Radiation asymmetry and MHD destabilization during the thermal quench after impurity shattered pellet injection

Abstract

The radiation response and the MHD destabilization during the thermal quench after a mixed species shattered pellet injection with impurity species neon and argon are investigated via 3D non-linear MHD simulation using the JOREK code. Both the n = 0 global current profile contraction and the local helical cooling at each rational surface caused by the pellet fragments are found to be responsible for MHD destabilization after the injection. Significant current driven mode growth is observed as the fragments cross low order rational surfaces, resulting in rapidly inward propagating stochastic magnetic field, ultimately causing the core temperature collapse. The thermal quench (TQ) is triggered as the fragments arrive on the q = 1 or q = 2 surface depending on the exact q profile and thus mode structure. When injecting from a single toroidal location, strong radiation asymmetry is found before and during the TQ as a result of the unrelaxed impurity density profile along the field line and asymmetric outward heat flux. Such asymmetry gradually relaxes over the course of the TQ, and is entirely eliminated by the end of it. Simulation results indicate that the aforementioned asymmetric radiation behavior could be significantly mitigated by injection from toroidally opposite locations, provided that the time delay between the two injectors is shorter than 1 ms. It is also found that the MHD response are sensitive to the relative timing and injection configuration in these multiple injection cases.

The JOREK non-linear extended MHD code and applications to large-scale instabilities and their control in magnetically confined fusion plasmas

“…In this regime, the TQ is typically triggered when shards reach the q = 2 surface, but it can be triggered even before for large and slow pellets. This behaviour has been observed in ASDEX Upgrade deuterium SPI simulations [263] and is also typically observed for any tokamak when simulating the injection of shattered pellets containing impurities [80].…”

Collisional-radiative non-equilibrium impurity treatment for JOREK simulations

“…The "marker-particles" considered in out treatment do not evolve according to their own respective Newton's law, but are simply advected with the fluid velocity field, hence the name. This simplification is in accordance with our previous assumption that the interspecies friction and the charge exchange is strong enough so that all species and their different charge states share the same velocity [29]. Currently, the additional diffusive transport of the fluid density is not yet reflected in the particle advection due to its sub-dominant role.…”

“…The amplitude and shape of such density source is detailed in Ref. [29]. The generated super-particles are equal in their weight (the amount of real particles represented by a given super-particle), and their generation location is obtained by the reject-sampling method according to the aforementioned source terms.…”

“…The formulation of our single and two temperature models under the CE assumption has been detailed in Ref. [29]. For the completeness of the paper, we again write down the governing equations with the particle moment contributions in this subsection, but we will only focus on the particle moment contributions instead of going over all the details.…”

“…2 of Ref. [29]. This is especially important if the temperature drop is not fast enough during the early injection period or during the Thermal Quench (TQ) when sudden outgoing heat flux reheats the outer plasma region, since the CE assumption tends to underestimate the line radiation contribution from the weakly ionized states.…”