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Description
The Helically Symmetric eXperiment (HSX) is a medium sized stellarator optimized to study confinement properties in a quasi-helically symmetric (QHS) configuration. The QHS configuration of HSX exhibits non-resonant divertor like behavior in its edge region [1]. In addition, HSX is equipped with a set of auxiliary coils leading to a wide range of possible magnetic configurations allowing for the study of island divertors. There are many unanswered questions about the structure and behavior of both island and non-resonant divertors in QHS optimized stellarators. This includes the 3D variation of plasma flows in the edge region, or the width λ_q of the of the power scrape layer [2].
In this work a versatile Langmuir probe is developed, benchmarked, and used to gather first data on HSX. The new probe consists of 2 triple probes at different radial locations, a Mach probe, and a unique implementation of thermocouples for measuring the overall heat flux on the central probe tip. This design allows for instantaneous measurement of spatially dependent ion density, flow velocity, electron temperature, floating potential, and total heat flux, potentially leading to ion temperature measurements not previously possible with Langmuir probes. The new probe was successfully benchmarked in a steady state DC multipole plasma source where both triple probes, and the Mach probe produced ion density and electron temperature values consistent with one another and consistent with previous measurements of the plasma parameters on this apparatus taking into account probe positioning.
First experiments using this probe on HSX demonstrated its robustness against plasma operation and provided first insight into the spatial variation of edge plasma parameters, observed super-thermal electron behavior during the pre-discharge phase, and was able to obtain first measurements of heat flux which will be presented. Moreover, the probe is ready for experiments during the upcoming 2026 experimental campaign of HSX starting in January during which detailed radial scans of the probe, as well as power and density scans of the plasma are foreseen. Results of these experiments will help to answer important questions about both island and non-resonant divertors.
[1] A. Bader et al., Phys. Plasmas 24(3), 032506 (2017)
[2] A. Bader et al., J. Plasma Phys. 91(2), E67 (2025)