H. Sugama

K." Tanaka

M. "Nunami

T.-H. Watanabe

National Institute for Fusion Science (NIFS-Repository)

Turbulent transport of the Large Helical Device(LHD) plasmas are investigated by means of nonlin-ear gyrokinetic simulations of ion temperature gradient(ITG) turbulence. In the present studies, the simulationswith full geometrical eﬀects of the LHD conﬁguration arecarried out, in which the transport levels comparable tothe experimental results are reproduced and reasonableagreements are also found in the wavenumber spectraof the density ﬂuctuations. Numerical analysis of theturbulence spectra on the two-dimensional perpendicu-lar wavenumber space clariﬁes the spectral transfer intoa high radial wavenumber region through the interac-tion with zonal ﬂows, which correlates with the turbulenttransport reduction. It is now widely recognized that theITG turbulent transport is determined by the competi-tive interaction between turbulence and zonal ﬂows. Theresultant transport levels are expressed in terms of a sim-ple linear relation between the transport coeﬃcient andthe ratio of the squared turbulent potential ﬂuctuationto the averaged zonal ﬂow amplitude.In linear gyrokinetic simulations1) in the LHD highion temperature discharge #883432), it was found thatthe density ﬂuctuations measured by phase contrastimaging (PCI) in the experiment have large amplitudesfor the radial positions and the poloidal wavenumberswhich correspond to ITG modes having large growthrates. Applying the gyrokinetic Vlasov ﬂux-tube codeGKV-X3), the nonlinear ITG turbulent transport simula-tions with adiabatic electrons are performed and the sat-uration levels of the turbulent ﬂuctuations, zonal ﬂows,and ion heat transport in the LHD discharge are evalu-ated. The radial proﬁle of the electron density is so ﬂat-tened in the discharge that the trapped electron mode isconsidered to be stabilized. A snapshot of the electro-static potential ﬂuctuations obtained from the simula-tions is shown in Fig.1, and a comparison of the ion heatﬂuxes Pi obtained from the simulations and the LHDexperiment is performed as in Fig.2. The simulation re-sults agree well with the anomalous part of experimentalPi, such that the observed diﬀerences are less than 10%for ρ = 0.46, 0.65, and about 30 % for ρ = 0.83. TheITG turbulence simulations also have shown the poloidalwavenumber spectrum of the potential ﬂuctuation whichis similar to that of the density ﬂuctuation given by thePCI measurements within experimental ambiguities inlow wavenumber regions4).Self consistent interaction between turbulence andzonal ﬂows causes the resultant transport level. In thespectral analyses of the squared potential ﬂuctuations inthe two-dimensional perpendicular wavenumber space,spectra for the experimental case in outer radial re-gion and the optimized LHD conﬁguration case (inward-shifted case) spread broader in the high radial wavenum-ber region which is caused by the spectral transfer fromthe low- to the high-radial wavenumber space throughthe interaction between zonal ﬂows and turbulence5).The higher wavenumber modes in the spreading spec-trum make less contribution to the transport while theystill contribute to the integrated power spectrum. In-deed, larger zonal ﬂows causing the spectrum spreadingare generated for the experimental case in outer radialregion and inward-shifted case. Correlations of the re-sultant transport level on the turbulence and the zonalﬂows are also found that any simulation results includingthe inward-shifted case are well represented by the rela-tion, χi/χGBi ∝ T /Z1/2, where the squared turbulent po-tential T = (1/2)∑kx,ky =0�|eφkx,kyR0/Tiρti|2� and thezonal ﬂow potential Z = (1/2)∑kx�|eφkx,0R0/Tiρti|2�.This relation is useful for modeling of the turbulent ionheat diﬀusivity applicable to the transport code analysesby estimating T and Z including eﬀects of equilibriumproﬁles and magnetic conﬁgurations.Fig. 1: Snapshot of the perturbed electrostatic poten-tials obtained from GKV-X simulations in the LHD high-Ti discharge.0.0 0.2 0.4 0.6 0.8 1.00.01.02.03.0ρP i(MW)ExperimentSimulationAnomalousFig. 2: Radial proﬁles of ion heat ﬂux obtained fromthe experiment (dotted curve) and the GKV-X simula-tions (symbols). The solid curve represents the anoma-lous part of the experimental result.1) M. Nunami et al., Plasma Fusion Res. 6 (2011)1403001.2) K. Tanaka et al., Plasma Fusion Res. 5 (2010) S2053.3) M. Nunami et al., Plasma Fusion Res. 5 (2010) 016.4) M. Nunami et al., Phys. Plasmas. 19 (2012) 042504.5) M. Nakata et al., Phys. Plasmas. 19 (2012) 022303.348§7. Turbulent Transport Simulations of a High Ion Temperature LHD PlasmaNunami, M., Watanabe, T.-H., Sugama, H.,  Tanaka, K.

Turbulent Transport Simulations of a High Ion Temperature LHD Plasma

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