Gyrokinetic Simulations Of Electric Current Driven By Turbulence In Tokamak Plasmas

The researches of the past decades have shown that the drift waves are the prominent candidates for the anomalous transport of particles,energy and momentum in the magnetic fusion devices.Because of the large ion-to-electron mass ratio,the momentum transport of electrons and ions have different effects on plasma confinement.The self-driven toroidal rotation in tokamak has been intensively researched,which is related to the turbulence-induced ion momentum transport.Even though the electron momentum transport has negligible impacts on the plasma rotation,it may significantly affect the plasma current in tokamaks.It was proposed firstly in 1988 that the turbulence produces the current by changing the electron parallel momentum.In recent years,the self-regulation system which includes turbulence,turbulence-induced current and the kink mode has been observed on the EAST device.Turbulence-induced current changes the plasma current density profile significantly and affects the magnetohydrodynamics(MHD)instabilities which provides a way to study the interaction between the turbulence and the MHD instabilities.Therefore,it is necessary to study the physical mechanism of turbulence-induced current and the turbulence-induced current density profile for the plasma confinement.In this work,the physical mechanisms and amplitude of the current induced by ion temperature gradient(ITG)turbulence and the collisionless trapped electron mode(CTEM)turbulence are studied systematically with the gyrokinetic code GEM.The first part of this work is the investigation of the physical mechanisms and magnitude of the ITG turbulence-induced current in the electrostatic and collisionless limits.It has been shown by the gyrokinetic simulations that the turbulence corrugates the current density profile by two physical mechanisms which is consistent with other works.One is the electron Reynold stress(also called electron momentum flux),which redistributes the current density profile but cannot produce a net current.The other is the turbulence acceleration(also called electron-ion momentum exchange),which can produce a net current.In the linear stage,the electron Reynold stress is proportional to the gradient of turbulent intensity,which grows rapidly near the rational surface,and its divergence has a fine-scale structure with a width about several ion Larmor radius.It has been demonstrated by the comparison between the magnitude of the divergence of electron Reynold stress and the turbulence acceleration that the electron Reynold stress plays a major role in the turbulence-induced current.The simulations that include multiple toroidal modes(multi-n simulation)are adopted to calculate the amplitude of the ITG turbulence-induced current.Multi-n simulation results show that the ITG turbulence-induced current changes the bootstrap current profile significantly near the rational surfaces.Especially,in the outer region of the tokamak,the amplitude of turbulence-induced current can reach 150%of the amplitude of the bootstrap current density.In the electrostatic limit,the magnitude of the turbulence-induced current is proportional to the plasma density in tokamak devices.The second part of this work is the investigation of the current induced by CTEM instability in the electrostatic limit.It has been shown by analyzing the perturbed electron distribution function that the barely passing electrons play a key role in the current generation in determining the magnitude and direction of the current density.The even parity component of the current phase space structure along parallel velocity direction leads to the net current.Two characteristic radial scales of the current density are separated by the wavelet transform method.One is the fine structure(a few ion Larmor radii)which is induced by the electron-wave resonance near the rational surface and the symmetry breaking of the turbulence intensity.The other is the mesoscale structure(tens of ion Larmor radii)which is related to the zonal flow shear.For the weakly driven CTEM case,the turbulence-induced current is dominated by the fine structures near the rational mode surfaces and the zonal flow shear has negligible effects on current generation.For the strongly driven CTEM case,the zonal flow shear regulates the current generation significantly for high n modes,while for low n modes,the fine structure of current density near the rational mode surfaces is dominant.For the multi-n nonlinear simulations of the strongly driven case,the current density structure is determined by the synergistic efects of zonal flow shear in the plasma(related to the mesoscale structure)and the symmetry breaking near rational mode surfaces(related to the fine scale structure).It is shown by the multi-n simulations that the CTEM turbulence-induced current density is about 50%of the bootstrap current density in magnitude near the rational surfaces for the strongly driven CTEM case.In conclusion,it has been shown by the gyrokinetic simulations that the turbulenceinduced current has a great influence on the local plasma current density profile.This may bring in impacts on the MHD instabilities,such as the neoclassical tearing modes.In the future fusion reactors,the effects of the turbulence-induced current need to be carefully considered.