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LOW-TEMPERATURE PLASMAS 33 1 Low-Temperature Plasmas INTRODUCTION During the last half century, low-temperature plasmas have made a dramatic impact on society, significantly improved the quality of life, and provided challenging scientific problems. Examples are the fluorescent lights that can be found in almost every home in America; high-power switches that control the electrical grid of the United States and divert electrical power on command; gas discharge lasers, including the red He-Ne laser, which was the first gas laser invented, and the high-power, infrared, CO2 lasers that are used daily in surgery and metal working; and plasma sources that provide positive and negative ions for ion-beam accelerators. These ion sources are used to implant ions into materials, including semiconductor chips for the computer industry, and to harden bearings to increase the life and reliability of high- performance engines. Provided the opportunity, the field of low-temperature plasmas will continue to make significant contributions. Based on the preceding paragraph, it is not surprising that low-temperature plasmas are important in many disciplines. Typically, they are high-pressure collision-dominated plasmas that have average electron energies of 1â10 eV. The purity of the gas is often important, and the physics and chemistry of the excited atomic states dominate the discharge characteristics. In industrial applications, the stability of the discharge frequently impacts the design and utility of the process, and the heterogeneous wall chemistry often impacts its reproducibility and reliability. Unfortunately, because basic research in this area has been neglected for
LOW-TEMPERATURE PLASMAS 34 many years, there is a severe lack of quantitative and experimental understanding of a wide range of phenomena that occur in low-temperature collision-dominated plasmas. Most low-temperature plasma applications involve complex reactions between electrons and a host of atomic, molecular, and ionic species. These species are found in highly excited states not encountered in nonplasma environments. Operation of plasmas in applications ranging from lasers to materials processing and lighting requires optimization of the densities of these species. Scientists modeling these systems require a broader range of diagnostics to characterize species densities in benchmark plasmas, and more powerful methods for measuring, calculating, or approximating the cross sections that dominate the rate equations. In some cases, such as microwave breakdown, the positive column of dc metal-vapor rare-gas discharges, and wall-stabilized arcs, researchers have obtained experimental data, theoretical understanding, and predictive models. However, much of this basic research was performed before 1960. In some cases with immediate industrial and government applications the information was updated in the 1970s, using modern experimental and modeling techniques. Examples include fluorescent lamps, high-intensity lamps, electron-beam- controlled discharge lasers, some specific plasma processes, and arcs (e.g., in discharge-limiting situations, such as transport in weakly ionized swarms and near thermal equilibrium). This research produced a significant improvement in the performance of devices using these plasmas. Recent research was driven by interest in high-power lasers for ballistic missile defense. The decline of interest in that use has severely reduced related funding. In other areas, there has been limited progress during the last 30 years, including understanding phenomena such as collisional discharges in magnetic fields in the presence of boundaries, transient discharges and sheaths, discharge stability, and plasma interactions with practical surfaces. For example, recently there has been much interest in the dc cathode fall, since modeling and experiments are much further ahead for bulk-phase plasmas than for cases, such as the cathode fall, in which plasma contact with surfaces is important. Lack of research support in the physics of low-temperature plasmas has resulted in a low level of training in collision-dominated low-temperature plasmas and in the training of engineers and physicists for plasma processing. No federal agency claims responsibility for this area. The existing support has emphasized short-term goals and work only on current government- and industry-related topics. In FY 1991, there were only two long-term projects, and neither is currently funded. It is our understanding that since the beginning of FY 1992, there has been essentially no low- temperature plasma research project with more than a one-year time scale, since research in this area is dominated by the current needs of the radio-frequency plasma processing and lighting industries. This short time scale severely discourages new, innovative, or thorough research. Novel experimental and modeling tech
LOW-TEMPERATURE PLASMAS 35 niques should be developed to explore new areas and provide more quantitative work in existing areas. A serious problem in low-temperature collision-dominated plasmas has been the lack of reproducible experimental verification of theoretical predictions. This is partly due to the critical dependence of the relevant phenomena on surface conditions and gas purity. It is also due to the fact that the models are too limited and qualitative to be tested or to be of general use. While gas purification techniques have been known for many years, the role of impurity effects in practical systems is often poorly understood. The problem of the reproducibility of practical surfaces is very difficult, and few useful and successful recipes exist. Even fewer techniques exist for characterizing practical surfaces with regard to their interactions with plasmas. As a result, most of the successful quantitative gas-discharge investigations are of phenomena that are relatively free of surface effects (i.e., microwave breakdown, the positive column, swarm transport, and near-equilibrium radiation). However, given adequate funds, more realistic models could be developed to investigate these complex phenomena with modern computer facilities. The Japanese government has long supported an active program in basic gas discharge research, particularly in its engineering schools. Japanese research is recognized internationally for its quality and impact. In spite of a major focus on plasma processing, many Japanese faculty still devote a significant fraction of their time and resources to basic, undirected research. In recent years the French government also has supported a large effort in low- temperature collision-dominated plasma research, which has produced a large fraction of the invited papers at recent international meetings. To change the situation in the United States will require strong support for research in applied physics and engineering in the area of the basic physics of low-temperature plasmas. Experimental programs emphasizing quantitative and reproducible results will be necessary to properly test the predictions of theoretical models. Improved understanding of these plasmas is necessary for applications such as plasma processing and environmental cleanup. This basic research can also be expected to yield innovative experimental techniques and novel modeling methods, and it will provide highly trained scientists and engineers in low-temperature plasma science. That low-temperature plasmas are crucial in so many technologies is both a strength and a weakness. These plasmas are indispensable in today's highly technical world, but since they are useful in many apparently disconnected disciplines, no agency has taken responsibility for research in low-temperature plasmas. This chapter focuses on the following important areas of low-temperature plasma physics: lighting, gas discharge lasers, plasma isotope separation, space propulsion, magnetohydrodynamics, and the use of plasmas for pollution control and reduction. Another major area is plasma processing, which was addressed in