Probe measurements of impurities in the plasma boundary of PLT

Carbon probes were exposed to the plasma edge in PLT at various distances from the plasma and to different numbers (1, 2, 3, 6, 12) of overlayed discharges. The type and areal density of impurities retained were determined using ion backscattering. The principal impurities with Z > 6 were found to be O, Fe, Cr, Ti, and Cu in decreasing order of abundance. Fluxes for these impurities at a position close to the wall were found to be respectively 14, 2.8, 0.8, 0.6 and 0.2 x 10/sup 15//cm/sup 2/ (+- 25%) respectively, compared to the value 2.2 +- .5 x 10/sup 17/ D/cm/sup 2/s for the deuterium flux at this position. Lateral spreading of the impurity deposition profile behind the probe slit allowed a lower limit to be placed on the impurity mean ion energy. Between the wall and radius of the ring limiter the impurity fluxes increased by a factor of approx. 2 compared to an increase in the D flux by a factor of approx. 5. Since impurity and deuterium fluxes were both measured the effect of sputtering by the incident deuterium on the impurity accumulation can be assessed