Physics of runaway electrons in tokamaks

Abstract

Of all electrons, runaway electrons have long been recognized in the fusion community as a distinctive population. They now attract special attention as a part of ITER mission considerations. This review covers basic physics ingredients of the runaway phenomenon and the ongoing efforts (experimental and theoretical) aimed at runaway electron taming in the next generation tokamaks. We emphasize the prevailing physics themes of the last 20 years: the hot-tail mechanism of runaway production, runaway electron interaction with impurity ions, the role of synchrotron radiation in runaway kinetics, runaway electron transport in presence of magnetic fluctuations, micro-instabilities driven by runaway electrons in magnetized plasmas, and vertical stability of the plasma with runaway electrons. The review also discusses implications of the runaway phenomenon for ITER and the current strategy of runaway electron mitigation.

Effect of plasma elongation on current dynamics during tokamak disruptions

“…In particular, future tokamaks with a large plasma current, such as ITER and SPARC (Greenwald et al 2018), will have a significant elongation. Experimental observations indicate that runaway beams are more easily produced in limiter or low-elongation discharges than in more elongated ones (Izzo, Humphreys & Kornbluth 2012;Hollmann et al 2013;Reux et al 2015;Breizman et al 2019). It is not clear if this is due to the elongation itself or to the difference of magnetic topology, i.e.…”

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

“…During the CQ phase of disruptions in reactor-scale devices, such as ITER, large RE currents are expected to form (Boozer 2015; Breizman et al. 2019). These energetic electrons are of particular concern, as they may give rise to localized power deposition and cause melting of plasma-facing components.…”

Spatiotemporal analysis of the runaway distribution function from synchrotron images in an ASDEX Upgrade disruption

“…2015; Breizman et al. 2019). Disruptions are notoriously hard to diagnose, and existing numerical models cannot simultaneously capture all aspects of their temporally and spatially multiscale nature, including the associated runaway dynamics.…”

“…Understanding runaway electron (RE) dynamics during tokamak disruptions is of utmost importance for the successful operation of future high-current tokamaks, such as ITER (Boozer 2015;Lehnen et al 2015;Breizman et al 2019). Disruptions are notoriously hard to diagnose, and existing numerical models cannot simultaneously capture all aspects of their temporally and spatially multiscale nature, including the associated runaway dynamics.…”