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.
Gyrokinetic particle simulation of fusion plasmas: path to petascale computing
Gyrokinetic particle simulation of fusion plasmas for studying turbulent transport on state-of-theart computers has a long history of important scientific discoveries. The primary examples are: (i) the identification of ion temperature gradient (ITG) drift turbulence as the most plausible process responsible for the thermal transport observed in tokamak experiments; (ii) the reduction of such transport due to the presence of zonal flows; (iii) the confinement scaling trends associated with size of the plasma and also with the ionic isotope species. With the availability of terascale computers in recent years, we have also been able to carry out simulations with improved physics fidelity using experimentally relevant parameters. Computationally, we have demonstrated that our lead Particle-in- Cell (PIC) code, the Gyrokinetic Turbulence Code (GTC), is portable, efficient, and scalable on various MPP platforms. Convergence studies with unprecedented phase-space resolution have also been carried out. Since petascale resources are expected to be available in the near future, we have also engaged in developing better physics models and more efficient numerical algorithms to take advantage of this exciting opportunity. For the near term, we are interested in understanding some basic physics issues related to burning plasmas experiments in International Thermonuclear Experimental Reactor (ITER) - a multi-billion dollar device to be constructed over the next decade. Our long range goal is to carry out integrated simulations for ITER plasmas for a wide range of temporal and spatial scales, including high-frequency short-wavelength wave heating, low-frequency meso-scale transport, and low-frequency large scale magnetohydrodynamic (MHD) physics on these computers.