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approvedabs_sherwood1.pdf2018-03-28 10:25:35Ben Zhu


Author: Ben Zhu
Requested Type: Pre-Selected Invited
Submitted: 2018-02-27 12:44:50

Co-authors: M.Francisquez, B.N.Rogers

Contact Info:
Dartmouth College
6127 Wilder Laboratory
Hanover, NH   03784

Abstract Text:
In global tokamak edge simulations of a shifted-circle magnetic geometry with an in-board limiter, the plasma density and temperatures are consistently poloidally asymmetric despite the (nearly) symmetric magnetic geometry and boundary conditions. In particular, in the closed flux region, the ion temperature profile tends to be up-down asymmetric (hotter at bottom) while the density exhibits similar asymmetric pattern but with opposite polarity (denser at top). We show that the compressibility (or, curvature) contribution of the ion transverse heat flux breaks the up-down symmetry of the ion temperature profile as predicted by neoclassical theory. The up-down asymmetric ion temperature further impacts other plasma quantities, ExB flow and turbulence via different physical mechanisms. For instance, the density is driven to be up-down asymmetric in order to satisfy the force balance. Analytical estimates suggest this up-down asymmetry $f_1/f_0propto nq^2/(T_i^{3/2}L_{T_i})$; therefore it is profound at the edge region where q is large, ion temperature is relatively low and its gradient is strong. This up-down asymmetry mechanism might also provide an explanation of the strongly asymmetric inboard up-shifted edge density observed in recent J-TEXT near density-limit experiment.

More strikingly, we find this up-down symmetry breaking introduces a new particle pinch mechanism other than the Ware effect, thermal-diffusion and turbulence equipartition theory: in our numerical studies a peaked radial equilibrium density profile is formed in the closed field line region even though the particles are sourced only locally near the last-closed flux surface. Once the transverse heat flux term is turned off, the density profile flattens as the plasma density and temperature become up-down symmetric. These very new results suggest that a robust density pinch is produced in our model and is most likely caused by physics associated with the poloidal up-down symmetry breaking.