Symmetry breaking effects of density gradient on parallel momentum transport: A new ρs* effect

Abstract

Symmetry breaking effects of density gradient on parallel momentum transport is studied via quasilinear theory. It is shown that finite ρs*(≡ρs/Ln), where ρs is ion sound radius and Ln is density scale length, leads to symmetry breaking of the ion temperature gradient (ITG) eigenfunction. This broken symmetry persists even in the absence of mean poloidal (from radial electric field shear) and toroidal flows. This effect, as explained in the text, originates from the divergence of polarization particle current in the ion continuity equation. The form of the eigenfunction allows the microturbulence to generate parallel residual stress via 〈k‖〉 symmetry breaking. Comparison with the E→×B→ shear driven parallel residual stress, parallel polarization stress and turbulence intensity gradient driven parallel residual stress are discussed. It is shown that this ρs* driven parallel residual stress may become comparable to E→×B→ shear driven parallel residual stress in small Ln region. In the regular drift wave ordering, where ρs*≪1, this effect is found to be of the same order as the parallel polarization stress. This ρs* driven parallel residual stress can also overtake the turbulence intensity gradient driven parallel residual stress in strong density gradient region whereas the later one is dominant in the strong profile curvature region. The parallel momentum diffusivity is found to remain undisturbed by this ρs* effect as long as the turbulence intensity inhomogenity is not important.