M. Wakatani

Y. Idomura

National Institute for Fusion Science (NIFS-Repository)

§33. Gyrokinetic Particle Simulation of ETG Mode in Negative Shear Tokamaks Wakatani, M., Idomura, Y. (Grad. School of Energy Sci., Kyoto Univ.) We have studied nonlinear dynamics of the ETG (electron temperature gradient) turbulence using a gyrokinetic finite element PIC code developed at Kyoto University. In a negative-sheared slab configuration, simulations are performed for non-resonant single-helicity perturbations, because the nonresonant NS (negative shear)-ETG mode is considered to playa significant role in the electron anomalous transport based on the mixing length estimate [1]. The principal results observed in the nonlinear simulation of the nonresonant NS-ETG mode are summarized as follows: (a) in the linear growth phase, a radially elongated vortex struc-ture predicted by the linear theory appears, (b) in the initial saturation phase, a saturation of the ETG mode is produced by an inverse (normal) wave energy cascade in the ky(kx ) space, which tends to generate E x B shear flow, (c) after the saturation of the ETG mode, the secondary insta-bility followed by a generation of the Er x B zonal flows occurs and the Er x B zonal flow region is extended in the radial direction, and (d) in the quasi-stationary phase, the quasi-steady Er x B zonal flows, which are stable to the K-H (Kelvin-Helmholtz) mode, is sustained, and a remarkable reduction of Xe (electron thermal transport) is ob-served in the Er x B zonal flow region. The observed Er x B zonal flow profile has a large amplitude VErxB rv 0.OI5vT; only in finite magnetic shear region in both sides of the qrnin-surface, although an inverse energy cascade in the ky space is observed also in a region of the qrnin-surface. Thus, the Er x B zonal flow profile is closely related to the q-profile. From the linear stability analysis of the K-H mode, it is found that the parallel electron dynamics, which comes from 342 an effect of the magnetic shear, has a stabilizing effect on the K-H mode. The observed Er x B zonal flow profile may be explained by the local critical Er x B flow velocity determined by the magnetic shear or the finite kll stabilization. It is considered that the K-H mode playa critical role in the formation of the Er x B zonal flow in the ETG turbulence. In the numerical result shown in Fig. 1, the quasi-steady Er x B zonal flow decays by changing the q-profile to reduce the magnetic shear. This result indicate that the K-H mode plays a role to destroy the Er x B zonal flow in a collisionless plasma. Thus it is considered that a quasi-steady Er x B zonal flow is determined by a competition between a flow generation process due to an in-verse energy cascade in the ky space and a flow destruction due to the K-H mode. 0.005 0.004 ~ 0.003 4' ~ 0.002 0.001 0 0 ,~ /V\ ! : : ; __ I"; 2000 k"pu==O.OOO --k;,P,e==0.086 .......... . 4000 I f2ci 6000 Fig.I. Time history of the ky of electrostatic poten-tial fluctuations, which is averaged over the Er x B zonal flow region. Here the zonal flow (kyPte = 0) and the dominant fluctu-ation (kyPte = 0.086) are shown. The q-profile is changed from Lne/ Lns = 0.609 to Lne/ Lns = 0.304 at to'i = 4.183, where Lne(Lns = J(2q~inR)/(q~inrrnin)) is a char-acteristic length of density gradient (mag-netic shear). Reference 1) Idomura, Y., Tokuda, S. and Wakatani, M., to be published in Phys. Plasmas 7 (2000) 

Gyrokinetic Particle Simulation of ETG Mode in Negative Shear Tokamaks

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