Dynamics of runaway electrons in tokamak plasmas

Abstract : One of the key features of a plasma is that the collisional friction resulting from Coulomb interactions decreases with the electron velocity. Therefore, in the presence of a parallel electric field larger than a critical value, electrons with sufficient velocity will be continuously accelerated. These so-called runaway electrons may reach energies on the order of several MeVs and cause serious damage to plasma facing components in ITER - the next large-scale tokamak. The runaway population may be multiplied through knock-on collisions, where an existing runaway electron can transfer a significant fraction of its energy to a secondary electron nearly at rest, while remaining in the runaway region. Understanding of the runaway electron formation processes is crucial in order to develop ways to mitigate them. In this context, modelling of runaway electron dynamics is performed using the 3-D linearized relativistic bounce-averaged electron Fokker-Planck solver LUKE, with a particular emphasis on knock-on collisions of fast electrons on thermal ones, which can lead to an avalanche of relativistic electrons. The theory of bounce-averaged knock-on collisions is derived, and the corresponding operator is implemented in the kinetic solver LUKE. The dependencies of the runaway electron growth rate on the electric field strength, density, temperature and magnetic configuration is investigated, in order to identify the relative importance between primary and secondary runaway generation, the latter resulting from the avalanche process. It is shown that avalanches of runaway electrons can be important even in non-disruptive regimes and this effect may become dominant in the build-up of the highly relativistic electron tail. Owing to their high magnetization, most of the knock-on electrons are born into the magnetic trapping domain in momentum space, which leads to a reduction of the runaway population off the magnetic axis. This accumulates the runaway electrons near the magnetic axis. The dynamics of the trapped electrons in the framework of runaway electron generation is investigated. Finally, the runaway electron formation in Ohmic discharges performed in the Tore Supra and COMPASS tokamaks is modelled with the LUKE code, using global plasma parameters such as parallel electric field and the toroidal MHD equilibrium calculated with the fast integrated modelling code, METIS. Details of the fast electron velocity distribution function are provided as well as quantitative comparison with non-thermal bremsstrahlung for the Tore Supra tokamak.
Complete list of metadatas

Cited literature [125 references]  Display  Hide  Download

https://pastel.archives-ouvertes.fr/tel-01212017
Contributor : Emelie Nilsson <>
Submitted on : Monday, October 5, 2015 - 9:28:47 PM
Last modification on : Monday, October 1, 2018 - 4:03:02 PM

Identifiers

  • HAL Id : tel-01212017, version 1

Collections

Citation

Emelie Nilsson. Dynamics of runaway electrons in tokamak plasmas. Physics [physics]. CEA, 2015. English. ⟨tel-01212017⟩

Share

Metrics

Record views

353

Files downloads

988