The nonlinear drift wave instabilityand its role in tokamak edge turbulence

Abstract

Drift wave turbulence, in general a balance between E×B drift turbulence in planes perpendicular to, anddissipative wave dynamics parallel to, a background magneticfield, is a hallmark example of nonlinearity in plasma physics. The turbulence generally has the same basic character in asheared magnetic field lying in closed surfaces whether linearinstabilities are present or not. Only when the linear forcingterms are dominant does this situation not prevail; it is notanalogous to neutral fluid turbulence where pure linear forcingis balanced by pure nonlinear mixing and decorrelation. Detailed computations show that two types of nonlinearity aresimultaneously present: advection of fluid vorticity by theE×Bflows, which tends to have a scattering character, andE×Badvection of pressure disturbances, which has the familiardiffusive mixing character. The vorticity nonlinearity excitesthe turbulence, acting against the mostly linear paralleldynamics which constrains it, while the pressure nonlinearityprovides dissipation via transfer to ever smaller scales. Thepractical result is that the saturated level of the turbulenceand the resulting averaged thermal energy transport arecontrolled principally by these nonlinear mechanisms even whenmoderate linear instabilities are present. The model is mostlyapplicable to tokamak edge turbulence, for which the linearforcing effects are sufficiently moderate that the nonlinearphysics is allowed to operate.