Rotating Langmuir probe for measurement of edge radial profiles on CASTOR tokamak

Langmuir probes are used for measuring local edge tokamak plasma parameters; in particular rake probes and reciprocating probes are used to determine plasma radial edge profiles, although they are not fully compatible with small tokamak experiments. An alternative concept is proposed here, which involves the use of a Langmuir probe that rotates in the poloidal plane, enabling temporal resolution in the milliseconds range. This probe has been tested in the CASTOR tokamak device. When the electron temperature is measured independently the ion saturation current and floating potential profile data from the probe roughly yield radial profiles of the electron density and the space potential. It turns out that the location of the plasma velocity shear layer in the edge can be quickly identified. A radial shift of the velocity shear layer has been observed during lower-hybrid-current-drive discharges. Edge fluctuation levels were found to be lower in lower-hybrid-current-drive discharges than in ohmic-heating discharges. 8 refs, 6 figs