharges produced immediately before and after with the usual Mo limiters, the "aluminium" discharges showed an increase of Te (by factors of 1.4 – 2 near the centre) and of energy confinement (by factors of 2 to 3 in electron energy/power input, depending on time of observation). H2 and He discharges showed practically identical effects. In plasma composition, the Mo concentration dropped significantly, but the Fe concentration decreased only slighly if at all; the A1 concentration was about 3 – 5% (i.e. high compared to the heavier metals), whereas oxygen, about 4 to 8% to start with, dropped to insignificance, probably as a result of A1 evaporation. The Zeff from resistivity increased 20 – 30% although the resistance dropped because of the higher Te. The improved Te and energy confinement are thought to be the result of cumulative effects of more favourable radial current and power input distributions rather than direct energy losses by radiation.
Energy balance in a low- high-density helium plasma in the ST Tokamak