Electron Temperature and Density Profiles, and SOL Parameter Scaling in W7-X Measured by Helium Beam Diagnostics

Heat and particle transport in the scrape-off layer (SOL) of Wendelstein 7-X (W7-X) plays a

critical role in determining divertor performance. Accurate measurements of electron

temperature (Te) and density (ne) are essential for understanding SOL transport and optimizing

exhaust conditions. This work presents results from helium beam diagnostics, which consist of a

gas injection system coupled with multiple spectrometers of varying spectral resolution. These

diagnostics provide simultaneous Te and ne measurements in both upper and lower divertor

regions (downstream) and, for the first time, in the midplane region (upstream).Electron

temperature and density are inferred using helium line ratio spectroscopy, based on a

collisional-radiative model of atomic helium. The helium beam diagnostic at W7-X has been

thoroughly validated and is routinely used to map plasma parameters, particularly within the

island divertor [1][2]. The Te and ne profiles reported in this study were obtained across a range

of plasma conditions, including detached and impurity-seeded discharges. We present

comparative analysis of upstream and downstream Te and ne profiles, along with an

investigation of scaling relationships across a range of conditions, such as heating power,

connection length, and field line pitch, among others.

[1] T. Barbui et al., Nucl. Fusion 60, 106014 (2020).

[2] Flom, E., et al. arXiv preprint arXiv:2312.01240 (2023).