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THEORETICAL AND COMPUTATIONAL PLASMA PHYSICS 161 Large-Orbit Effects on Plasma Stability The influence of large-orbit particles in a plasma on low-frequency stability was computed by the Vlasov formalism in the form of a modified energy principle. The observed stability of field-reversed configurations has been attributed to this effect. A formal theory of interaction of a dilute species of energetic particles (described by the Vlasov equation) with magnetohydrodynamic Alfvén waves (described by fluid equations) has been developed and applied, with quantitative success, to tokamak plasmas and to the magnetosphere. The complex geometry of tokamaks, which is periodic the short-way-around the doughnut, altered the Alfvén wave propagation and attenuation bands as periodic media generally do. Almost-undamped Alfvén wave modes emerged that could be destabilized by energetic particles with velocities comparable to the Alfvén speed. This development forms the basis on which to expect challenging physics when thermonuclear reactions take place in magnetically confined plasmas. Three-Dimensional Magnetohydrodynamics Three-dimensional resistive magnetohydrodynamic simulations have successfully modeled turbulent generation of toroidal flux in force-free reversed- field pinch experiments. Three-dimensional resistive magnetohydrodynamics further gives a good account of magnetic reconnection in tokamaks and associated magnetic oscillations, including spontaneous formation of singular current sheets. However, a few troubling enigmas remain to be explored. Numerical Simulation of Plasma Processes The numerical study of plasmas has advanced markedly during the past decade, with applications to the ionosphere, the magnetosphere, solar flares, solar pulsations, stellar convection, nonlinear magnetohydrodynamics, gyrokinetics, and so on. (See Plate 8.) The progress has been due to a combination of improvements in algorithms and the advent of cheaper more powerful computers, both supercomputers and workstations, that provide great power, rapid turnaround, and networking at very modest cost. The computational discovery of nonlinear coherences that compensate for linear damping of microinstability modes in tokamaks calls into question the use of quasilinear correlation functions to estimate transport consequences of microinstabilities in tokamaks. Nonlinear Laser-Plasma Interaction Virtually all of the many instabilities driven by intense electromagnetic waves interacting with plasma were identified theoretically and studied in laser- plasma experiments during the past decade. Key nonlinear signatures predicted
