Reduction of Turbulent Transport with Zonal Flows Enhanced in Helical Systems

T.-H. Watanabe, H. Sugama, S. Ferrando-Margalet
2008 Physical Review Letters  
Gyrokinetic Vlasov simulations of the ion temperature gradient turbulence are performed in order to investigate effects of helical magnetic configurations on turbulent transport and zonal flows. The obtained results confirm the theoretical prediction that helical configurations optimized for reducing neoclassical ripple transport can simultaneously reduce the turbulent transport with enhancing zonal-flow generation. Stationary zonal-flow structures accompanied with transport reduction are
more » ... y identified by the simulation for the neoclassically optimized helical geometry. The generation of the stationary zonal flow explains a physical mechanism for causing the confinement improvement observed in the inward-Magnetically confined toroidal plasmas involve two distinctive transport processes: that is, the neoclassical and turbulent diffusions. The neoclassical transport is strongly influenced by geometry of the toroidal field and is especially important for determining confinement properties in helical systems [1], where particles trapped in helical ripples cause significant radial particle and heat fluxes. The turbulent transport driven by microinstabilities [2] are widely observed in fusion plasma experiments while the E B zonal flows have been investigated in numerous theoretical, numerical, and experimental studies as an attractive mechanism for regulating the turbulent transport [3] [4] [5] [6] . So far, the two transport processes have been separately treated in conventional frameworks although it has been argued in recent theories that there exists a close relation between the neoclassical and anomalous transport through generation of zonal flows in helical systems [7] [8] [9] [10] [11] . Gyrokinetic theoretical studies of zonal flows driven by the ion temperature gradient (ITG) turbulence [7-9] show that a high-level zonal flow can be maintained for a longer time by reducing bounce-averaged radial drift velocity of helical-ripple-trapped particles. This means that optimization of the three-dimensional magnetic configuration for reducing the neoclassical ripple transport [12 -14] simultaneously enhances the residual zonal flows which can lower the anomalous transport as well [7] [8] [9] [10] [11] . In fact, it is observed in the Large Helical Device (LHD) [15] experiments that the anomalous transport decreases in the inward-shifted plasma configuration optimized for reducing the neoclassical transport [16] . Thus, it is desired to validate the theoretical prediction of the transport reduction in the neoclassically optimized helical configurations by gyrokinetic simulations. Preliminary simulations of the ITG turbulence and the zonal flows in helical systems were performed [17] by means of the gyrokinetic Vlasov simulation code GKV [18] and demonstrated stronger generation of zonal flows in a model configuration for the inward-shifted LHD plasma. However, the stationary zonal-flow structure effectively regulating the turbulent transport was not clearly observed, and the obtained i in the inward-shifted case was higher than that in the standard one in contrast to the LHD experimental results. This is attributed to discrepancies in the linear ITG growth rates and the zonal-flow response between the numerical and experimental conditions. In our recent study [9] on the linear ITG stability and the zonal-flow response relevant to the LHD experiments, we have employed more accurate confinement field models. Stabilizing effects of the smaller safety factor and the stronger magnetic shear associated with the inward plasma shift decrease the difference of the ITG mode growth rates between the two cases. Also, specified changes in the helical field components, the safety factor, and the aspect ratio clearly show that an initially given zonal flow keeps a higher level for a longer time in the inward-shifted configuration than that in the standard one. Both favorable changes in the zonal-flow response and the linear stability are expected to contribute to reduction of the ITG turbulent transport in the inward-shifted LHD configuration. Indeed, in this Letter, the nonlinear GKV simulation implemented with the relevant magnetic field parameters confirms generation of large zonal flows enough to reduce the ion heat transport in the inward-shifted plasma. Stationary zonalflow structures are clearly shown in the present simulation for the helical system with the neoclassical optimization. The obtained results agree with our theoretical analysis and are consistent with observation of better confinement in the inward-shifted LHD plasma [16] . The nonlinear gyrokinetic equation for the perturbed ion gyrocenter distribution function in the low-electrostatic limit is numerically solved by the GKV code [18] as a partial differential equation defined on the fivedimensional phase space. We introduce a collision model given by the gyrophase average of the Lenard-Bernstein
doi:10.1103/physrevlett.100.195002 pmid:18518454 fatcat:6ggsk4vzpncpxcvuw5hhs626qa