Experimental study of the effect of gas puff modifications on the density profile in the HSX stellarator*

A detailed understanding of the formation and shape of the density profile in the Helically Symmetric eXperiment (HSX) is essential since plasma turbulence has been observed to depend strongly on the normalized density gradient length [1]. Here, we present a comparison of prior data from gas-puff modulation experiments [2] with forward modeling results based on the pySTRAHL [3] and pyFIDASIM [4] codes to model density profile evolution and fueling, respectively. Additionally, new experiments will be conducted where the fueling location will be changed from the tips of the elongated flux-surface cross-sections to the mid-plane, bringing the fueling source closer to, or further away from the magnetic axis. In preparation for these experiments, significant modifications have been made to the HSX vacuum vessel. These changes include physical cleaning, DC glow discharges, and bake-out. Through these efforts, we expect reduced outgassing from the walls so that gas-puffing primarily fuels and modulates the plasma density. For the new experiments, pyFIDASIM predictions suggest that changes to the fueling location will alter the plasma density profile. Comparisons between forward-modeled data and experimental results via multi-channel microwave interferometry are presented and analyzed.

[1] J. C. Smoniewski, Ph.D. thesis, University of Wisconsin - Madison, 2021.

[2] C. Deng, poster in Proceedings of American Physical Society Division of Plasma Physics, Fort Lauderdale, Florida (2019) 

[3] C. Swee et al., Plasma Phys. Control. Fusion 64 015008 (2022).

[4] B. Geiger et al., Plasma Phys. Control. Fusion 62 105008 (2020).