Transition of vortex structure and transport in slab ETG turbulence

Full text: Electron temperature gradient (ETG) turbulence is considered as one of the possible candidates causing the anomalous electron heat transport in a core region of magnetic fusion plasmas. Although the ETG mode with the adiabatic ion response is isomorphic to the ion temperature gradient (ITG) mode, the nonlinear saturation mechanism of the ETG turbulence is considered to be different from that in the ITG turbulence, and it has been still investigated. In this study we have carried out gyrokinetic Vlasov simulations of the ETG turbulence in a two-dimensional slab geometry with high phase-space resolution. Then, we found spontaneous transition of vortex structures from turbulent to coherent states accompanied with significant reduction of the electron heat flux. In the coherent state (t=3000LT /vte), the amplitude of potential fluctuation is nearly the same as that in the turbulent state (t=1250 LT/vte) while levels of the turbulent transport and fine-scale potential fluctuations decrease significantly. We have also found that the transport reduction is related to the phase matching of the potential and temperature fluctuations rather than the changes of each amplitude. Furthermore, it is found that the coherent vortex structure is approximately described by a traveling wave solution of Hasegawa-Mima equation including temperature gradient derived from the gyrokinetic equation. More detailed results including the parameter dependence of the transition onset will be also reported. (author)