Gabriele Partesotti

• The present Ph.D. thesis addresses the issue of radiation asymmetries for power exhaust in the plasma of Wendelstein 7-X (W7-X), currently the largest and most advanced stellarator device. First, 3D transport simulations using the EMC3-EIRENE code are used to set expectations on the features of plasma radiated power at W7-X. The computed plasma emissivity is found to be toroidally asymmetric, with peaking often localized near the target interaction region in connection to particle sources. To investigate the radiation distribution in experimental plasma discharges, the newly installed imaging bolometer diagnostic is integrated into the existing W7-X bolometer setup and made operational. The observed divertor radiated power pattern is consistent with the modeling results, providing a first experimental confirmation of toroidal radiated power asymmetries at W7-X. Then, a method involving 2D tomography and toroidal mapping is developed to combine multiple bolometer diagnostics sampling the plasma from several vantage points and characterize the toroidal emissivity gradients. The findings indicate considerable deviations from the assumption of constant parallel emissivity, with significant consequences for power exhaust and global power balance assessments. Finally, two solutions are implemented to reduce the uncertainty in the total radiated power estimate. First, new Compact Bolometer Camera (CBC) diagnostics are designed and installed to efficiently identify poloidal and toroidal radiation asymmetries. Second, an alternative proxy for the total radiated power is optimized to include the measured asymmetry information. The combination of increased diagnostic coverage and optimized proxy markedly improves the accuracy of the radiated power estimate, reducing its associated error below reactor requirements.