Dependence of Impurity Transport on Absorbed Power in the HSX Stellarator

Results from the HSX laser blow-off experiment show impurity confinement times from a 50 kW plasma to be 1.5 ms with on-axis heating (core electron temperature T_e~1000 eV) and 2.1 ms with off-axis heating (core T_e~500 eV). A laser blow-off impurity injection system is used to rapidly deposit a small, controlled quantity of aluminum into the confinement volume. Global confinement time (τ) has been calculated from the exponential decay of the line-integrated impurity radiation measured using AXUV photodiodes. Experiments are done in hydrogen plasmas of line-average density 3e+12 cm^-3 with core electron temperatures in the range of 500-1500 eV. The heating location has been varied from on-axis to off-axis to change the absorbed ECH power (P). Analysis of the confinement time dependence on the absorbed power, τ~P^x, suggests that x is on the order of -1. The spatially one-dimensional impurity transport code STRAHL is used to obtain experimental impurity transport coefficients, diffusivity and convective velocity.