Understanding the reduction of the edge safety factor during hot VDEs and fast edge cooling events

Abstract

In the present work a simple analytical approach is presented in order to clarify the physics behind the edge current density behaviour of a hot plasma entering in contact with a resistive conductor. As it has been observed in recent simulations [1], when a plasma enters in contact with a highly resistive wall, large current densities appear at the edge of the plasma. The model shows that this edge current originates from the plasma response, which attempts to conserve the poloidal magnetic flux (Ψ) when the outer current is being lost. The loss of outer current is caused by the high resistance of the outer current path compared to the plasma core resistance. The resistance of the outer path may be given by plasma contact with a very resistive structure or by a sudden decrease of the outer plasma temperature (e.g. due to a partial thermal quench or due to a cold front penetration caused by massive gas injection). For general plasma geometries and current density profiles the model shows that given a small change of minor radius (δa) the plasma current is conserved to first order (δI p = 0 + O(δa 2 )). This conservation comes from the fact that total inductance remains constant (δL = 0) due to an exact compensation of the change of external inductance with the change of internal inductance (δL ext + δL int = 0). As the total current is conserved and the plasma volume is reduced, the edge safety factor drops according to q a ∝ a 2 /I p . Finally the consistency of the resulting analytical predictions is checked with the help of free-boundary MHD simulations.

First predictive simulations for deuterium shattered pellet injection in ASDEX Upgrade

“…This peaking of the current after the TQ appears since the edge is cooled first and the current is lost from the stochastic region and re-induced in the still hot plasma center like described in Ref. [29]. The q-profile is altogether only weakly perturbed.…”

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

“…The change of the edge current density and edge safety factor during a VDE when the plasma moves into the PFCs is discussed in [282]. Analytical theory and JOREK non-linear simulations show excellent agreement.…”

Fast plasma dilution in ITER with pure Deuterium Shattered Pellet Injection

“…It can be seen in the j φ profile evolution shown in Fig. 5 that current is removed from the edge and partly re-induced further into the core, with a skin current progressing towards the core, as described in [24]. Accordingly, I p decreases and l i (3) increases.…”

“…This effect, which is captured by 3D non-linear MHD modelling [17], is due to the fact that, via radiation, impurities cool the electrons down to T e ∼ 10 eV or below on a timescale which is much shorter than the time it takes for shards to reach the plasma center, τ travel ≡ v s /a, where v s is the shards velocity and a the plasma minor radius. This T e drop increases the plasma resistivity η (which scales like T −3/2 e ) dramatically, which in turn modifies the current density distribution [24], destabilizes MHD modes and leads to the TQ.…”