Additional ECRH mitigates thermal quenches induced by tungsten TESPEL injection in LHD

Massive injection of impurities, e.g., due to unforeseen mechanical failures, into the core region of fusion plasmas can lead to their termination. Resulting thermo-mechanical loads may affect both the integrity of plasma-facing components and safe plasma operation. In order to examine the mechanism of impurity-induced plasma termination in stellarators, experiments were conducted in LHD by intentionally injecting impurities. To this end, TESPEL, loaded with various defined amounts of tungsten, exceeding typical tracer amounts, was used. 3×1017 tungsten atoms are found to be sufficient to induce radiative collapses of plasmas with diamagnetic energy of ∼500 kJ. The termination is governed by the formation of inwards-propagating cold fronts. Additional ECRH was applied in response to the TESPEL injection with the goal of mitigating detrimental heat loads. It was found that additional electron heating prolonged plasma termination up to ∼100 ms. This investigation documents a high resilience of helical plasmas against strong perturbations and suggests approaches to mitigate the criticality of heat loads in fusion grade stellarator plasmas.