Two-fluid tokamak equilibria with reversed magnetic shear and sheared flow

Abstract

The aim of the present work is to investigate tokamak equilibria with reversed magnetic shear and sheared flow, which may play a role in the formation of internal transport barriers (ITBs), within the framework of two-fluid model. The study is based on exact self-consistent solutions in cylindrical geometry by means of which the impact of the magnetic shear, s, and the "toroidal" (axial) and "poloidal" (azimuthal) ion velocity components, v iz and v iθ , on the radial electric field, E r , its shear, |dE r /dr|, and the shear of the E × B velocity, ω E×B ≡ |d/dr(E × B/B 2 )|, is examined. For a wide parametric regime of experimental concern it turns out that the contributions of the v iz , v iθ and pressure gradient (∇P i ) terms to E r , |E ′ r | and ω E×B are of the same order of magnitude. The contribution of the ∇P i term is missing in the framework of magnetohydrodynamics (MHD) [G. Poulipoulis et al. Plasma Phys. Control. Fusion 46 (2004) 639]. The impact of s on ω E×B through the ∇P i term is stronger than that through the velocity terms; in particular for B z = constant, the contributions of the ∇P i and velocity terms to ω E×B at the point where dE r /dr = 0 are proportional to (1 − s)(2 − s) and (1 − s), respectively. The results indicate that, alike MHD, the magnetic shear and the sheared toroidal and poloidal velocities act synergetically in producing electric fields and therefore ω E×B profiles compatible with ones observed in discharges with ITBs; owing to the ∇P i term, however, the impact of s on E r , |E ′ r | and ω E×B is stronger than that in MHD.

Magnetohydrodynamic counter-rotating vortices and synergetic stabilizing effects of magnetic field and plasma flow

“…It may be noted here that certain synergetic action of the magnetic field and flow for the formation of internal transport barriers in terms of reversed magnetic shear and flow shear was found theoretically in Refs. [11,12] and observed recently in JET (Ref. [29]).…”