Inter-machine comparison of the termination phase and energy conversion in tokamak disruptions with runaway current plateau formation and implications for ITER

Abstract

The termination of the current and the loss of runaway electrons following runaway current plateau formation during disruptions have been investigated in the JET, DIII-D and FTU tokamaks. Substantial conversion of magnetic energy into runaway kinetic energy, up to ~10 times the initial plateau runaway kinetic energy, has been inferred for the slowest current terminations. Both, modelling and experiment suggest that, in present devices, the efficiency of conversion into runaway kinetic energy is determined to a great extent by the characteristic runaway loss time, t diff , and the resistive time of the residual ohmic plasma after the disruption, t res , increasing with the ratio t diff /t res. It is predicted that, in large future devices such as ITER, the generation of runaways by the avalanche mechanism will play an important role, particularly for slow runaway discharge

On the avalanche generation of runaway electrons during tokamak disruptions

“…No discussion has been made of the mechanisms driving the runaway seed or the termination phase of the disruption, when the plasma current and the runaway electrons are lost, and conversion of the magnetic energy of the runaway plasma into runaway kinetic energy can occur, determining the final runaway heat loads onto the plasma facing components. 26 These have been evaluated for ITER disruptions with the models presented here in Ref. 27 and will be the subject of a detailed analysis in a future publication.…”

Physics of runaway electrons in tokamaks

“…A 0D coupled-circuit modeling indicates that longer RE loss times at the wall will cause greater conversion of magnetic to kinetic energy, while fast loss τ Loss < τ W all , τ Ω (where τ Ω is the background plasma resistive time scale) will result in the RE current dominantly being converted into Ohmic current and vessel current [269]. This overall trend was confirmed with comparison to JET, DIII-D, and FTU data [269]. It is essential that the RE plateau current can be converted into wall current (and then dissipated resistively in the wall) for sufficiently fast loss.…”

Disruptions in ITER and strategies for their control and mitigation

“…Depending on the timescale of the runaway loss, a significant fraction of this magnetic energy can be converted into kinetic energy as shown in Fig. 3a [17,18].…”