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

Journal article, 2012

Symmetry breaking effects of density gradient on parallel momentum transport is studied via quasilinear theory. It is shown that finite (equivalent to rho(s)/L-n), where rho(s) is ion sound radius and L-n 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(parallel to)> symmetry breaking. Comparison with the (E) over right arrow x (B) over right arrow shear driven parallel residual stress, parallel polarization stress and turbulence intensity gradient driven parallel residual stress are discussed. It is shown that this rho(s)* driven parallel residual stress may become comparable to (E) over right arrow x (B) over right arrow shear driven parallel residual stress in small L-n region. In the regular drift wave ordering, where rho(s)* << 1, this effect is found to be of the same order as the parallel polarization stress. This rho(s)* 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 rho(s)* effect as long as the turbulence intensity inhomogenity is not important.