The stellarator Wendelstein 7-X: Towards long pulse, high performance operation with the help of multi-institutional partnerships

A major goal of the superconducting stellarator Wendelstein 7-X is the exploitation of the reactor potential of the HELIAS concept. Its modular magnetic field configuration is optimized to provide improved neoclassical and fast ion confinement, enhanced MHD stability, and the feasibility of a divertor concept. During the last maintenance phase, a fully water-cooled graphite divertor and plasma facing components was realized, allowing for long-pulse operation with the goal to increase the energy turnaround to 18GJ over the next operation campaigns. The heating power of the neutral beam injection system was doubled with the commissioning of two additional injectors, providing in total up to 7MW of heating power. A new pellet injector system was installed to allow for steady-state core fueling. Additionally, a lot of new or strongly enhanced diagnostics system were installed. With these enhancements, plasma operation restarted in September 2022 for a period of 6 months. Key objectives for the campaign were the demonstration of long-pulse operation with an energy turnaround exceeding 1GJ and studies of enhanced plasma confinement and its relation to neoclassical and turbulent transport phenomena. Two different long pulse scenarios could be developed: In a pure ECR heated plasma with a power level of 3-4 MW in O2 polarization, a detached divertor scenario with a fraction of radiated power of more than 90% could be operated for a length of 100 s, corresponding to 350 MJ of energy turnaround. Fully detached divertor scenarios could be achieved with and without impurity seeding (typically N or Ne). For an attached divertor, pulses with a length of up to 8 min and energy turnaround of 1.3GJ were achieved. The studies of plasma performance could connect to the previously achieved record confinement scenario, however instead of pellet fueling using NBI to obtain central density peaking. The large number of modified system could only be achieved due to the strong European and international collaborations. These kind of collaborations hold also for the program development in the framework of expert groups and topical task forces, which is made possible by our one-team structure, including an open science approach with clear agreements about ownerships and publication rules.