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NONNEUTRAL PLASMAS 58 Electron-Beam Ion Traps Studies of electron-beam ion traps as plasma devices are in their infancy, and progress in this area could lead to performance enhancements by several orders of magnitude. This would enable new kinds of experiments in atomic, nuclear, and surface physics. An enhanced ion source based on the electron- beam ion trap is expected to be useful for surface modification and nanotechnology. In most cases, plasma physics issues are the key to these developments. For example, a thousandfold increase in the x-ray emission rate from an electron-beam ion trap might be achieved by increasing the total electron-beam current, the current density, and the space charge neutralization (i.e., ion density). The total beam current is likely to be limited by instabilities such as those that occur in backward-wave oscillators. The current density is likely to be limited by the brightness of future electron guns, and the (poorly understood) super-emissive, hollow-cathode discharge is a leading candidate for an electron gun. The ion density will be limited by a two-stream instability. The performance of electron-beam ion traps is also limited by discharges and instabilities involving trapped secondary electrons. These phenomena are not understood to the degree necessary to design a reliable next-generation device. Progress has been made only by trial and error. It is possible that the ion output could be enhanced by an even larger amount simply by making the trap longer, but success will again depend on understanding plasma properties of these devices. A plasma research program in this area might also spin off benefits for other electron-beam devices (e.g., klystrons, traveling-wave tubes, and free- electron lasers) and for other plasma devices (e.g., electron-cyclotron-resonance ion sources and the pure electron or pure ion plasmas described above). In addition to issues related to the electron beam itself, it is known that many electron-beam devices are affected by trapped ions. New types of devices could also evolve from the present experimental configurations of electron-beam ion traps, which, for example, might provide new and inexpensive laboratory sources of x-rays and highly charged ions and microwave devices with trapped ions designed into their operation. Radiation Sources The increased understanding of single-component plasmas is likely to have significant impact on the development of beam-type microwave devices, particularly for use in high-power and high-frequency applications. Such applications are discussed in more detail in the section on beams and radiation sources. Pressure Standard in Ultrahigh-Vacuum Regime A pure electron plasma confined in a Penning trap can potentially be used to develop a primary pressure standard in the ultrahigh-vacuum regime (<10-5Pa).