The trapped electrons relax to a well-defined equilibrium state in which the average rotation frequency of the electron plasma is independent of radius. When a neutral gas is present, collisions between electrons and neutrals perturb the plasma and modify the rotation frequency and electron distribution function. By using a reference value for the elastic momentum transfer cross section of the neutrals, the neutral gas density consistent with the observed evolution of the electron plasma can be determined and used to develop a pressure standard.
In the past two decades, much progress has been made in the understanding of nonneutral and single-component plasmas. New experimental configurations have been discovered and exploited, leading to a better understanding of the underlying physical principles of plasma confinement, approach to equilibrium, and in some cases, mechanisms of plasma transport. There are many potential scientific and technological uses of such plasmas. These opportunities result, at least in part, from the excellent confinement properties that distinguish single-component plasmas from neutral plasmas and enable true thermal equilibrium states to be achieved. Therefore, plasmas with controlled departures from equilibrium also can be created. This allows a study of nonequilibrium plasma phenomena with a degree of precision unachievable in other plasma systems.
Experiments in nonneutral plasmas, such as those described above, can be exploited to address forefront problems in atomic, molecular, and optical physics and in fluid dynamics, as well as in plasma physics. Consequently, it is expected that this will continue to be a vital and productive area in plasma physics research for the foreseeable future. Since these experiments can typically be done with a relatively modest expenditure of resources, they are ideally suited to a university setting.
In addition to the intrinsic scientific value of research in nonneutral plasmas, there are many important applications of these plasmas. Several examples, discussed above and in Chapter 5, "Beams, Accelerators, and Coherent Radiation Sources," include beam-type microwave devices, such as gyrotrons and free-electron lasers, precision clocks and mass spectrometers, and future generations of ion sources.
The progress in this area has benefited greatly by steady support from a dedicated program at the Office of Naval Research and support from the National Science Foundation and the Department of Energy. It is the conclusion of the panel that research on nonneutral plasmas should be considered a vital part of a healthy and vigorous plasma science program in the United States in the next decade. Therefore, the panel recommends that continued strong support be given to research on nonneutral plasmas and to the development of technological applications.