Quasi-Linear Theory for Small Larmor Radius Universal Instabilities

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Abstract
A class of universal instabilities is discussed. The basic physical mechanism of these instabilities is illustrated with a simple physical picture, and the large amount of energy available to drive these instabilities is pointed out. A small Larmor radius quasi-linear theory is developed for a class of unstable ion waves whose phase velocities along the magnetic field lie between the thermal velocities of ions and electrons. In the approximation of localized perturbations, the final state quantities of the plasma are given without specifying the detailed instability mechanism. An essential feature is that the quasi-linear theory predicts an anomalous ``displacement'' of a group of resonant particles rather than any anomalous diffusion. The theory is applied to a special type of the current-driven universal instability. Its quasi-linear consequences are explored.

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