Generation and suppression of runaway electrons in disruption mitigation experiments in TEXTOR

Abstract

Abstract. Runaway electrons represent a serious problem for the reliable operation of the future experimental tokamak ITER. Due to the multiplication factor of exp(50) in the avalanche even a few seed runaway electrons will result in a beam of high energetic electrons that is able to damage the machine. Thus suppression of runaway electrons is a task of high importance, for which reason we present here a systematic study of runaway electrons following massive gas injection in TEXTOR. Argon injection can cause generation of runaways carrying up to 30% of the initial plasma current, while disruptions triggered by injection of helium or of mixtures of argon (5, 10, 20%) with deuterium are runaway free. Disruptions caused by argon injection finally become runaway free for very large amounts of injected atoms. The appearance/absence of runaway electrons is related to the fraction of atoms delivered to the plasma center. This so called mixing efficiency is deduced from a 0D model of the current quench. The estimated mixing efficiency is: 3% for argon, 15% for an argon/deuterium mixture and about 40% for helium. A low mixing efficiency of high-Z impurities can have a strong implication for the design of the disruption mitigation system for ITER. However, a quantitative prediction requires a better understanding of the mixing mechanism.

Runaway generation during disruptions in JET and TEXTOR

“…To achieve temperatures below this limit, the impurity density and thus the electron density has to be too high to allow for detectable runaway generation (see also discussion in [15]). The limit '2' is an empirical limit taken from figure 3 for high magnetic fields.…”

“…Thus, a mitigation technique has to provide a reliable suppression of this avalanche mechanism. Presently, massive gas injection is discussed as a technique to mitigate forces and heat loads and also to suppress runaway generation [20][21][22][23][24][25][26]15]. However, the latter aim might have severe implications which are not easily overcome.…”

“…The mixing efficiency is defined as the ratio between impurity density in the current quench plasma and the mean density of atoms injected before the energy quench (amount of atoms divided by the vessel volume). The impurity density has been determined by modelling the current quench in TEXTOR disruptions [15]. The mixing efficiency is a function of the gas species (figure 8): 3-6% for pure Argon injection, 15-30% for the Argon/Deuterium mixture and above 35-70% for Helium injection.…”

“…The runaway generation rate is calculated according to the standard equations, which are given for example in [5,15]. For a maximum electric field of E = 50V/m a minimum diffusion coefficient of D pt = 2 × 10 −6 m 2 /m follows from the above considerations.…”

Measurements of the runaway electron energy during disruptions in the tokamak TEXTOR

“…[20][21][22] In order to mitigate the generation of REs, several techniques are studied. Massive gas injection [23][24][25][26] and pellet injection 27,28 aim at the suppression of the RE generation due to a steep increase of the electron density. In the case of gas, typically more than 10 22 atoms must be injected.…”