Gyrokinetic deltaf particle simulation of collisionless trapped electron mode and toroidicity-induced Alfven eigenmode.

摘要

In toroidal magnetically confined plasmas, the turbulent transport driven by collisionless trapped electron modes (CTEM) is systematically investigated using three-dimensional gyrokinetic deltaf particle-in-cell simulations. Scalings with local plasma parameters are studied. Mode coupling theory and gyrokinetic turbulence simulation are used to study the nonlinear saturation mechanisms of CTEM turbulence. Turbulence simulations show that the importance of zonal flow is parameter sensitive, but is well characterized by the E x B shearing rate formula. The importance of zonal flow is found to be sensitive to temperature ratio, magnetic shear and electron temperature gradient. For parameter regimes where zonal flow is unimportant, zonal density (a purely radial density perturbation) is generated and is found to be the dominant saturation mechanism. In fact, CTEM turbulence saturates at physically reasonable levels with or without zonal flow. This is in stark contrast to ion-temperature-gradient driven turbulence where the zonal flow has an order of magnitude effect on the saturation level. A toroidal mode coupling theory is developed that agrees well with simulation in the initial nonlinear saturation phase (before fully developed turbulence ensues). The theory predicts nonlinear generation of the zonal density and then the feedback and nonlinear saturation of the unstable mode. The spectrum change from the linear stage to the nonlinear stage is also observed in CTEM turbulence and is reported here.;We have also utilized the gyrokinetic deltaf particle-in-cell code to investigate the toroidicity-induced Alfven eigenmode (TAE) in tokamak plasmas. The gyrokinetic code is reduced to a two-fluid model and is successfully benchmarked with an analytical theory [1] and an MHD eigenmode analysis for TAE mode frequency and mode structure. After the gyrokinetic energetic particles are added, the TAE is driven unstable. The scaling of the growth rate with the central energetic particle pressure agrees well with the MHD theory.