Impurity Transport Experiments at the HSX Stellarator

The laser blow-off technique is used to inject aluminum atoms into the confined region of HSX. To study the radial propagation of the injected impurities, photodiode arrays take time-resolved measurements of the impurity radiation. The spatially one-dimensional impurity transport code STRAHL is used to calculate a time-dependent plasma emissivity profile. A synthetic diagnostic code then integrates the modeled emissivity profile along the experimental lines-of-sight, providing simulated intensity signals. An optimization algorithm with the impurity diffusivity (D) acting as free parameters fits the model to experimental data. Results show D values two orders of magnitude more than neoclassically-predicted calculations from the PENTA code. For plasmas with an absorbed ECH power (P) of ~11.3 kW, the average impurity confinement time (τ) is ~1.3 ms while an average τ of 3.0 ms is observed when reducing P to 6.3 kW. A systematic study of the power shows a τ ∝ P^-1 dependence, similar to the ISSO4 scaling. Thus, neoclassical diffusion alone is insufficient to explain these results and suggest a substantial impact of turbulence on the impurity confinement.