Flow-shear stabilization of ion temperature gradient-driven instability in a tokamak: Slab theory

Abstract

A simple physical model for the flow-shear stabilization of ion temperature gradient (ITG)-driven instability in the presence of dissipative trapped electrons is presented. Reduced fluid equations and magnetic sheared slab geometry are adopted. From analytical analyses and numerical calculations, it was found that the coupling between the sheared flows and non-adiabatic electrons will result in the stabilization of both toroidal sheared flow and poloidal sheared flow on hybrid dissipative trapped electron ITG mode. This implies that the confinement improvement in tokamaks can be obtained not only through the poloidal sheared rotation in the edge region of tokamaks for the H-mode (high-confinement mode), but also through the toroidal sheared rotation in the core region for further confinement improvement associated with internal transport barrier (ITB) and negative shear (or reversed magnetic shear). When the poloidal and toroidal sheared flows are simultaneously considered, it was found that in the presence of poloidal sheared flow, the toroidal sheared flow is a stabilizing or destabilizing effect on hybrid dissipative trapped electron ITG mode according to the relative sign of the poloidal and toroidal sheared flows. However, for sufficiently large value of the toroidal sheared flow, the toroidal sheared flow is always destabilization on the hybrid dissipative electron ITG mode regardless of the relative sign of these sheared flows. These conclusions are consistent with the experimental observations on the Japanese Tokamak-60U (JT-60U) [Y. Koide et al., Phys. Rev. Lett. 72, 3662 (1994)], where large toroidal rotation shear or jump was observed across the surface q=3 (the internal transport barrier location). Toroidal sheared flow stabilization therefore appears to offer favorable prospects for high confinement operation of future fusion reactor.