Time: Tuesday, Apr. 7, 10:00 am
Venue: Online
Speaker: Shengming Li
Title: Nonlinear Gyrokinetic Simulations of Isotope Effect on ITG-TEM Driven Turbulent Transport
Isotope effect on the turbulent transport is an important issue in toroidal burning plasmas for several decades. The scaling of the confinement with the isotope mass A_i have been studied experimentally on many devices. Gyro-Bohm scaling predicts that the turbulent diffusivity of ion energy flux increases with √(A_i ), which is in contrast to experiments.
In this work, nonlinear simulations using Gyrokinetic Toroidal Code (GTC) are carried out to study isotopic dependence of hydrogen isotope (H, D and T) on typical electrostatic turbulence driven by ion temperature gradient (ITG) mode and trapped electron mode (TEM). Simulations show that in linear growth stage, the TEM instability grows very fast with wavelength shorter than that of ITG. It is also found that the growth rate γ for energy flux increases with A_i in TEM but decreases in ITG. In nonlinear stage, both TEM and ITG saturate at long wavelength and the saturation level has a strong isotope dependence between H, D and T when kinetic electrons are retained in the simulation. The average radial wave vector of the zonal flow
Speaker: Tyler E. Bagwell
Title: Shock and Discontinuity Formation in the Front of an Expanding Coronal Magnetic Arcade in Two-Fluid Simulations
Abstract:
Dynamic evolution of a magnetic arcade associated with photospheric shearing motions is investigated by an ideal two-fluid (electron-ion) code. The two-fluid numerical simulations produce conspicuous differences compared to earlier ideal and resistive MHD simulations beyond the inner arcade region. The decoupling motion between electrons and heavier ions during the arcade expansion induces a growing charge separation and strong electric field gradients in the front of the expanding arcade. These electric fields provide an additional force, along with the magnetic and thermal pressures, that drive the growth of propagating discontinuities in the plasma and field values that eventually exceed the phase velocity of the MHD fast mode and evolve into perpendicular fast-like shocks. After the outflow velocity of the ions exceeds that of the electrons in the shock, preferential heating of the ions is observed. Finally, parameter tests indicate that (1) the propagation speed of the discontinuities before exiting the inner arcade region is independent of the maximum shear speed; (2) slower shearing speeds produce weaker shocks with weaker adiabatic heating; (3) the ion to electron mass ratio, m_i/m_e, affects the strength of the charge separation linearly, but has a mild affect on the propagation speed; and (4) the speed of light to Alfvén speed ratio, c/v_0, does not affect the propagation speed but does affect the magnitude of the charge separation at the shock surface.