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Energy exchange between electrons and ions by ion temperature gradient turbulence

Tetsuji Kato et al. summarized their research as “Energy exchange between electrons and ions by ion temperature gradient turbulence,” and their paper has been published in Issue 6 of Physics of Plasmas. In addition, this paper has been selected as an Editor’s Pick.

【Abstract】
Microturbulence in magnetic confined plasmas contributes to energy exchange between particles of different species as well as the particle and heat fluxes. Although the effect of turbulent energy exchange has not been considered significant in previous studies, it is anticipated to have a greater impact than collisional energy exchange in low collisional plasmas, such as those in future fusion reactors. In this study, gyrokinetic simulations are performed to evaluate the energy exchange due to ion temperature gradient (ITG) turbulence in a tokamak configuration. The energy exchange due to the ITG turbulence mainly consists of the cooling of ions in the grad-B curvature drift motion and the heating of electrons streaming along a field line. It is found that the ITG turbulence transfers energy from ions to electrons regardless of whether the ions or electrons are hotter, which is in marked contrast to the energy transfer by Coulomb collisions. This implies that the ITG turbulence should be suppressed from the viewpoint of sustaining the high ion temperature required for fusion reactions since it prevents energy transfer from alpha-heated electrons to ions as well as enhancing ion heat transport toward the outside of the reactor. Furthermore, linear and nonlinear simulation analyses confirm the feasibility of quasilinear modeling for predicting the turbulent energy exchange in addition to the particle and heat fluxes.

Figure: The wavenumber spectra of terms in the entropy balance equation for electrons (a) and ions (b) in the case of Te/Ti=1.0. They are evaluated in the steady state of turbulence obtained by nonlinear simulation. The ratios of the turbulent particle and heat transport fluxes and the turbulent energy exchange to the squared amplitude of the electrostatic potential obtained by linear and nonlinear simulations are shown for electrons (c) and ions (d). The nonlinear entropy transfer terms are negative in the wavenumber regions colored in sky blue. Dashed lines in (c) and (d) represent the ratios obtained by linearly unstable modes with kx=0.

The numerical simulations were performed on Plasma Simulator of NIFS.

【Publication】

T. Kato, H. Sugama, T.-H. Watanabe, and M. Nunami, “Energy exchange between electrons and ions in ion temperature gradient turbulence”, Phys. Plasmas 31, 062510(2024).

DOI: 10.1063/5.0204022

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