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

Research of turbulent transport by gyrokinetic analyses
in collisional plasmas for tokamak devices
S. Toda1,2, M. Nunami1,3, N. Kasuya4
1
National Institute for Fusion Science, National Institutes of Natural Sciences2
Graduate Institute for Advanced Studies, SOKENDAI
3
Graduate School of Science, Nagoya University
4
Research Institute for Applied Mechanics, Kyushu University
e-mail (speaker): toda.shinichiro@nifs.ac.jp
Turbulent transport is one of the most important subjects
in research of toroidal plasmas. The purpose of this studyis to quantify turbulent transport in toroidal plasmas.
Microscopic plasma instabilities are studied using
electromagnetic
gyrokinetic
simulations.
Linearcalculations are used to investigate the type of instability
and to evaluate the conditions for instability excitation.
The saturation level of the instability or the turbulent
level is determined from nonlinear calculations. The
trapped electron mode (TEM) where the normalized
electron collision frequency is less than the unity [1,2] iscalled the dissipative-TEM, where the ions do not need
to be in the banana regime. This mode is known to be
driven unstable in the presence of density [3] and/ortemperature gradients. In previous studies of the ion
temperature gradient (ITG) and TEM plasmas, the TEM
is mainly excited by the density and/or temperature
gradients in the collisionless plasmas. The plasma
experiments at the PLATO device at the ResearchInstitute for Applied Mechanics, Kyushu University havebeen started. The plasma profiles in the PLATO arepredicted using the integrated code, the TASK code [4].
The electron collision frequency is typically less than theelectron bounce collision frequency and the ion collision
frequency is larger than the unity in these plasmas,
therefore, the dissipative TEM is predicted to be unstable.
Not many studies of the dissipative-TEM using thegyrokinetic simulation have been done so far.
A local gyrokinetic simulation by the GKV code [5]using the model collision operators, such as the Sugama(S) [6] and Lenard-Bernstein (LB) collision model
operators, is performed in this study. In this article, theinstabilities are predicted to be driven by thedissipative-TEM and the ion temperature gradient (ITG)
mode in the predicted parameter regions for the PLATO.
First, the dominant mode and the condition under which
the instability occurs are studied by the linear gyrokineticsimulations. Next, the nonlinear simulation demonstrates
the time evolution of the electrostatic potentialfluctuations. By the nonlinear simulation results, theturbulent levels of the fluxes and diffusivities arequantified. The S collision operator was used for theresidue level of zonal flows with the collisional effect. In
these analyses, the residue level of zonal flows using theS collision operator is found to be close to theanalytically predicted one. On the other hand, the linear
response of zonal flows using LB collision operator
shows over-damping [7]. Therefore, in this study, the S
collision operator is used for the local gyrokinetic
simulation.
The gyrokinetic simulation is performed for the tokamak
plasmas in the collisional regime. The dissipative-TEM,
which is excited by the density gradient in this article, isstudied. The linear simulation results are compared in thecases of the S and LB collision model operators. Using
the S collision model operator, the plasma instabilities
are found to be excited by the TEM and ITG mode at
ρ=0.47 and ρ=0.61 by the linear simulation results. Onthe other hand, only the TEM is unstable at ρ=0.81. The
ITG mode is excited for 0.026<keρe<0.052 at ρ=0.47 andfor 0.030<keρe<0.060 at ρ=0.65 only by using the Scollision operator. The mode changes from the TEM tothe ITG mode when the poloidal wave number increases,
where the electrons form the banana orbit and the ions donot. Using the LB collision model operator, the only
TEM is obtained. The residue level of zonal flows andthe zonal flow decay time [8] using the S collision
operator are found to be larger than those using the LB
collision operator. The turbulent transport is evaluated by
the nonlinear gyrokinetic simulation results. The effectsof zonal flows using two kinds of the collision operators
are studied in the nonlinear simulation results. In theprevious studies, the reduced models are constructed [9]for the ITG turbulence in helical plasmas. These reduced
models, which are constructed by the linear simulation
results, quickly reproduce the nonlinear simulation
results for the turbulent transport, thus, these reduced
models enable us to simulate the integrated transport
simulation in helical plasma [10]. The simulation results
for the TEM and ITG mode plasmas in this study are thebasis when the reduced models for the TEM and ITG
mode turbulence in tokamaks will be constructed.
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