Correlation between radiated power and target heat flux in the island divertor of W7-X

Radiative losses from the plasma represent a key component of global power balance in a magnetic confinement fusion device. Measurements of the plasma radiation and its spatial distribution are essential for physical studies and to ensure safe machine operation. Here, an imaging bolometer diagnostic is employed to characterize the plasma radiated power in the island divertor of the large Wendelstein 7-X (W7-X) stellarator device. In attached plasmas, the divertor radiation pattern exhibits magnetic field-aligned stripes and a toroidally non-uniform character highly correlated with the target power load deposition pattern of the given magnetic field configuration. Toroidal uniformity of the plasma emissivity is partly recovered in detached discharges. Similar results are found in EMC3-EIRENE simulations of carbon impurity radiation, which indicate that the steep toroidal emissivity gradients observed in the experiment (factor ×2 change over 10 cm parallel distance) are supported by toroidally localized particle sources.