[{"type":"article-journal","title":"ECCD-induced sawtooth crashes at W7-X","issued":{"date-parts":[["2020","10"]]},"volume":"60","issue":"10","page":"Article no: 106021","container-title":"Nuclear fusion","DOI":"10.1088\/1741-4326\/aba72b","author":[{"family":"W7-X Team"},{"family":"Zanini","given":"M."},{"family":"Laqua","given":"H. P."},{"family":"Thomsen","given":"H."},{"family":"Stange","given":"T."},{"family":"Brandt","given":"C."},{"family":"Braune","given":"H."},{"family":"Brunner","given":"K. J."},{"family":"Fuchert","given":"G."},{"family":"Hirsch","given":"M."},{"family":"Knauer","given":"J."},{"family":"H\u00f6fel","given":"U."},{"family":"Marsen","given":"S."},{"family":"Pasch","given":"E."},{"family":"Rahbarnia","given":"K."},{"family":"Schilling","given":"J."},{"family":"Turkin","given":"Y."},{"family":"Wolf","given":"R. C."},{"family":"Zocco","given":"A."},{"family":"Gantenbein","given":"Gerd"},{"family":"Huber","given":"Martina"},{"family":"Illy","given":"Stefan"},{"family":"Jelonnek","given":"John"},{"family":"Kobarg","given":"Thorsten"},{"family":"Lang","given":"Rouven"},{"family":"Leonhardt","given":"Wolfgang"},{"family":"Mellein","given":"Daniel"},{"family":"Papenfu\u00df","given":"Daniel"},{"family":"Scherer","given":"Theo"},{"family":"Thumm","given":"Manfred"},{"family":"Wadle","given":"Simone"},{"family":"Weggen","given":"J\u00f6rg"}],"ISSN":"0029-5515, 1741-4326","abstract":"The optimised superconducting stellarator W7-X generates its rotational transform by means of\r\nexternal coils, therefore no toroidal current is necessary for plasma confinement. Electron\r\ncyclotron current drive experiments were conducted for strikeline control and safe divertor\r\noperation. During current drive experiments periodic and repetitive crashes of the central\r\nelectron temperature, similar to sawtooth crashes in tokamaks, were detected. Measurements\r\nfrom soft x-ray tomography and electron cyclotron emission show that the crashes are preceded\r\nby weak oscillating precursors and a displacement of the plasma core, consistent with a\r\n(m, n)=(1, 1) mode. The displacement occurs within 100\u03bcs, followed by expulsion and\r\nredistribution of the core into the external part of the plasma. Two types of crashes, with\r\ndifferent frequencies and amplitudes are detected in the experimental program. For these\r\nnon-stationary parameters a strong dependence on the toroidal current is found. A 1-D heuristic\r\nmodel for current diffusion is proposed as a first step to explain the characteristic crash time.\r\nInitial results show that the modelled current diffusion timescale is consistent with the initial\r\ncrash frequency and that the toroidal current rise shifts the position where the instability is\r\ntriggered, resulting in larger crash amplitudes.","kit-publication-id":"1000125250"}]