The effect of limiter conditioning on the Tokamak Fusion Test Reactor edge plasma

Measurements by moveable Langmuir probes and edge spectroscopy diagnostics have documented the conditioning effect of low density helium-initiated discharge sequences on the Tokamak Fusion Test Reactor (TFTR) edge plasma. Langmuir probe measurements show in general that the edge electron density n{sub e} decreases by less than a factor of 2 while the edge electron temperature T{sub e} doubles. Radial profiles to the plasma boundary show that the density scrape-off length increases somewhat while the temperature scrape-off length decreases substantially. The particle flux density is unaffected. The spectral emission of C 2 decreases by a factor of 2, a much smaller change than that exhibited by the D{sub {alpha}} signal. These results complement previous accounts of the conditioning technique. Comparisons of these He conditioning measurements are made to edge measurements during a deuterium density scan experiment, showing many similarities, and to an existing edge model of the conditioning process, showing qualitative agreement. 20 refs., 5 figs.