An active feedback recovery technique from disruption events induced by m = 2, n = 1 tearing modes in ohmically heated tokamak plasmas. (April 2015)

• Record Type: Journal Article
• Authors: Zanca, P.; Paccagnella, R.; Finotti, C.; Fassina, A.; Manduchi, G.; Cavazzana, R.; Franz, P.; Piron, C.; Piron, L.
• Is Part Of: Nuclear fusion. Volume 55:Number 4(2015:Apr.)
• Page Start: S128
• Page End: S202
• Publication Date: 2015-04
• Subjects: Nuclear fusion -- Periodicals; 621.48405
• DOI: 10.1088/0029-5515/55/4/043020
• Languages: English
• ISSNs: 0029-5515

Abstract

<abstract> <title>Abstract</title> <p>We present experimental results of magnetic feedback control on the <italic>m</italic> = 2, <italic>n</italic> = 1 tearing mode in RFX-mod operated as a circular ohmically heated tokamak. The feedback suppression of the non-resonant <italic>m</italic> = 2, <italic>n</italic> = 1 resistive wall mode (RWM) in <italic>q</italic>(<italic>a</italic>) < 2 plasmas is a well-established result of RFX-mod. The control of the tearing counterpart, which develops in <italic>q</italic>(<italic>a</italic>) > 2 equilibrium, is instead a more difficult issue. In fact, the disruption induced by a growing amplitude <italic>m</italic> = 2, <italic>n</italic> = 1 tearing mode can be prevented by feedback only when the resonant surface <italic>q</italic> = 2 is close to the plasma edge, namely 2 < <italic>q</italic>(<italic>a</italic>) < 2.5, and the electron density does not exceed approximately half of the Greenwald limit. The RFX-mod data show very clearly that a large amplitude <italic>m</italic> = 2, <italic>n</italic> = 1 tearing mode significantly limits the operative region of plasma parameters even in the presence of an efficient magnetic feedback. A combined technique of tearing mode and <italic>q</italic>(<italic>a</italic>) control has been therefore developed to recover the discharge from the most critical conditions: the potentially disruptive tearing mode is converted into the relatively benign RWM by suddenly decreasing <italic>q</italic>(<italic>a</italic>) below 2. The experiments successfully demonstrate the concept. The <italic>q</italic>(<italic>a</italic>) control has been performed through the plasma current, given the capability of the toroidal loop-voltage power supply of RFX-mod. We also propose a path for decreasing <italic>q</italic>(<italic>a</italic>) by acting on the plasma shape, which could be applied to medium size elongated tokamaks.</p> </abstract>