Study of TEMs and Quasi-Coherent Modes via Synthetic and Experimental Reflectometer Measurements at the HSX Stellarator*

Transport in the Helically Symmetric Experiment (HSX), a quasi-helically symmetric stellarator with reduced neoclassical transport, is dominated by trapped electron mode (TEM) turbulence. A monostatic perpendicular incidence reflectometer is used to measure density fluctuations and study TEMs in HSX [1]. A full-wave synthetic diagnostic using CUWA [2] has been implemented to model the HSX reflectometer to determine the capabilities of the diagnostic and directly compare experiment and simulation data. Findings from the synthetic diagnostic are presented here. The results show that the reflectometer is sensitive to poloidal wavenumbers under 1.5 cm^-1 and radial wavenumbers under 4 cm^-1, with very little dependence on local curvature, density gradient, or probing frequency. The reflectometer outputs scale linearly with density fluctuation level in the physically relevant range, and this scaling is characterized to relate reflectometer signal to electron density fluctuation levels. Synthetic diagnostic measurements of GENE-simulated density fluctuations are compared with experiment, with both showing strong coherent mode activity and supporting the link between observed quasi-coherent modes and the large-scale fluctuations seen in gyrokinetic simulations of HSX.

[1] X. Han et al., Plasma Phys. Control. Fusion 67, 045011 (2025)

[2] P. Aleynikov et al., Comput. Phys. Commun. 241, 40-47 (2019)