Experimental study of reduced TEM growth rate configurations on the HSX stellarator*

Transport in the Helically Symmetric Experiment (HSX) is dominated by trapped-electron-mode (TEM) turbulence [1]. In an effort to identify HSX configurations with reduced TEM-driven transport, a database of HSX coil-current configurations was explored using the gyrokinetic code GENE. This investigation identified flux-surface elongation and quasi-symmetry as beneficial linear TEM stability metrics [2]. Conversely, preliminary nonlinear gyrokinetic simulations predict comparable or higher heat fluxes in elongated configurations compared to the standard HSX configuration (QHS). One elongated configuration is the so called Well configuration, which was implemented during the most recent HSX experimental campaign. Thomson scattering and stored energy measurements indicate the Well configuration has poorer overall confinement than the QHS configuration, although the reason for this reduction remains an open question. A detailed comparison of energy confinement times, heat fluxes, and thermal diffusivities in the Well and QHS configurations will be presented. In addition, a large amplitude global quasi-coherent mode at 40-70kHz was observed using reflectometry in Well experiments with sufficiently high plasma density. This mode will be further characterized, and candidates for its origin will be considered.

[1] B.J. Faber et al., Phys. Plasmas 22, 072305 (2015)

[2] M.J. Gerard et al., Phys. Plasmas 31, 052501 (2024)