After the successful short pulse operation phase 1 (OP1) of the stellarator Wendelstein 7-X with maximal plasma energies of 200 MJ, the upcoming long pulse OP2 aims at stepwise higher energies up to 18 GJ. With the knowledge of partly unexpected experimental heat load distributions on plasma facing components (PFC), their allowable loads were re-evaluated which consequently lead to some adaptation work.
The divertor target modules TM5 h and TM6 h were loaded more than expected during OP1. This triggered a detailed transient and stationary analysis which revealed a high sensitivity of the thermal response to the heating power distribution.
At the baffles, cracks were detected just before assembly for OP1 at the braze joints between CuCrZr heat sinks which carry the graphite tiles, and the stainless steel (SS) cooling pipes. Due to large temperature differences between parts of the pipes and support structure the braze becomes heavily stressed during the high thermal loads in OP2. Multiple optimization analyses led to the decision to release thermomechanical stresses by loosening the connections between the heat sinks and steel structures of the modules.
New stainless steel wall protection panels to be positioned behind the divertor pumping gaps for stationary OP2-loads of 100 kW/m² are being manufactured. Calculations were performed to optimize the intricate cooling channels and to lead as well as confirm the mechanical design for all working cases, the CFD calculation, static strength, ratcheting performance and fatigue assessment are performed in sequence.
The findings of these analyses lead also to adaptation of the plasma operating instructions for OP2.