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NONNEUTRAL PLASMAS 53 ics, including tests of leading theories of atomic structure and crucial tests of extrapolations of previous, lower-Z measurements. Other important experiments include x-ray observations of magnetic octopole decay in atomic spectra; the first use of x-ray polarization to probe the hyperfine interaction in highly charged ions; the first direct measurements of ionization cross sections for highly charged ions; the first measurements of dielectronic recombination cross sections in ions that are important in hot fusion plasmas; the first excitation functions for x-ray lines used in the analysis of high-temperature tokamak and astrophysical plasmas; measurements of line overlaps for x-ray laser design; and measurements of metastable lifetimes in regions of the electromagnetic spectrum inaccessible to other techniques. Confinement of Antimatter Trapping techniques similar to those described above for pure electron plasmas have proved to be an efficient way to accumulate and store antimatter particles such as positrons and antiprotons. Single-component positron plasmas, a few cubic centimeters in volume, with a temperature of 300 K and Debye screening lengths less than 1 mm have now been created in the laboratory by accumulating positrons from a radioactive source. Recently, antiprotons from the low-energy antiproton storage ring at CERN in Geneva have been captured by a similar trapping scheme. The antiprotons were cooled to 4 K by collisions with an electron plasma confined in the same cryogenic trap. These experiments have provided a controlled way to study antimatter interactions with ordinary matter and to study the properties of the antimatter particles themselves. Examples include precision measurements of the mass of the antiproton and positron annihilation phenomena relevant to atomic and molecular physics and to gamma-ray astronomy. RESEARCH OPPORTUNITIES Continued progress is expected in the areas identified above in "Recent Advances in Nonneutral Plasmas." In addition, the following topics, while not comprehensive or mutually exclusive, represent important research opportunities in the physics of nonneutral plasmas.
