Observation of impurity accumulation and concurrent impurity influx in PBX

Impurity studies in L- and H-mode discharges in PBX have shown that both types of discharges can evolve into either an impurity accumulative or nonaccumulative case. In a typical accumulative discharge, Z_eff peaks in the center to values of about 5. The central metallic densities can be high, n_met/n_e ≈ 0.01, resulting in central radiated power densities in excess of 1 W/cm³, consistent with bolometric estimates. The radial profiles of metals obtained independently from the line radiation in the soft X-ray and the VUV regions are very peaked. Concurrent with the peaking, an increase in the impurity influx coming from the edge of the plasma is observed. At the beginning of the accumulation phase the inward particle flux for titanium has values of 6 × 10¹⁰ and 10 × 10¹⁰ particles/cm² s at minor radii of 6 and 17 cm. At the end of the accumulation phase, this particle flux is strongly increased to values of 3 × 10¹² and 1 × 10¹² particles/cm² s. This increased flux is mainly due to influx from the edge of the plasma and to a lesser extent due to increased convective transport. Using the measured particle flux, an estimate of the diffusion coefficient D and the convective velocity ν is obtained.