Separation of electrons from their atoms or molecules forms a neutral plasma. To avoid recombination through subsequent electron-ion collisions, the neutral plasma must be hot or dilute or short-lived, or supplied with a lot of energy to encourage continuing ionization. However if the species can be spatially separated, the opportunity for recombination is removed, and the resulting nonneutral plasma can be made cold and dense and long-lived, with very little coupling to its environment.
While neutral plasmas have been studied for about a century as an outgrowth of electrical conduction in gases, the study of nonneutral plasmas has been more recent. The reason is clear: a single species plasma has a net electrical charge and tries to fly apart through electrical repulsion. It was the development of charged particle traps in the 1950's that opened the way for non-neutral plasma confinement. In the past twenty years or so, a growing realization of the unusual properties of non-neutral plasmas has emerged from experimental and theoretical investigation. For example, electron plasmas have been radiation-cooled in cold-bore high-field magnets to about 10 Kelvin, while ion plasmas have been laser-cooled to much lower temperatures, below the transition where crystal structures appear. By adding angular momentum, ion plasmas have been confined for weeks and compressed to near the Brillouin density limit.
There is an interesting relation between the physics of intense beams and the physics of non-neutral plasmas. A beam is just a non-neutral plasma which happens not to be at rest in the laboratory reference frame. When the plasma has a collective Coulomb potential energy orders of magnitude greater than its kinetic energy, a beam physicist would describe this as "space charge domination", and view the plasma in its trap as a super-intense beam which has been arrested in its flight and imprisoned in the laboratory for convenient study. Electron plasmas resemble a chunk of the intense electron beam used for cooling in a storage ring. Potential applications may include plasma targets for atomic physics use, or processing devices for slow products of nuclear reactions.