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Abstract
An impurity-driven drift-wave instability is observed to be destabilized by the reversed density gradient of a heavy, singly-ionized impurity-ion population in a Q-machine plasma. The dispersion relation is derived from the Vlasov equation, including collisions, in the velocity range v(,I) < v(,i) < (omega)/k(,z) << v(,e). The dependences of frequencies and growth rates upon azimuthal mode number, magnetic field, and impurity fraction are all in good agreement with the local, linear, slab-model dispersion relation. By pulsing the impurity-ion injection, the evolution of the instability is investigated through the linear exponential growth phase, into nonlinear saturation, whereupon strong, radially outward anomalous diffusion is driven by a wave-particle interaction. The relationship between the anomalous diffusion coefficient and the wave amplitude strongly supports some aspects of the nonlinear drift-wave turbulence theory of Dupree. The experimental results verify the minimum fluctuation level required for drift-wave-induced transport which, according to Dupree, results from the onset of transverse trapping of the ions by the wave electric field. In the strong-turbulence limit, the anomalous diffusion coefficient is proportional to the normalized wave amplitude.
