Turbulent transport of heat and particles in a high ion temperature discharge of the Large Helical Device

Abstract

Turbulent transport in a high ion temperature discharge of the Large Helical Device (LHD) is investigated by means of electromagnetic gyrokinetic simulations, which include kinetic electrons, magnetic perturbations, and full geometrical effects. Including kinetic electrons enables us to firstly evaluate the particle and the electron heat fluxes caused by turbulence in LHD plasmas. It is found that the electron energy transport reproduces the experimental result, and that the particle flux is negative. The contribution of magnetic perturbation to the transport is small because of very low beta. The turbulence is driven by the ion temperature gradient (ITG) instability, and the effect of kinetic electrons enhances the growth rate larger than that from the adiabatic electron calculation. The ion energy flux is larger than that observed in the experiment, while the flux is close to the experimental observation when the temperature gradient is reduced 20% in the simulation. This significant sensitivity of the energy flux implies that the profile in the experiment is close to the critical temperature gradient. The critical gradient for turbulent energy flux is similar to that for the linear instability, i.e., the Dimits shift is small. This is because the zonal flow in the LHD is weaker than that in tokamaks.

A Reduced Transport Model for Ion Heat Diffusivity by Gyro-Kinetic Analysis with Kinetic Electrons in Helical Plasmas

“…The values ofχ i by the simulation with the kinetic electron are from two to three times larger than those by the simulation The normalized electron and ion temperature gradients are artificially altered at 0.8 and 1.2 times the experimental values at ten radial points. This is because the ion energy flux decreases with the reduced gradient by 20% and close to the experimental value [22]. The parameter range in which the nonlinear simulation has been performed is shown in Table 1.…”

“…The averaged value of the ion energy flux in the time interval 50 < t < 100 is 0.12 MW/m 2 . In [22], the averaged value of the ion energy flux in the time interval 50 < t < 80 is about 0.13 MW/m 2 , when the Fourier mode numbers in thek x andk y directions are 169 and 43. Even if the number of the Fourier modes is small, the close value of the ion energy flux is obtained in this study.…”

“…How to apply the reduced model of the turbulent ion heat diffusivity from the gyro-kinetic simulation with the adiabatic electron to the transport code has been shown in helical plasmas [21]. The simulation with the kinetic electron shows the larger ion energy flux than the experimental results in the high-T i LHD discharge [22]. On the other hand, the electron and ion energy fluxes of the simulation results with the kinetic electron are close to those of the experimental results in the low-T i #88343 discharge [23].…”

“…The simulation result with the adiabatic electron in this low-T i discharge shows that the ITG mode becomes stable around ρ(= r/a) = 0.5. The effect of the kinetic electron induces the enhancement of the linear growth rate of ITG modes [4,22]. To show the reduced transport model for the turbulent ion heat diffusivity, the effect of the kinetic electron on the plasma instability should be included.…”

“…To reduce computer resources and perform the nonlinear analysis in the wide plasma parameter region, we take the smaller number for the Fourier modes and the smaller number of the grid points in this simulation than in the case in [22]. The total Fourier mode numbers in thek x and k y directions are 9 and 6 in the regions −0.5 ≤k x ≤ 0.5 and 0.0 ≤k y ≤ 0.5, wherek x (= k x ρ i ) andk y (= k y ρ i ) are the radial and poloidal wavenumbers.…”