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 [1–3, 6, 12] of overlayed discharges. The type and areal density of impurities retained were determined using ion backscattering. The principal impurities were found to be O, Fe, Cr, Ti, and Cu which had fluxes of 14, 2.8, 0.8, 0.6 and 0.2 × 10¹⁵/cm² (±25%), respectively, near the radius of the wall. At the same position the deuterium flux was 2.2 ± 0.5 × 10¹⁷ D/cm² s. 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 ~2 compared to an increase in the D flux by a factor of ~5. The observed nonlinear impurity retention versus the number of discharges is consistent with the values determined for the impurity and D fluxes and the expected effects of sputtering. The observed large flux of iron at the radius of the ring limiter suggests that this may be a primary source of metal impurities in PLT.