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

Bruce D. Scott
Max-Planck-Institut für Plasmaphysik, Germany

Self-sustained drift wave turbulence is a hallmark example of nonlinearity and nonlinear instability in plasma physics. New computational studies at very high resolution are presented in 3D slab and toroidal flux tube geometry. Detailed diagnosis of the computations shows both the vorticity and pressure nonlinearities are involved, the first catalysing the drive and the second providing dissipation via diffusive mixing. Comparative study between the slab and toroidal models show the turbulence to have the same basic character in both, with the transitions to MHD ballooning occurring only at unrealistically large collisionality or above the MHD beta limit. 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. Tokamak edge turbulence results from this mechanism, irrespective of any considerations arising from linear theory.