First numerical analysis of runaway electron generation in tungsten-rich plasmas towards ITER

TL;DR

This study extends disruption simulation models to include tungsten impurities, revealing that tungsten concentration significantly influences runaway electron generation, with hot-tail mechanisms being the dominant source in tungsten-rich plasmas, especially relevant for ITER safety.

Contribution

First numerical analysis of runaway electron generation in tungsten-rich plasmas, comparing fluid and kinetic models, and identifying dominant generation mechanisms relevant for ITER safety.

Findings

W tungsten concentration below 10-3 does not cause significant RE.

Higher tungsten levels can lead to very high RE currents.

Hot-tail mechanism dominates RE generation in tungsten-rich disruptions.

Abstract

The disruption and runaway electron analysis model code was extended to include tungsten impurities in disruption simulations with the aim of studying the runaway electron (RE) generation. This study investigates RE current sensitivity on the following plasma parameters and modelling choices: tungsten concentration, magnetic perturbation strength, electron modelling, thermal quench time and tokamak geometry: ITER-like or ASDEX-like. Our investigation shows that a tungsten concentration below 10-3 does not cause significant RE generation on its own. However, at higher concentrations it is possible to reach a very high RE current. Out of the two tested models of electrons in plasma: fluid and isotropic (kinetic), results from the fluid model are more conservative, which is useful when it comes to safety analysis. However, these results are overly pessimistic when compared to the isotropic model, which is based on a more reliable approach. Our results also show that the hot-tail RE generation mechanism is dominant as a primary source of RE in tungsten induced disruptions, usually providing orders of magnitude higher RE seed than Dreicer generation. We discuss best practices for simulations with tungsten-rich plasma, present the dependence of the safety limits on modelling choices and highlight the biggest shortcoming of the current simulation techniques. The obtained results pave the way for a wider analysis of tungsten impact on the disruption dynamics, including the mitigation techniques for ITER in the case of strong contamination of the plasma with tungsten.