Density Peaking by Parallel Flow Shear Driven Instability

Abstract

A theory to describe coupled dynamics of drift waves and D'Angelo modes is presented. The coupled dynamics is formulated by calculating fluctuation energy evolution. When drift waves dominate, turbulence production is due to release of free energy in density profile. Drift waves in turn exert Reynolds stress to drive secondary axial flows. When parallel flow shear is strong, D'Angelo modes dominate. Turbulent production occurs from release of free energy in parallel flow shear. D'Angelo modes can generate a secondary structure in density profile and can peak density profile. It is shown that when D'Angelo modes are unstable, they necessarily contribute to an inward particle flux, that compete against an outward, down-gradient flux. Net inward, up-gradient particle flux can result for strong flow shear, which can lead to density peaking in plasmas. Application to laboratory and astrophysical plasmas is discussed.