First X3-operation and density feedback experiments with ECRH at the stellarator Wendelstein 7-X

The 10 MW 140 GHz electron cyclotron resonance heating (ECRH) system at the superconducting stellarator Wendelstein 7-X is foreseen to maintain reactor-relevant plasmas for 30 minutes at 2.5 T. However, a gyrotron can fail due to a variety of errors, and restarting it requires a few seconds. For this reason, a FPGA-based feedback control loop was implemented that automatically increases the power of the remaining gyrotrons when the total power is reduced until another gyrotron is activated. The feedback loop can also be applied to the measured density to control the density pumpout in NBI-fueled discharges by adjusting the ECRH power. With an ECRH power stabilized to 5 MW, a detached gas-fueled plasma with moderate density was operated up to 6 minutes with an overall energy turnaround of 1.8 GJ. The first reactor-relevant long-pulse discharges at central densities well above the X2 cutoff of 1.2 x 10²⁰ m^-3 were realized with the aid of the new continuous pellet injector. Here, the metallic reflector tiles of the optimized multipath O2 scenario showed their steady state capabilities in pulses up to 45 s. A similar multipath X3 scenario was successfully demonstrated for the first time at W7-X at a reduced magnetic field of 1.8 T. The critical part here was the plasma breakdown using a detuned gyrotron at 101 GHz supported by 4MW NBI heating [1]. With an envisaged X3 heating power of up to 16 MW, an average plasma beta of >4% should be achieved in the next campaigns to demonstrate the improved fast particle confinement of W7-X.