Progress in understanding disruptions triggered by massive gas injection via 3D non-linear MHD modelling with JOREK

Abstract

3D non-linear MHD simulations of a D 2 massive gas injection (MGI) triggered disruption in JET with the JOREK code provide results which are qualitatively consistent with experimental observations and shed light on the physics at play. In particular, it is observed that the gas destabilizes a large m/n = 2/1 tearing mode, with the island O-point coinciding with the gas deposition region, by enhancing the plasma resistivity via cooling. When the 2/1 island gets so large that its inner side reaches the q = 3/2 surface, a 3/2 tearing mode grows. Simulations suggest that this is due to a steepening of the current profile right inside q = 3/2. Magnetic field stochastization over a large fraction of the minor radius as well as the growth of higher n modes ensue rapidly, leading to the thermal quench (TQ). The role of the 1/1 internal kink mode is discussed. An I p spike at the TQ is obtained in the simulations but with a smaller amplitude than in the experiment. Possible reasons are discussed.

Runaway electrons and ITER

“…The physics of thermal quenches, whether naturally occurring or produced by impurity injection, has much in common [14]. The breakup of surfaces and the associated rapid thermal quench due to massive-gas or shattered-pellet injection have been seen in a number of simulations [5][6][7][8].…”

“…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].…”

“…where J is the Jacobian of canonical coordinates, equation (8) for the electric field gives an evolution equation for the poloidal flux averaged over a surface of constant toroidal flux, equation (6), which is¯∥…”

“…This Alfvénic effect defines the duration of a reconnection event, which appears to be of order 1 ms in large tokamaks. Nardon et al saw an avalanche of tearing modes during the initiation of a thermal quench in their JOREK simulations [8] even though the code was not run with the spatial resolution required for reconnection on an Alfvénic time scale.…”

Disruption mitigation by injection of small quantities of noble gas in ASDEX Upgrade

“…The effect of rotation on the asymmetry is suppressed and the effect of toroidal spreading of the impurities -due to parallel thermal velocity -is limited by the low plasma temperature in this dedicated experiment. This data-set is particularly suited for the benchmark of 3-D non-linear MHD codes, such as JOREK, which is being further developed for disruption modelling [10]. In previous experiments [11], it was found that plasmas at larger energies could not be stopped by the RMPs and therefore the radiation asymmetry was affected by a residual, difficult to measure, rotation.…”