A First Study of Impurity Behavior During Externally Induced Radial Electric Fields in the TJ-II Stellarator

A fine-grained mobile pyrolytic graphite limiter was biased to generate radial electric fields in the plasma edge region of the TJ-II stellarator to improve confinement operation modes. Indeed, for the range of voltages applied (up to ±300 V), spectroscopic data indicate that limiter biasing does not induce significant external influxes of impurities. Also, after boronization of the vacuum chamber, increases of ~100% in electron density, together with reductions of the order of 40% in Z_eff, are observed during limiter biasing. Here, we report on the first study of impurity behavior in the TJ-II during externally induced radial electric fields. For this, different spectroscopic methods were employed, and the results obtained were compared to assess impurity behavior and to evaluate the effectiveness of such biasing on plasma confinement in TJ-II.