Runaway electron confinement modelling for rapid shutdown scenarios in DIII-D, Alcator C-Mod and ITER

Abstract

MHD simulations of rapid shutdown scenarios by massive particle injection in DIII-D, Alcator C-Mod and ITER are performed in order to study runaway electron transport during mitigated disruptions. The simulations include a runaway electron (RE) confinement model using drift-orbit calculations for test particles. A comparison of limited and diverted plasma shapes is studied in DIII-D simulations, and improved confinement in the limited shape is found due to both spatial localization and reduced toroidal spectrum in the nonlinear MHD activity. C-Mod simulations compare shutdown scenarios in which impurity (Ar) fueling is concentrated in the edge versus the core, and find the confinement of REs in the core is maintained until the onset of the m = 1 n = 1 mode, which is delayed in the case of edge deposition, relative to core deposition. But, the overall RE loss fraction is 100% regardless of Ar fueling profile. A comparison of simulations across the three devices points to a trend of increased RE confinement with increasing device size, wherein all REs are lost in C-Mod, all are confined in ITER, and a partial loss is observed in DIII-D. This trend is related to a reduction in the fluctuating field amplitude near the plasma edge during the thermal-quench-induced MHD activity. The result bodes poorly for RE mitigation strategies in ITER that rely on MHD deconfinement of runaway electrons.

20 years of research on the Alcator C-Mod tokamak

“…MHD simulations were consistent with a model where these electrons were lost through the break-up in the field structure by the large instabilities that accompany the disruption. 374 While initially promising, experiments on larger devices suggest that the confinement of fast electrons improves with device size and thus provides no relief of this problem for ITER. A more promising result was the observation that the critical electric field required to generate significant runaway populations was 5-10 times higher than previously predicted.…”

Theory of tokamak disruptions

“…Non-axisymmetric with perfectly conducting walls Disruption simulations using NIMROD assume a perfectly conducting wall, [8][9][10] which is an inconsistent boundary condition for a plasma being driven into a wall. The NIM-ROD disruption simulations focus on effects that arise from the breakup of the internal magnetic surfaces, such as disruption mitigation by massive gas injection.…”

“…Although the axisymmetric simulations of ITER disruptions assume force balance, the non-axisymmetric simulations, which include magnetic islands, [6][7][8][9][10] not only retain Alfvénic effects, which would be important only if force balance failed, but more importantly have a tight intertwining of the equilibrium and transport calculations. Existing non-axisymmetric simulations use unphysical boundary conditions though the effect is mitigated by specifying important plasma parameters heuristically rather than determining their dependence through boundary conditions.…”

Runaway electrons and ITER

“…Increasing the plasma density by impurity injection is likely to produce a large scale breakup of magnetic surfaces when the impurities reach the q = 2 surface. This is seen in simulations of massive gas and shattered pellet injection [5][6][7] into plasmas in which the current is carried by near-thermal electrons. Because the growth rate of tearing modes is determined by near-thermal electrons [44], little difference is expected in the conditions or speed of surface breakup depending on whether the current carriers are near-thermal or relativistic electrons though a relativistic-electron current evolves towards regions of low density rather high temperature.…”

“…Non-intercepting flux tubes are seen in numerical simulations [5][6][7][8] of massive gas and shattered pellet injection-particularly near the magnetic axis and sometimes in the cores of magnetic islands. Nonintercepting flux tubes are also seen in models of the effects of non-axisymmetric magnetic fields [17,18] and are found to provide excellent confinement of relativistic electrons [17,19].…”