Properties of freely decaying and driven turbulence of fusion plasmas using gyrokinetic particle simulation
In a recent work, the effects of nonlinear self-consistent profile relaxation on the ITG turbulence in global delta-f particle simulations were studied [1] using Gyrokinetic Toroidal Code (GTC) [2]. In contrast to several other gyrokinetic turbulence models, GTC shows that the turbulence is insensitive to minute or drastic profile changes and reaches the similar flux levels irrespective of whether or not it is a driven or freely decaying turbulence. In the present work, we address the spectral properties of gyrokinetic turbulence and argue that correct inclusion of parallel velocity space nonlinearity may bring forth certain important late-time features such as Geodesic Acoustic Modes like (GAM) structures.
Nonlinear turbulent transport in magnetic fusion plasmas
For more than a decade, the study of microturbulence driven by ion temperature gradient (ITG) drift instabilities in tokamak devices has been an active area of research in magnetic fusion science for both experimentalists and theorists alike. An important impetus for this avenue of research was the discovery of the radial streamers associated with the ITG modes in the early 1990s using the particle-in-cell (PIC) simulation method. Subsequently, ITG simulations using codes with increasing realism have been made possible by the dramatic increase in computing power. Notable examples were the demonstration of the importance of nonlinearly generated zonal flows in regulating ion thermal transport and the transition from Bohm to gyroBohm scaling with increased device size. In this paper, we will describe an interesting nonlinear physical process, as well as the resulting turbulent transport, that is associated with the interactions between the nonlinear parallel acceleration of the ions and the zonal flow modes. This study was carried out by utilizing a fully parallelized three-dimensional PIC code in global toroidal geometry on the most advanced, modern, massively parallel supercomputers.
Role of nonlinear toroidal coupling in electron temperature gradient turbulence
Global gyrokinetic particle simulation and nonlinear gyrokinetic theory find that electron temperature gradient (ETG) instability saturates via nonlinear toroidal coupling, which is a nonlocal interaction in the wave vector space that transfers energy successively from unstable modes to damped modes preferentially with lower toroidal mode numbers. The electrostatic ETG turbulence is dominated by nonlinearly generated radial streamers. The length of the streamers scales with the device size, which is longer than the distance between mode rational surfaces and electron radial excursions. Both fluctuation intensity and transport level at saturation are independent of the streamer length, and are much smaller than the mixing length estimates.