Study of ion-to-electron temperature ratio at the edge of Wendelstein 7-X using a fast swept ball-pen probe

In the Wendelstein 7-X (W7-X) stellarator, a ball-pen probe (BPP) has been routinely employed to measure T$_i$ in the island scrape-off layer (SOL) across large parts of the recent 2024/2025 campaigns. The high temporal resolution of 25 μs allows us to resolve Ti fluctuations, their probability density functions, and modulation via low-frequency magnetohydrodynamics (MHD) modes. We present a unique comparison of the ion temperature measured using BPP with a more conventional retarding field analyzer (RFA). A good agreement between the two diagnostics is observed, when fast T$_i$ measurements are reduced to the same temporal resolution (5 ms) using ‘RFA-like’ averaging. Using an RFA-like interpretation of the fast BPP data, we find a linear decrease in SOL T$_i$ and T$_e$ with line-integrated density. The SOL ion-to-electron temperature ratio τ$_{i,e}$ ranges between τ$_{i,e}$= 1 and 3 with smaller τ$_{i,e}$ values observed for higher densities (collisionalities) and at positions close to the LCFS. The upstream BPP T$_1$measurements are compared to the average C$^{2+}$ T$_1$ obtained by coherence imaging spectroscopy (CIS), in the divertor region, magnetically mapped to the BPP. ... mehrThe downstream T$_1$ measured by
CIS is about half the upstream T$_1$ and exhibits a similar inverse scaling with line-integrated density. Finally, we benchmark the experimentally obtained SOL T$_1$ and T$_e$ against the EMC3–Eirene modeling and find good agreement for high density plasmas, but significantly diverging results for low density plasmas.