Y. Xiao

Gyrokinetic particle simulation of beta-induced Alfvén eigenmode

The beta-induced Alfven eigenmode (BAE) in toroidal plasmas is studied using global gyrokinetic particle simulations. The BAE real frequency and damping rate measured in the initial perturbation simulation and in the antenna excitation simulation agree well with each other. The real frequency is slightly higher than the ideal magnetohydrodynamic (MHD) accumulation point frequency due to the kinetic effects of thermal ions. Simulations with energetic particle density gradient show exponential growth of BAE with a growth rate sensitive to the energetic particle temperature and density. The nonperturbative contributions by energetic particles modify the mode structure and reduce the frequency relative to the MHD theory. The finite Larmor radius effects of energetic particles reduce the BAE growth rate. Benchmarks between gyrokinetic particle simulation and hybrid MHD-gyrokinetic simulation show good agreement in BAE real frequency and mode structure.

Microturbulence in DIII-D tokamak pedestal. I. Electrostatic instabilities

Gyrokinetic simulations of electrostatic driftwave instabilities in a tokamak edge have been carried out to study the turbulent transport in the pedestal of an H-mode plasma. The simulations use annulus geometry and focus on two radial regions of a DIII-D experiment: the pedestal top with a mild pressure gradient and the middle of the pedestal with a steep pressure gradient. A reactive trapped electron instability with a typical ballooning mode structure is excited by trapped electrons in the pedestal top. In the middle of the pedestal, the electrostatic instability exhibits an unusual mode structure, which peaks at the poloidal angle θ=±π/2. The simulations find that this unusual mode structure is due to the steep pressure gradients in the pedestal but not due to the particular DIII-D magnetic geometry. Realistic DIII-D geometry appears to have a stabilizing effect on the instability when compared to a simple circular tokamak geometry.

New Paradigm for Turbulent Transport Across a Steep Gradient in Toroidal Plasmas

First principles gyrokinetic simulation of the edge turbulent transport in toroidal plasmas finds a reverse trend in the turbulent transport coefficients under strong gradients. It is found that there exist both linear and nonlinear critical gradients for the nonmonotonicity of transport characteristics. The discontinuity of the transport flux slope around the turning gradient shows features of a second order phase transition. Under a strong gradient the most unstable modes are in nonground eigenstates with unconventional mode structures, which significantly reduces the effective correlation length and thus reverse the transport trend. Our results suggest a completely new mechanism for the low to high confinement mode transition without invoking shear flow or zonal flow.

Verification of nonlinear particle simulation of radio frequency waves in tokamak

Nonlinear simulation model for radio frequency waves in fusion plasmas has been developed and verified using fully kinetic ion and drift kinetic electron. Ion cyclotron motion in the toroidal geometry is implemented using Boris push in the Boozer coordinates. Linear dispersion relation and nonlinear particle trapping are verified for the lower hybrid wave and ion Bernstein wave (IBW). Parametric decay instability is observed where a large amplitude pump wave decays into an IBW sideband and an ion cyclotron quasimode (ICQM). The ICQM induces an ion perpendicular heating, with a heating rate proportional to the pump wave intensity.

Nonlinear dynamics of beta-induced Alfvén eigenmode in tokamak

The beta-induced Alfven eigenmode (BAE) excited by energetic particles in toroidal plasmas is studied in the global gyrokinetic simulations. It is found that the nonlinear BAE dynamics depends on the deviation from the marginality. In the strongly driven case, the mode exhibits a bursting state with fast and repetitive chirping. The nonlinear saturation is determined by the thermal ion nonlinearity and has no clear dependence on the linear growth rate. In the weakly driven case, the mode reaches a nearly steady state with small frequency chirping. The nonlinear dynamics is dominated by the energetic particle nonlinearity. In both cases, the nonlinear intensity oscillation and frequency chirping are correlated with the evolution of the coherent structures in the energetic particle phase space. Due to the radial variation of the mode amplitude and the radially asymmetric guiding center dynamics, the wave-particle interaction in the toroidal geometry is much more complex than the conventional one-dimensional...

Turbulent transport of toroidal angular momentum in fusion plasmas

Global nonlinear gyrokinetic simulations of ion temperature gradient (ITG) and collisionless trapped electron mode (CTEM) turbulence find significant spinning up of a plasma in the directions opposite for CTEM and ITG turbulence. The outward momentum convection by the particle flux could be strong enough to overcome the inward momentum pinch and reverse the radial direction of the convective momentum flux. Momentum pinch velocity shows no explicit dependence on background temperature, while it is significantly affected by steepening the background density. Convective momentum fluxes are generally smaller in the CTEM turbulence than the ITG turbulence, while the intrinsic Prandtl number is similar or slightly larger in the CTEM turbulence.

Properties of toroidal Alfvén eigenmode in DIII-D plasma

Linear properties of the toroidal Alfven eigenmode (TAE) excited by energetic particles (EP) in a DIII-D tokamak experiment have been studied in global gyrokinetic particle simulations treating self-consistently kinetic effects of EP, thermal ions, and electrons. Simulation results of the TAE frequency and mode structure agree very well with the experimental measurements. The non-perturbative EP contribution induces a radial localization of the TAE mode structure, a break-down of mode radial symmetry, as well as a frequency dependence on the toroidal mode number. The simulations further demonstrate the dependence of the growth rate and mode structure on EP pressure gradients. The in-out asymmetry of the mode structure and the experimental identification of the poloidal harmonics have also been clarified.

Parallel ion compressibility effects on kinetic ballooning mode for different magnetic shears

Various gyrokinetic simulations suggest that the kinetic ballooning mode (KBM) instability is sensitive to the numerical implementation of equilibrium magnetic configuration in tokamaks. In this work, the gyrokinetic code GTC is employed to investigate the KBMs sensitivity to equilibrium plasma profiles. An outward radial shift of the radial mode is found for the normal magnetic shear case, but there is no shift if the shear is negative. The simulation results are explained by a linear eigenmode theory. It is found that the observed phenomenon is an effect of the parallel ion compressibility.

Gyrokinetic simulation of dissipative trapped electron mode in tokamak edge

The gyrokinetic simulation using the gyrokinetic toroidal code (GTC) is carried out for the dissipative trapped electron mode (DTEM), which is an important source for the electrostatic turbulence in the pedestal of tokamak plasmas. The DTEM instability is identified for the edge plasmas, and its dependence on the wavelength and collisional frequency is obtained by both simulation and theory. It is shown for the first time that the linear gyrokinetic simulation results are fully consistent with that from the analytic theory with edge parameters. This suggests that the GTC code can simulate accurately the DTEM instability in the pedestal. It provides a useful benchmark for verifying gyrokinetic simulation of edge plasmas.

Microturbulence in DIII-D tokamak pedestal. III. Effects of collisions

Gyrokinetic simulations of the H-mode pedestal in DIII-D discharge 145701 find that the kinetic ballooning mode (KBM) is the most unstable mode for low toroidal numbers (n ≤ 25) and that the trapped electron mode (TEM) dominates over the KBM at higher toroidal mode numbers for realistic pressure gradients in the pedestal. Collisions reduce the TEM growth rate but have little effects on the KBM. KBM has the conventional ballooning mode structure peaking at the outer mid-plane, while TEM has an unconventional mode structure peaking at the top and bottom of the poloidal plane.