Plasma Science: From Fundamental Research to Technological Applications (1995)
Chapter: Electron Plasmas
Electron Plasmas
Much progress has been made in understanding the basic physics of single-component plasmas, including critical aspects of the stability, confinement, and equilibrium of these plasmas. It was shown theoretically that the conservation of canonical angular momentum implies, in the absence of external torques, that a single-component plasma can be confined indefinitely. Soon afterward, it was demonstrated that the confinement of pure electron plasmas for several minutes to hours is relatively easily achievable in laboratory experiments. The confinement times are sufficiently long that the plasma approaches a state of thermal equilibrium.
The existence of these thermal equilibrium states in confined, single-component plasmas distinguishes them from neutral plasmas. A magnetically confined neutral plasma does not remain in a state of spatially isolated, local thermal equilibrium, because collisions between the electrons and ions lead to a diffusive expansion of the plasma across magnetic field lines. In addition, in a neutral plasma there is typically free energy (associated with the relative cross-field flow of electrons and ions) available to drive collective instabilities that produce enhanced transport across the field lines. Such instabilities pose a challenge to the achievement of high-quality confinement in electrically neutral plasmas of interest in fusion. In contrast, a confined, single-component plasma that has come to thermal equilibrium is in a state of minimum free energy and hence is stable. It is also a great advantage theoretically to be able to use thermal equilibrium statistical mechanics to describe the equilibrium state.
Theory predicted that, in a strong magnetic field and at low temperature, the relaxation of the particle velocities to a thermal equilibrium distribution would be constrained by an adiabatic invariant, and as a consequence, the relaxation rate would be exponentially small. Subsequent experiments confirmed this prediction, and now there is good agreement between theory and experiment over eight orders of magnitude in effective magnetic field strength and five orders of magnitude in the scaled relaxation rate.
The well-controlled nature of these plasmas has also permitted precise studies of nonequilibrium states unachievable in other plasmas. For a sufficiently low-density nonneutral plasma, in the limit that transport along magnetic field lines is rapid compared to transport perpendicular to the field, the plasma is described by similar equations (in an isomorphic sense) to those describing an inviscid classical fluid in two dimensions. Charge-density perturbations in a single-component plasma are analogous to vortices in a fluid, and vortex dynamics is an important subject of long-standing interest in fluid dynamics. Recently, this analogy has begun to be exploited to test models of coherent structures and vortex merger with a precision not possible in classical fluids. For example, since the effective viscosity of a pure electron plasma is less by orders of magnitude than the viscosity of a classical fluid, the trajectories of a pair of vortices
