The thermal equilibria of a two-dimensional guiding-center model for a single-species plasma bounded by a cylindrical conductor are considered in the microcanonical ensemble. The same description applies to identical point vortices in a two-dimensional, ideal fluid surrounded by a circular streamline. The statistically dominant configurations are displaced asymmetrically from the axis, for sufficiently large energies at specified canonical angular momentum. The transition between symmetric and asymmetric states resembles a second-order phase transition, and occurs at negative temperatures. It is related to a bifurcation in the mean-field (Vlasov) description. The theory is compared with Monte Carlo simulations of microcanonical ensembles of guiding centers.

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The nonlinear evolution of collisionless drift instabilities in a shearFree magnetic field has been studied by means of gyrokinetic particle simulation as well as numerical integration of model mode-coupling equations. The purpose of the investigation is to identify relevant nonlinear mechanisms responsible for the steady-state drift wave fluctuations.It is found that the saturation of the instability is mainly caused by the nonlinear E * B convection of the resonant electrons and their associated velocity space nonlinearity. The latter also induces energy exchange between the competing modes, which, in turn, gives rise to enhanced diffusion. The nonlinear E x B convection of the ions, which contributes to the nonlinear frequency shift, is also an important ingredient for the saturation.

The Hamiltonian formalism for surface waves associated with the method of Watson & West (1975) is extended to handle the case of spatially varying bottom depth. This description models moderately nonlinear waves over a wider range of scales than Boussinesq-type approximations. A pseudospectral simulation code has been developed using this formalism in two horizontal dimensions. Computations using the model compare well with measurements of waves over a bar, diffractive focusing by topography, and shoaling of solitary waves. Waveforms are computed accurately until near the point of breaking.