Runaway dynamics in the DT phase of ITER operations in the presence of massive material injection

Abstract

A runaway avalanche can result in a conversion of the initial plasma current into a relativistic electron beam in high-current tokamak disruptions. We investigate the effect of massive material injection of deuterium–noble gas mixtures on the coupled dynamics of runaway generation, resistive diffusion of the electric field and temperature evolution during disruptions in the deuterium–tritium phase of ITER operations. We explore the dynamics over a wide range of injected concentrations and find substantial runaway currents, unless the current quench time is intolerably long. The reason is that the cooling associated with the injected material leads to high induced electric fields that, in combination with a significant recombination of hydrogen isotopes, leads to a large avalanche generation. Balancing Ohmic heating and radiation losses provides qualitative insights into the dynamics; however, an accurate modelling of the temperature evolution based on energy balance appears crucial for quantitative predictions.

MHD stability and disruptions in the SPARC tokamak

“…A simple, physically motivated zero-dimensional scaling law suggests that the required number of neon atoms for full TQ mitigation n Ne,crit scales like n Ne,crit ∝ W th V p /R (W th is the pre-disruption thermal energy, V p is the plasma volume and R is the major radius) and predicts that the order of 10 21 assimilated neon atoms are required for full TQ mitigation in SPARC (Lehnen et al 2017). This number is expected to be reduced for higher-Z noble gases; however, higher-Z gases might raise the CQ electric field and provide more electrons for avalanche multiplication (Vallhagen et al 2020). Until further analysis can be done, we assume that the mitigation gases will be neon and a low-Z gas such as hydrogen, deuterium or helium.…”

“…This number is expected to be reduced for higher- noble gases; however, higher- gases might raise the CQ electric field and provide more electrons for avalanche multiplication (Vallhagen et al. 2020). Until further analysis can be done, we assume that the mitigation gases will be neon and a low- gas such as hydrogen, deuterium or helium.…”

Effects of magnetic perturbations and radiation on the runaway avalanche

“…2011; Vallhagen et al. 2020) or similar codes. Introducing the effect of transport losses into such a model, including the effects of impurities and allowing for partial screening, would allow us to quantify the reduction given by transport of the total number of runaway electrons at the end of a disruption.…”

“…2017; Vallhagen et al. 2020). In the simulations presented here, we neglect the hot-tail generation occurring in a rapidly cooling plasma.…”

“…Such simplified disruption modelling has been used to estimate the post-disruption runaway population in the presence of massive material injection (Martín-Solís, Loarte & Lehnen 2017; Vallhagen et al. 2020; Linder et al. 2020).…”