Summary of the edge physics results from the first operation phase of the Wendelstein 7-X stellarator

Wendelstein 7-X (W7-X) is the largest and most optimised superconducting stellarator world-wide, which aims at demonstrating high heating power steady state plasma operation. In its first operation phase (OP1.1), most of the in-vessel graphite wall armor had not yet been installed and instead of the 10 discrete island divertor modules only 5 inboard limiters were used. Plasma purity and wall outgassing were continuously improved by ECRH and GDC wall conditioning. Low ECR heated plasmas of up to 6 s (0.6 MW, 1 gyrotron) were created, as were shorter-lived higher power discharges (4.3 MW, 6 gyrotrons). Te > 8 keV, Ti > 1.5 keV and ne = 3 10¹⁹ m^-3 were achieved simultaneously. The power loads to the limiters reached up to 5 MW/m² in steady state (< 60% ECRH input power), with toroidal asymmetries up to a factor 2. The SOL widths were on the order of 1-2 cm. Langmuir probe arrays integrated into one limiter showed significant top/bottom asymmetries in Te and ne, as did IR camera measurements in the power load distribution across the limiters, which suggest ExB drifts at the plasma edge. IR camera observations revealed indications for enhanced transport due to frequent bursts with poloidal mode numbers of about 15 seen on the limiter surface, while fast video camera images show filamentary structures elongated along the magnetic field lines, which rotated poloidally, consistent with the ExB drift velocity.