ICRH assisted breakdown study on JET

Ion cyclotron (IC) wave assisted breakdown has the potential to increase the robustness of plasma initiation during the ITER pre-fusion operation phase. Studies were performed at JET at ITER relevant loop electric field, E loop ≲ 0.33 Vm − 1 , and a range of toroidal fields, including at the low toroidal field of 1.7 T for which breakdown had not been achieved previously on JET. The study covered a range of H₂ and D₂ gas prefill pressures and timings, pumping conditions, and residual impurity levels. IC assisted breakdown was achieved for a lower and wider range of gas prefill pressures. IC assisted breakdown works by activating wall pumping before the current rise, changing the relation between fuelling and torus pressure in this phase compared to Ohmic breakdown. IC assisted breakdown enables plasma initiation with a higher level and significantly wider range of injected plasma prefill gas. As the injected prefill gas is the controlled parameter, this significantly improves the robustness of plasma initiation operationally. IC assistance is found to be more robust at ITER-like E loop , succeeding with higher low-Z impurity content. Moreover, it does not introduce an impurity source that may hamper the subsequent burn though and current ramp-up phase. For both the IC assisted and pure Ohmic breakdown, the initial current rise rate is found to scale with n e / E loop . The results and implications for ITER are presented.