Dr. Ben Zhu, Dartmouth College

"Global Two-Fluid Study of Tokamak Edge Turbulent Transport"

Abstract: A flux-driven global code, Global Drift Ballooning (GDB) model, based on the drift-reduced Braginskii equations is developed to study the low frequency turbulence at the tokamak edge region. In this model, profiles of plasma density, electron and ion temperature, electric potential, magnetic flux and parallel flow are self-consistently evolved across the entire edge region: from plasma sources in the inner core to plasma sinks in the outer-most scrape-off layer (SOL). GDB has successfully simulated realistic Alcator C-Mod L- and H-like plasmas in a simple shifted circular configuration, and reproduced the $alpha_{d}-alpha_{mhd}$ turbulence phase space diagram, in agreement with the previous local studies. It is also used to study the interaction between turbulence, global profiles, and the spontaneous $E{times}B$ shear flow that is not captured in the previous local studies. In particular, we find that the spontaneous formation of the $E{times}B$ drift in the electron diamagnetic drift direction in the closed-flux region can be explained based on the quasi-steady-state ion continuity relation $
abla {cdot} n vec{v}_i {approx} 0$. Another interesting phenomenon exhibits in GDB simulations is the up-down asymmetric plasma profiles caused by the transverse heat flux. The temperature gradient driven transverse heat flux heats the ions at the bottom while cools them on the top, and vice versa for electrons. Since the electrons have a faster parallel thermal diffusion, the up-down asymmetric pattern on electron temperature is weaker than ion temperature. As a result, density profile is driven to be up-down asymmetric due to the total plasma pressure is up-down symmetric enforced by the force balance constraint. Analysis shows this effect is profound for cold, dense plasma with finite temperature gradient; it might be the explanation to the formation of strongly asymmetric density profile when discharge approaches to the density limit observed in experiments back from the 80s. Furthermore, the symmetry breaking also affects the spontaneous generated $E{times}B$ flow, resulting two asymmetric convective cells with a net inward particle flux which is typically two orders larger than the thermal-diffusion theory predicts.