Experiments with a limiter of variable radius in the TFR tokamak

The characteristics of TFR plasmas have been investigated for limiter radii from a=12.5 to a=19cm and for two limiter materials, molybdenum and graphite. The central electron temperature is roughly proportional to a. The energy confinement time scales approximately as a2. It is not clear whether this is true geometrical scaling or the consequence of a particular behaviour of the radiation losses. The concentration of light impurities decreases with increasing plasma-to-wall distance. The same is true for heavy elements (molybdenum) layered on the wall when a light element (graphite) is used as the limiter material. (author)