Scaling of the MHD perturbation amplitude required to trigger a disruption and predictions for ITER

Abstract

The recycling of D ions impinging onto a W divertor surface is a key input parameter into the power and momentum balance at the target boundary during SOL modeling. It is described by the ratio R of the flux of recombining D 2 molecules to the incident ion flux. In steady-state plasmas where the surface is in equilibrium with the incident flux R equals one due to particle conservation. However during transient events such as ELMs the evolution of R with time is not straight forward to predict. Therefore detailed diffusiontrapping calculations were performed taking into account the variations in power influx and particle energy during an ELM. They showed that in contrast to the naive expectation, that the ELM would deplete the surface and subsequently lead to "pumping" (R ≪ 1) of the incident flux by the empty surface, R ≈ 1 or even R > 1 occurs. This paper will first describe how the ELM was approximated in the 1D diffusion-trapping code and will then discuss the evolution of R during an ELM and in the inter ELM phase. Also an analytical picture of R will be developed which allows to qualitatively understand the evolution of R as calculated by the diffusion-trapping code.

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

“…Indeed, the I p spike in the simulations is too weak, suggesting that tearing modes are not large enough, and simulations with larger tearing modes (which display a larger I p spike) produce a full stochastization of the magnetic field so that the convective core mixing plays a less important role. An interesting question which could be the object of future work is whether the TQ triggering picture found here may explain the exper imental finding that the TQ is triggered at a distinct locked mode amplitude [13]. The work with JOREK on D 2 MGI simulations shall in any case be pursued, aiming for quantitative validation.…”

Runaway electrons and ITER

“…Rapid thermal quenches can occur either naturally [13] or during mitigated disruptions [14,15]. In simulations of experiments, rapid thermal quenches are associated with fast magnetic reconnections, which break the magnetic surfaces over much of the plasma volume causing a rapid flattening of the / ∥ j B profile, a large reduction in the internal inductance i , and an upward spike in the plasma current on a 1 ms time scale.…”

“…But, this requires a carefully orchestrated strategy for cooling, which preserves magnetic surfaces. If natural thermal quenches can be avoided by preemptive plasma termination as suggested by [13], then surface-preserving plasma cooling could be the primary strategy for avoiding damaging relativistic electrons.…”

“…The fast breakup of magnetic surfaces can arise as part of a natural plasma evolution [13] or be forced [14,15]. A forced breakup often arises from efforts to mitigate halo currents and heat loads through the injection of impurities.…”

“…Some time, called the trigger time, is required for the field lines to become sufficiently complicated to trigger a reconnection. Magnetic perturbations that resulted in a thermal quench in JET [13] were seen on a number of time scales, but a slow natural growth, ≈ 400 ms, could occur before the fast reconnection was triggered. Once reconnection begins, it causes a magnetic relaxation in one region.…”