Formation of coherent vortex streets and transport reduction in electron temperature gradient driven turbulence

Vortex structures in slab electron temperature gradient (ETG) driven turbulence are investigated by means of a gyrokinetic simulation with high phase-space resolution. Depending on parameters that determine the eigenfrequency of the linear ETG modes, two different flow structures, i.e., statistically steady turbulence with a weak zonal flow and coherent vortex streets along a strong zonal flow, are observed. The former involves many isolated vortices and their mergers with complicated motion and leads to steady electron heat transport. When the latter is formed, phase difference and high wavenumber components of potential and temperature fluctuations are reduced, and the electron heat transport decreases significantly. It is also found that the phase matching with the potential fluctuation is correlated with the reduction in the imaginary part of the perturbed distribution function, and it occurs not only for the temperature fluctuation but also for any nth velocity moments. A traveling wave solution of a Hasegawa-Mima type equation derived from the gyrokinetic equation with the ETG agrees well with the coherent vortex streets found in the slab ETG turbulence.