Electron thermal diffusivity differences due to magnetic geometry on the HSX stellarator*

Electron thermal diffusivity profiles χ_e have been experimentally determined on the Helically Symmetric eXperiment (HSX) over a scan of magnetic configurations defined by adding or subtracting toroidal field. HSX is a Quasi-Helically Symmetric (QHS) stellarator with a flexible magnetic geometry owing to a set of auxiliary coils surrounding its main field coils. The performed configuration scan suggests that additional toroidal field (decreased well depth and rotational transform) enhances electron heat flux relative to the standard QHS configuration. These diffusivity profiles are obtained by fitting electron temperature T_e and density n_e data from a 10-channel Thomson Scattering system and assuming a steady-state energy balance and diffusive transport model. Due to the sensitivity of χ_e to the fit of n_e and gradient of T_e, Gaussian Process Regression (GPR) has been used to better estimate the uncertainty in the fit. GPR is the first step in upgrading the Thomson system to a Bayesian probability theory based analysis technique, allowing for further confidence in determining sensitive transport coefficients.