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

Non-Maxwellian electron distribution functions in magnetized fusion plasmas
G. Giruzzi1, M. Fontana2, F.P. Orsitto3, E. de la Luna4, R. Dumont1, S. Mazzi1 and JET contributors5
1
CEA, IRFM, 2 Tokamak Energy, 3 ENEA Fusion Department, 4 CIEMAT, 5 See J Mailloux et al.,Nucl. Fusion 62, 042026 (2022)
e-mail (speaker): gerardo.giruzzi@cea.fr
Non-Maxwellian distribution functions are ubiquitousin magnetically confined fusion plasmas. Investigation of
their properties is not only important for predicting andunderstanding the plasma behaviour in fusion experiments,
but also for optimizing scenarios of plasma discharges. Inaddition, the study of these distributions can reveal
physics features or exploit properties that are analogous tothose observed in other laboratory or astrophysicalplasmas. Well-known examples are the runaway andslideaway electron distributions produced by a strong
electric field, as well as electron and ion distributions
associated to non-inductive current drive by waves orneutral particle injection. Other sources of nonMaxwellian electron distributions, such as spatial
diffusion due to magnetic turbulence, energetic ionscollisional relaxation on electrons, or damping of MHD
modes, offer less familiar, but very interesting examples,with analogies to inertial fusion or magnetosheathplasmas.
The main drivers of non-Maxwellian electron
distributions in magnetically confined plasmas are firstpresented and their roles in fusion experiments arediscussed. Examples of computed distribution functionsof various kinds and of measurements connected with
their presence are presented. Then, novel results areshown of a new class of non-Maxwellian distributions thathave been revealed in the high-performance JET
experiments of recent campaigns (both in Deuterium andin Deuterium-Tritium). The experimental finding that has
allowed to discover these distributions is a discrepancy
between electron temperature measurements by Electron
Cyclotron Emission (ECE) and by Thomson Scattering(TS), which appears only in high-temperature phases(typically, for temperatures higher than 5 keV). Such a
discrepancy has been observed for more than 30 years,
first in TFTR [1], then in JET [2], but so far neverunderstood.
In order to perform a systematic analysis of this effect,a model for bipolar perturbations of the electron
distribution function has been developed, allowing
analytic calculation of the EC emission and absorption
coefficients [3]. The model is characterised by three
perturbation parameters: the momentum p0 at which it iscentred, its width  in momentum and its amplitude f0. The
analysis carried out using this model shows that thediscrepancy observed on an extensive data base of recent
JET discharges [4] is an evidence of electron distributionfunctions presenting a bipolar distortion in the vicinity ofthe electron thermal velocity. An example of thecomparison between experiment and model for a specificdata set (Deuterium Baseline discharges with Ne
injection) is shown in Fig. 1.
In order to investigate the cause of this perturbation,
two distinct physical mechanisms have been studied. 1)
Fokker-Planck computations with a newly developed
kinetic code, using a full integro-differential collision
operator, have demonstrated, for the first time, thatcollisional relaxation on the electrons of energetic ion tails(always present in JET high-performance discharges) canbe responsible precisely for this kind of bipolar distortions.
2) Linear and non-linear gyrokinetic computations
performed with the GENE code have shown that Kinetic
Ballooning Modes (present in JET at high electron beta)
can also produce a bipolar distortion of the electron
distribution function around the thermal velocity. This issimilar to the effect observed in the magnetosheath [5] anddue to the Landau damping of Kinetic Alfvén Waves [6].
[1] G. Taylor et al., EC-9 workshop proceedings, p. 485(1995)[2] E. de la Luna et al., Rev. Sci. Instr. 74, 1414 (2003)[3] G. Giruzzi et al., 21st workshop on ECE and ECRH,
2022. https://doi.org/10.1051/epjconf/202327703005[4] M. Fontana et al., 21st workshop on ECE and ECRH,
2022. https://doi.org/10.1051/epjconf/202327703006[5] C.H.K. Chen et al., Nat. Commun. 10, 740 (2019)[6] S.A. Horvath et al., Phys. Plasmas 27, 102901 (2020)

Fig. 1: ECE radiation temperature measured by aMartin-Puplett interferometer vs TS temperature
measured by LIDAR technique. Data (with error bars)
and model estimates for the two harmonics. Perturbation
parameters are shown at the top.