Assessment of potential heat flux overload of target and first wall components in Wendelstein 7-X finite-beta magnetic configurations and choice of locations for temperature monitoring

Within the framework of R&D activities on the Wendelstein 7-X (W7-X) stellarator machine, the assessment of heat loads onto the plasma facing components (PFCs) is an important aspect. So far, W7-X was operated in short pulses without water cooling of the PFCs. Presently, the device is being prepared for future operation phases with water cooling. The target plates, which receive the highest heat loads, are monitored by a thermography system. The rest of the first wall (heat shield) is not designed to receive convective particle loads, and it is in part poorly monitored by the infrared cameras. Recent studies have shown that for many plasma configurations there are locations on the heat shield which do receive significant convective heat flux. This is in particular true for the heat shields adjacent to the target plates (called baffles) and for configurations with finite plasma pressure, where the magnetic configuration is modified by plasma currents. On the other side, the heat flux limit to the baffle tiles had to be reduced from 0.50 to 0.25 MW/m(2). In this work an evaluation of the heat flux to targets and baffles in plasma configurations with finite plasma pressure is presented. To this end, Field Line Diffusion (FLD) calculations have been performed to obtain the heat load pattern distributions for the considered magnetic configurations. The results have been assessed statistically to achieve a measure of certainty in the prediction of an overload in a certain location. The possibility of overloads onto the baffles due to plasma radiation has also been investigated. The results of the entire analysis show that local temperature monitoring by thermocouples in a rather limited number of locations will be sufficient to avoid heat flux overloads of the baffles and heat shields in all magnetic configurations considered.