H. Bouvain

Transitions to core-electron-root-confinement (CERC) regime are examined in the presence of low-order rational values of the rotational transform (ι) in Wendelstein 7-X. Experiments at densities and heating powers near the bifurcation from pure ion-root to CERC conditions revealed transitions at stationary plasma conditions but evolving plasma currents. Analysis of the effects due to the ramping plasma current indicates a change of the ι profile temporarily crossing low-order rational values. The transition occurs within a narrow range of plasma currents and, supported by simulations of the shielding current evolution, is consistent with the observed rise in central electron temperature. Prior to these transitions, low-frequency fluctuations were detected, ceasing once higher electron temperatures were reached, and the central radial electric field became positive. Experimental and numerical analyses suggest that these fluctuations are not instabilities, but rather sidebands of zonal flow oscillations, Doppler-shifted by the ambient electric field. Although the oscillations are not the trigger, their systematic characterization offers deeper insight into the underlying physics governing the transition and emphasizes the role of the evolving ι.

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.