Speaker
Description
The plasma exhaust concept of the Wendelstein 7-X (W7-X) stellarator is based on the island divertor configuration, which exploits the interaction of magnetic islands with ten discrete water-cooled targets. This translates into a scrape-off layer (SOL) with long connection lengths and small field line pitch angles, affecting the balance of the different transport channels, favouring the perpendicular (radial, binormal) over the parallel.
In the most recent campaigns, the operational space of W7-X was extended including input powers above $7$ MW (i.e. $P_{SOL} > 6.5$ MW). Under these conditions, line-averaged densities $\langle n_e \rangle$ of up to $11.3 \cdot 10^{19}$ m$^{-3}$ were sustained while maintaining the plasma attached (i.e. $f_{rad}=P_{rad}/P_{in} < 0.45$). This increased range of densities allowed the observation of features that had not been detected in lower-power/density attached experiments. Bolometer measurements show variations in the toroidal radiation distribution that strongly reduce with increasing $\langle n_e \rangle$ rather than with $f_{rad}$, as implied by low-power observations. At constant $\langle n_e \rangle$, radiation asymmetries are stronger with increasing power. Moreover, higher divertor densities were observed at higher power, indicating a more beneficial, nearly-quadratic, scaling with respect to the last-closed-flux-surface density compared to what previously measured. These increased divertor densities were accompanied by higher neutral pressures and stronger parallel flow velocities. Additionally, the SOL density profiles showed steeper radial gradients inside the magnetic islands, correlated with increased density/temperature fluctuations, therefore possibly driving higher perpendicular (mostly radial) transport. This change in transport resulted in heat loads being deposited on plasma-facing components not designed to withstand them. This prevented safe attached operation ($f_{rad}< 0.45$) within the mid-density range ($6$ to $9 \cdot 10^{19}$ m$^{-3}$) in some of the most common W7-X magnetic configurations for heating powers above $4 – 5$ MW.
Due to the challenges of operating under the described attached conditions, significant effort was invested in developing a feedback-control system on the radiated power. Its successful implementation enabled the first attempts at combined scenarios with both high core performance ($T_i > 1.5$ keV) and acceptable divertor heat loads. However, the improved performance of these experiments is associated with suppressed core turbulence, which reduces the typical diffusive impurity flushing. This results in a strong accumulation of both intrinsic and seeded impurities in the core, ultimately limiting the plasma stored energy via radiative cooling. This highlights the need to further optimise impurity seeding and divertor density in order to minimise the impact on core performance.