Abstract
Sawtooth-like crashes were observed during electron cyclotron current drive experiments for strikeline controls at the optimised superconducting stellarator Wendelstein 7-X (W7-X). The majority of the crashes did not have a relevant impact on plasma performance. However, a limited number of events, characterised by a large plasma volume affected by the instability, have been related to a strong deterioration performance and even to the premature termination of the plasma. The hot plasma core expelled during these sawtooth crashes can reach the plasma edge, where plasma surface interaction can occur and impurities can be released. The x-ray tomography shows a strong radiation increase starting from the edge and moving towards the inner plasma regions. This results in the cooling down and shrinking of the plasma, which eventually leads to a poor coupling of the ECRH to the electrons, that can in turn result in a plasma loss. A relation between the size and amplitude of the sawtooth crashes and the impurity increase is reported.
Highlights
Four VUX/XUV spectrometers (HEXOS) [29, 30], detecting from 2.5 to 160 nm were used to study the emission of impurities, such as carbon, oxygen and nitrogen after the sawtooth crashes, which led to a deterioration of the stored plasma energy
electron cyclotron current drive (ECCD) experiments show that a low shear stellarator, like W7X, is very sensitive to localised toroidal currents and sawtooth crashes are observed during ECCD experiments
In this work we report on a series of experiments where strong density increases were detected after sawtooth crashes
Summary
Magnetohydrodynamic (MHD) instabilities play a crucial role in magnetic confined plasmas, since they can lower the plasma performance and even lead to the termination of the plasma discharge, limiting the maximum achievable plasma. Toroidal currents, which are generally observed in W7-X, are small compared to tokamaks, fast current dissipation can induce electromagnetic forces in the vessel Since this was not part of the original design of the device, understanding and possibly preventing such events or mechanically strengthening in- or near-vessel components is important for avoiding damages. It has been shown that it was possible to control the strikeline position either by driving counter current, in order to keep Itor ∼ 0 kA [13] or by accelerating the convergence of the bootstrap current itself by driving co-current for a limited amount of time During these experiments, repetitive sawtooth-like crashes were observed.
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