Turbulent relaxation in magnetohydrodynamic plasma

Abstract

Magnetohydrodynamic (MHD) clump instability is considered as a turbulent relaxation mechanism for current carrying plasma. Predictions of the instability are compared with results from laboratory fusion experiments as well as computer simulations. For the tokamak, the predictions include safety factor [q(a)≤3] and Lundquist number [S≥104] thresholds, a magnetic helicity conserving radial electric field profile, and a voltage spike time of 0.1τRS−1/3, where τR is the resistive time. It is argued that the voltage spike that signals a tokamak disruption is due to the global conservation of magnetic helicity during the instability. In the reversed field pinch (RFP), clump steady states corresponding to B0z field reversed Taylor states at pinch parameters θ≥1.2, for S≥102, and magnetic fluctuation levels δB scaling as S−3/8 and θ3 are predicted. Reasonable agreement is obtained. The MHD clump instability appears to provide a unified picture of tokamak disruptions, where it occurs intermittently in its growth phase, and RFP ‘‘quiescent’’ states, where it is continuously present in its steady state turbulence phase.