Thermal and structural analysis of W-7X first wall graphite tiles under direct NBI loads

Thermo-mechanical analysis of the Wendelstein 7-X neutral beam dump is presented focusing on failure phenomena resulting from strong impulsive thermal loading of the graphite tiles. One of the plasma heating systems of the Wendelstein 7-X stellarator is Neutral Beam Injection (NBI). There are two NBI boxes with currently a total injected hydrogen beam power of 7 MW and a pulse length up to 5 s. Each NBI box can be operated with four positive ion neutral injectors (PINI). A total of three PINIs were used simultaneously in the last operation campaign in 2022. NBI is highly important for the W7-X physics program, thus increasing NBI power is planned in the next operation phase. A portion of the NBI power passes through the plasma, creating a region of the first wall (FW) with a significantly increased heat load. This region, called the beam dump, could be a limiting factor for the NBI pulse length and thus it requires a precise remodeling of all constituents before the 2024/2025 operation phases to prevent repeated structural damage to this area. Since the graphite tiles are attached by multiple bolts to the CuCrZr heat sinks, the thermal expansion of graphite can induce significant stress in the graphite. The safety limiting factor is the induced thermal stress in the graphite tile due to the NBI beam power load. ANSYS Mechanical model was developed, including the bolted connections, and analyzed to examine the effect of the heat load on the FW components and to evaluate the maximal allowed duration of the NBI beam. A coupled transient simulation of the thermal and structural analysis is performed. The calculation used the NBI heat loads on the FW surfaces estimated by the BEAMS3D code. This paper presents the results of the numerical analysis and the related operating limits.