Abstract
Massive material injection has been proposed as a way to mitigate the formation of a beam of relativistic runaway electrons that may result from a disruption in tokamak plasmas. In this paper we analyse runaway generation observed in eleven ASDEX Upgrade discharges where disruption was triggered using massive gas injection. We present numerical simulations in scenarios characteristic of on-axis plasma conditions, constrained by experimental observations, using a description of the runaway dynamics with a self-consistent electric field and temperature evolution in two-dimensional momentum space and zero-dimensional real space. We describe the evolution of the electron distribution function during the disruption, and show that the runaway seed generation is dominated by hot-tail in all of the simulated discharges. We reproduce the observed dependence of the current dissipation rate on the amount of injected argon during the runaway plateau phase. Our simulations also indicate that above a threshold amount of injected argon, the current density after the current quench depends strongly on the argon densities. This trend is not observed in the experiments, which suggests that effects not captured by zero-dimensional kinetic modelling – such as runaway seed transport – are also important.
Highlights
Disruptions in tokamak plasmas may lead to the formation of a beam of relativistic, so-called runaway electrons (RE), which has the potential to severely damage plasma-facing components (Hender et al 2007)
The plasma currents, temperatures and densities of future devices such as ITER will be significantly larger than what can be achieved in current experiments, and simulations are necessary to foresee the effectiveness of massive material injections for disruption mitigation under such conditions
We have presented kinetic modelling of runaway generation and dissipation in argon-induced disruptions in ASDEX Upgrade, where the initial on-axis free electron temperature, the free electron density, the amount of injected argon and the initial current density have been based on experimental data
Summary
Disruptions in tokamak plasmas may lead to the formation of a beam of relativistic, so-called runaway electrons (RE), which has the potential to severely damage plasma-facing components (Hender et al 2007). To be applicable for predictions for ITER and beyond, theoretical tools must first be validated against existing experimental data to ensure that they capture the relevant physics One such kinetic modelling tool is CODE (COllisional Distribution of Electrons), briefly described in § 2 and in detail in the paper by Stahl et al (2016). Since spontaneous disruptions do not occur reproducibly, we instead model disruptions which were deliberately triggered by an MGI, resulting in a scenario which is similar to the desired scenario in the important aspect that a runaway current is formed and dissipated in the presence of partly ionized high-Z materials Such scenarios have been considered in the recent paper by Linder et al (2020), where the ASTRA-STRAHL 1.5-dimensional (1.5-D) transport code (Dux et al 1999; Fable et al 2013) was used, including reduced kinetic models for Dreicer and avalanche For a full understanding of the disruption scenario, kinetic simulation must be combined with other tools, most importantly modelling spatial dynamics, 3-D MHD (magnetohydrodynamic) evolution and the atomic physics needed to determine ionization states
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