Radial transport of fluctuation energy in a two-field model of drift-wave turbulence

A theory of spatial propagation of turbulence, referred to as turbulence spreading, is developed for the two-field model of drift wave turbulence. Markovian closure expressions for the flux of kinetic and internal fluctuation energies are systematically derived. Simplified closure expressions are used to obtain two coupled reaction-diffusion equations for kinetic and internal energy. The efficacy of various nonlinear interaction mechanisms for spreading is analyzed systematically. Spreading of internal energy is predicted to "lead" that of kinetic energy. The important role of zonal flow damping in spreading is identified, but zonal flows are shown not to be the dominant agents of turbulence spreading.