Phase-space structure in plasma turbulence
Résumé
Plasma turbulence driven by the ion temperature gradient (ITG) is theoretically studied with high-resolution Eulerian kinetic simulations. A spectral analysis of the velocity distribution function in the slab ITG turbulence clarifies how the entropy variable associated with the fine-scale structure of the distribution function is produced by the turbulent heat transport in the presence of the temperature gradient, transferred from macro to microscales in the velocity space through phase-mixing processes, and dissipated by collisions. The entropy spectral function is analytically derived and confirmed by the simulation result. It is shown that the entropy spectrum obeys a power law in the range that is free from instability sources and collisional dissipation. The Eulerian gyrokinetic simulation of the toroidal ITG turbulence yields the ion thermal diffusivity in the steady turbulent state, in which the balance between the entropy production by the ion thermal transport and the collisional dissipation is verified. A formula for a long time behavior of the zonal flow potential in helical systems is analytically derived, by which collisionless zonal flow dynamics in tokamaks and helical plasmas are compared. A good agreement between the formula and the gyrokinetic simulation results is obtained.
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