The mechanism of self-sustainment in collisional drift wave turbulence

Although collisional drift waves in a sheared slab configuration are linearly damped, the corresponding turbulence is self-sustaining if initialized at an electrostatic potential fluctuation amplitude of eφ/T≳0.3ρs/Ln, much less than that of observed fluctuations. Within the context of two-dimensional sheared slab magnetic geometry, computational investigations into the amplitude threshhold and the nonlinear mode structure reveal the shear-induced self-organization that is responsible for efficiently tapping the free energy in the temperature and density gradients. They also show that the turbulence is nondiffusive in character, since the important assumptions behind turbulence-as-diffusion are all violated. Many important features of experimentally observed tokamak edge fluctuations are reproduced by these single resonant surface nonlinear dynamics, suggesting that model components not treated may have an effect more additive than qualitative. The ingredients giving rise to self-organized turbulence, shear-localized long-wavelength modes of some width Δ0λ, and isotropic intermediate wavelength modes with λ∼Δ0, in direct, coherent interaction, are at least potentially shared by most of the viable candidates for anomalous tokamak transport. Consequently, such transport is unlikely to bear much resemblance to the simpler models currently in use.