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(EX/P4-13) Characterization of Axisymmetric Disruption Dynamics toward VDE Avoidance in Tokamaks

Y. Nakamura¹⁾, R. Yoshino¹⁾, R.S. Granetz²⁾, G. Pautasso³⁾, O. Gruber³⁾, S.C. Jardin^4)
 
1) Japan Atomic Energy Research Institute, Naka Fusion Research Establishment, Naka, Ibaraki, Japan
²⁾ Plasma Science and Fusion Center, MIT, Cambridge, Massachusetts, USA
³⁾ MPI für Plasmaphysik, EURATOM Association, Garching, Germany
⁴⁾ Princeton Plasma Physics Laboratory, Princeton University, New Jersey, USA

Abstract.  Disruption experiments on Alcator C-Mod and ASDEX-Upgrade tokamaks and axisymmetric MHD simulations using the TSC have explicated the underlying mechanisms of Vertical Displacement Events (VDEs) and a diversity of disruption dynamics. First, the neutral point, which is known as an initial vertical plasma position advantageous to VDE avoidance, is shown to be fairly insensitive to plasma shape and current profile parameters, while the VDE rate significantly depends on those parameters. Secondly, it is clarified that a rapid flattening of the plasma current profile frequently seen at the thermal quench drags a single null-diverted, up-down asymmetric plasma vertically toward divertor, whereas the dragging effect is absent in up-down symmetric limiter discharges. As a consequence, the occurrence of downward-going VDEs predominates over the upward-going ones in bottom-diverted discharges, being consistent with experiments in ASDEX-Upgrade. Together with the attractive force that arises from passive shell currents induced by the current quench and vanishes at the neutral point, the dragging effect explains many details of the VDE dynamics over the whole period of disruptive termination.

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