The nonlinear drift wave instability and its role in tokamak edge turbulence

Drift wave turbulence, in general a balance between E x B drift turbulence in planes perpendicular to, and dissipative wave dynamics parallel to, a background magnetic field, is a hallmark example of nonlinearity in plasma physics. The turbulence generally has the same basic character in a sheared magnetic field lying in closed surfaces whether linear instabilities are present or not. Only when the linear forcing terms are dominant does this situation not prevail; it is not analogous to neutral fluid turbulence where pure linear forcing is balanced by pure nonlinear mixing and decorrelation. Detailed computations show that two types of nonlinearity are simultaneously present: advection of fluid vorticity by the E x B flows, which tends to have a scattering character, and E x B advection of pressure disturbances, which has the familiar diffusive mixing character. The vorticity nonlinearity excites the turbulence, acting against the mostly linear parallel dynamics which constrains it, while the pressure nonlinearity provides dissipation via transfer to ever smaller scales. The practical result is that the saturated level of the turbulence and the resulting averaged thermal energy transport are controlled principally by these nonlinear mechanisms even when moderate linear instabilities are present. The model is mostly applicable to tokamak edge turbulence, for which the linear forcing effects are sufficiently moderate that the nonlinear physics is allowed to operate.