#70

Study of Suprathermal Electron Dynamics by Modelling the Electron Cyclotron Emission
Oral
Remote

Abstract

Electrons gyrating around magnetic field lines emit electron cyclotron emission (ECE) ra-
diation at the frequency Ωc = neB/(γm), where e is the elementary charge, B is the magnetic
field amplitude, m is the electron rest mass, γ is the relativistic factor and n is the harmonic
number [2]. In the presence of a magnetic field amplitude gradient, as in a tokamak, the ori-
gin of emission is related to the frequency of the emission and a temperature profile can be
reconstructed by scanning a range of frequencies, which is a common application of ECE di-
agnostics. In certain scenarios, the plasma may contain a significant fraction of suprathermal
particles whose energies exceed that of the thermal particles by several orders of magnitude,
affecting both temperature measurements and profile reconstructions. Measuring ECE at var-
ious frequencies along vertical lines of sight (VECE) [1] , along which the magnetic field B
is constant, means that any variation in the observed radiation comes from a variation in the
electron relativistic factor γ. On the other hand, reconstructing the 3D bounce-averaged guid-
ing center electron distribution function from ECE measurements is an ill-conditioned problem
and a more promising approach consists of constructing an equivalent synthetic ECE diagnostic
providing simulations that can be directly compared to measurements.
In this contribution, a new general synthetic ECE diagnostic that includes the effect of
suprathermal electrons is constructed: the Yoda code. This code is able to calculate: i) the EC
emission and (re)absorption based on any numerical electron distribution function calculated
by any first-principle kinetic code (such as the 3-D bounce-averaged relativistic Fokker-Planck
code Luke [4]) for an arbitrary line of sight simulated using the c3po [6] ray-tracing code
(which also models the detection system); ii) the transport of EC radiated intensity along the
propagation path. In this work, the Yoda code is validated for thermal plasmas against the
ECE synthetic diagnostic spece [5], and two direct applications to TCV tokamak electron
cyclotron current drive (ECCD) experiments are demonstrated with good agreement between
the experimental vertical ECE measurements and synthetic intensity trends. This work has the
potential to open new paths in the understanding of fast electron dynamics in tokamaks using
synthetic ECE and synthetic HXRS [3] as constraints in first principle kinetic simulations.
References
[1] A. Tema Biwole et al. “The Vertical ECE of TCV”. submitted to review of scientific
instruments. 2023.
[2] M Bornatici et al. “Electron cyclotron emission and absorption in fusion plasmas”. In:
Nuclear Fusion 23.9 (1983), p. 1153.
[3] D. Choi et al. “Study of suprathermal electron dynamics during electron cyclotron cur-
rent drive using hard x-ray measurements in the TCV tokamak”. In: Plasma Physics and
Controlled Fusion 62.11 (2020), p. 115012.
[4] J Decker and Y Peysson. DKE: A fast numerical solver for the 3D drift kinetic equation.
EUR-CEA-FC-1736. Euratom-CEA, 2004.
[5] D. Farina et al. “SPECE: a code for Electron Cyclotron Emission in tokamaks”. In: AIP
Conference Proceedings. Vol. 988. 1. American Institute of Physics. 2008, pp. 128–131.
[6] Y. Peysson, J. Decker, and L. Morini. “A versatile ray-tracing code for studying rf wave
propagation in toroidal magnetized plasmas”. In: Plasma Physics and Controlled Fusion
54.4 (2012), p. 045003

Copyright © 2024 Chalmers Plasma Theory group