Runaway electron generation during tokamak start-up

TL;DR

This paper introduces a new simulation tool called STREAM to analyze runaway electron generation during tokamak start-up, revealing how plasma conditions influence runaway formation and its impact on plasma initiation.

Contribution

The study presents STREAM, a self-consistent 0D burn-through simulation model that predicts runaway electron formation during tokamak start-up, incorporating plasma density, temperature, and electric field evolution.

Findings

Dreicer generation is key in runaway electron formation.

Runaway electrons can hinder plasma burn-through and current ramp-up.

Early gas fueling can suppress runaway generation.

Abstract

Tokamak start-up is characterized by low electron densities and strong electric fields, in order to quickly raise the plasma current and temperature, allowing the plasma to fully ionize and magnetic flux surfaces to form. Such conditions are ideal for the formation of superthermal electrons, which may reduce the efficiency of ohmic heating and prevent the formation of a healthy thermal fusion plasma. This is of particular concern in ITER where engineering limitations put restrictions on the allowable electric fields and limit the prefill densities during start-up. In this study, we present a new 0D burn-through simulation tool called STREAM (STart-up Runaway Electron Analysis Model), which self-consistently evolves the plasma density, temperature and electric field, while accounting for the generation and loss of relativistic runaway electrons. After verifying the burn-through model, we investigate conditions under which runaway electrons can form during tokamak start-up as well as their effects on the plasma initiation. We find that Dreicer generation plays a crucial role in determining whether a discharge becomes runaway-dominated or not, and that a large number of runaway electrons could limit the ohmic heating of the plasma, thus preventing successful burn-through or further ramp-up of the plasma current. The runaway generation can be suppressed by raising the density via gas fuelling, but only if done sufficiently early. Otherwise a large runaway seed may have already been built up, which can avalanche even at relatively low electric fields and high densities.