The ecology of flows and drift wave turbulence in CSDX: A model

Abstract

This paper describes the ecology of drift wave turbulence and mean flows in the coupled drift-ion acoustic wave plasma of a CSDX linear device. A 1D reduced model that studies the spatiotemporal evolution of plasma mean density n¯, and mean flows v¯y and v¯z, in addition to fluctuation intensity ε, is presented. Here, ε=〈ñ2+(∇⊥ϕ̃)2+ṽz2〉 is the conserved energy field. The model uses a mixing length lmix inversely proportional to both axial and azimuthal flow shear. This form of lmix closes the loop on total energy. The model self-consistently describes variations in plasma profiles, including mean flows and turbulent stresses. It investigates the energy exchange between the fluctuation intensity and mean profiles via particle flux 〈ñṽx〉 and Reynolds stresses 〈ṽxṽy〉 and 〈ṽxṽz〉. Acoustic coupling breaks parallel symmetry and generates a parallel residual stress Πxzres. The model uses a set of equations to explain the acceleration of v¯y and v¯z via Πxyres∝∇n¯ and Πxyres∝∇n¯. Flow dynamics in the parallel direction are related to those in the perpendicular direction through an empirical coupling constant σVT. This constant measures the degree of symmetry breaking in the 〈kmkz〉 correlator and determines the efficiency of ∇n¯ in driving v¯z. The model also establishes a relation between ∇v¯y and ∇v¯z, via the ratio of the stresses Πxyres and Πxzres. When parallel to perpendicular flow coupling is weak, axial Reynolds power PxzRe=−〈ṽxṽz〉∇v¯z is less than the azimuthal Reynolds power PxyRe=−〈ṽxṽy〉∇v¯y. The model is then reduced to a 2-field predator/prey model where v¯z is parasitic to the system and fluctuations evolve self-consistently. Finally, turbulent diffusion in CSDX follows the scaling: DCSDX=DBρ⋆0.6, where DB is the Bohm diffusion coefficient and ρ⋆ is the ion gyroradius normalized to the density gradient |∇n¯/n¯| −1.