Relation among ITG Turbulence, Zonal Flows, and Transport in Helical Plasmas

Abstract

Relation among the ion temperature gradient (ITG) turbulence, zonal flows, and the transport in helical plasmas is investigated by nonlinear gyrokinetic simulations. Local gyrokinetic simulations for helical field configurations are carried out employing various parameters such as the density and temperature gradients and local shears. From the simulation results, we construct a simple model function to represent ion heat diffusivity in terms of the turbulent fluctuations and zonal flow amplitude in helical plasmas. Turbulent transport has been considered to be one of the most critical issues in the magnetically confined fusion plasma research. For the study of plasma confinement properties, it is very important to evaluate the transport level associated with turbulence caused by microinstabilities such as the ion temperature gradient (ITG) mode. To date, a large number of gyrokinetic simulation studies have been made for clarifying the turbulent transport physics [1]. Owing to recent great progress in the development of computational resources, direct comparisons of numerical simulation with experimental data are possible. However, only a few validations of the gyrokinetic simulations in helical systems such as the Large Helical Device (LHD) [2] have been performed, in contrast to the case for tokamaks in which studies against the experiments have been extensively promoted [3]. This is because gyrokinetic simulations for non-axisymmetric systems require a higher spatial resolution than those for tokamaks. In addition to the physical clarification and validation, it is also necessary to construct a reduced transport model for integrated simulations [4]. Therefore, we should clarify the intermediate relation bridging the turbulence simulation and transport modeling.In our previous paper [5], nonlinear local flux-tube gyrokinetic Vlasov simulations of ITG turbulent transport were carried out by using the GKV-X code [6] for the high ion temperature (high-T i ) LHD plasma in shot number 88343 [7]. The simulation results reproduced the turbulent ion heat transport level and the wavenumber spectra of the density fluctuations observed in the experiment. Figure 1 shows that the simulation results are in reasonable agreement with the experimental values of turbulent ion heat

A reduced model for ion temperature gradient turbulent transport in helical plasmas

“…In our previous papers, 3,16 we performed gyrokinetic Vlasov simulations for ITG turbulent transport in the Large Helical Device 17 (LHD) plasma with the high ion temperature 18 by using the local flux-tube code, GKV-X. 19 In the GKV-X simulations, the nonlinear gyrokinetic equation for the perturbed ion gyrocenter distribution function 20,21 in the low-b electrostatic limit…”