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Plasmoids formation during helicity injection for startup in toroidal fusion plasmas

Author: Fatima Ebrahimi
Requested Type: Consider for Invited
Submitted: 2015-01-19 15:09:04

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Princeton University
C Site T 152 MS-28 - PPPL - PO
Princeton, NJ   08543
USA

Abstract Text:
Magnetic reconnection, which energizes many processes in nature, is shown to have a fundamental role in the plasma start up and current formation in NSTX/NSTX-U. In previous work, in a systematic approach using MHD simulations, the fundamental reconnection mechanism that leads to the generation of closed flux surfaces in a transient Coaxial Helicity Injection (CHI) discharge in NSTX was explained. It was found that during transient CHI closed flux surfaces expand in the NSTX global domain through a local Sweet-Parker type reconnection with an elongated current sheet in the injector region. Here, formation of an elongated Sweet-Parker current sheet and a transition to plasmoid instability has for the first time been predicted by simulations in a large-scale toroidal fusion plasma in the absence of any pre-existing instability. Simulations have been performed in a realistic geometry with a toroidal guide field and using experimental poloidal coil currents. It is demonstrated that during CHI discharges at high Lundquist number (S), the elongated
current sheet formed through a forced reconnection can break up and a transition to a spontaneous reconnection occurs, i.e. plasmoid instability. Consistent with theory, the fundamental characteristics of the plasmoid instability, including 1) the break up the elongated current sheet, 2) the increasing number of plasmoids with S, 3) the reconnection rate as it becomes nearly independent of S, have been observed in the simulations. The latter has importance for the fast flux closure observed in the experiments. The requirements for full flux closure in NSTX-U, as obtained in the simulations will be presented. As the MHD description is valid in these CHI reconnection experiments, these predictive MHD simulations are believed to be relevant to the actual experiment as well as having implications for astrophysical reconnection. Work supported by DOE-FG02-12ER55115.

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