7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Anomalous electron diffusion in magnetized plasma with magnetic islands andfield stochasticity
Evdokiya Kostadinova1, Bradley Andrew1, Jessica Eskew1, Dmitri Orlov2, Mark Koepke3,Frederick Skiff4, and Max Austin5
1Physics Department, Auburn University, 2 Center for Energy Research, UC San Diego,
3
Department of Physics and Astronomy, 4 West Virginia University, Department of Physics andAstronomy, University of Iowa, 5 Institute for Fusion Studies, The University of Texas at Austin
e-mail (speaker): egk0033@auburn.eduEnergetic electrons (EEs) in magnetized plasmas are
sub-populations of particles whose energy is orders ofmagnitude higher than the bulk electrons. These particles
are known to exhibit anomalous diffusion in the
cross-field direction, which results in non-Maxwellianenergy distribution functions. While EEs are ubiquitousin laboratory conditions (tokamaks and stellarators) andin space plasma (the solar wind and the Earth’s
magnetotail), their origin and dynamics are poorly
understood. Here we investigate experimentally how theproduction, trapping, and release of energetic electrons inmagnetized plasma are affected by the features of themagnetic field topology in magnetized plasmas using
data from the DIII-D tokamak [1] and a Fraction
Laplacian Spectral (FLS) model [2]. Specifically, we
focus on the role of magnetic island chains and regionsof magnetic field stochasticity.
In the examined DIII-D experiments [1], coil
perturbations are used to create and manipulate islands atdesired flux surfaces, while electron cyclotron heating
and current drive (ECH/ECCD) pulses are employed to‘tag’ electrons within various locations inside and outside
the island chains. The properties of suprathermal
electrons are determined from Thomson Scattering,
electron cyclotron emission, gamma ray imaging, andscintillator (hard X-ray) measurements. The experiments
examine how the distribution of suprathermal electronschanges as a function of island width, location, structure,and dynamics.
Two types of energetic particles are distinguished anddiscussed: nonrelativistic suprathermal electrons andrelativistic runaway electrons. The magnetic field
topology in each discharge is reconstructed with field
line tracing codes, which are also used to determine thelocation and scale of magnetic islands and stochasticregions. Comparison of simulations and experimentssuggests that suprathermal transport is suppressed when
the tagging is performed at a smaller radial location thanthe location of the q=1 island chain and enhanced
otherwise. We further demonstrate that increasing thewidth of the stochastic region within the edge plasma
yields enhancement of the suprathermal electrontransport.
To understand the physical mechanisms leading toanomalous electron diffusion observed in theseexperiments, we employ a Fraction Laplacian Spectral(FLS) model [2]. In the FLS model, the probability foranomalous electron transport as a function of magnetic
field topology is determined from the spectrum of thecorresponding Hamiltonian. We construct Hamiltonian
operators corresponding to the magnetic field topology
in the examined DIII-D experiments. Nonlocal
interactions due to magnetic islands are modeled by afractional Laplacian operator, while random fluctuationsof the field due to coil perturbations are represented by astochastic potential term in the Hamiltonian.
The spectral properties of each Hamiltonian areinvestigated for fractions representative of each diffusion
regime. Comparison of these calculations to DIII-D data
reveals the presence of at least two types of EEs:
runaway electrons, best described by a Lévy process, andsuprathermal, but non-relativistic, electrons, best
described by trapping with nonlocal jumps. As thesefindings are in agreement with experimental results, wepropose that the FLS approach can be used as a powerful
reduced model for the study of nonthermal electrontransport in magnetized plasmas.
[1] Kostadinova, E. G., et al. (submitted to JPP), arXiv
preprint arXiv:2303.02009 (2023).
[2] Padgett, J. L., et al., J. Phys. A. 53.13 (2020):
135205.