In recent years, there has been a wealth of new science involving low-energy antimatter (i.e., positrons and antiprotons) at energies ranging from 10 2 to less than 10 −3 eV. Much of this progress has been driven by the development of new plasma-based techniques to accumulate, manipulate and deliver antiparticles for specific applications. This article focuses on the advances made in this area using positrons. However many of the resulting techniques are relevant to antiprotons as well. An overview is presented of relevant theory of single-component plasmas in electromagnetic traps. Methods are described to produce intense sources of positrons and to efficiently slow the typically energetic particles thus produced. Techniques are described to trap positrons efficiently and to cool and compress the resulting positron gases and plasmas. Finally, the procedures developed to deliver tailored pulses and beams (e.g., in intense, short bursts, or as quasi-monoenergetic continuous beams) for specific applications are reviewed. The status of development in specific application areas is also reviewed. One example is the formation of antihydrogen atoms for fundamental physics [e.g., tests of invariance under charge conjugation, parity inversion and time reversal (the CPT theorem), and studies of the interaction of gravity with antimatter]. Other applications discussed include atomic and materials physics studies and study of the electron-positron many-body system, including both classical electron-positron plasmas and the complementary quantum system in the form of Bose-condensed gases of positronium atoms. Areas of future promise are also discussed. The review concludes with a brief summary and a list of outstanding challenges.

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Positron annihilation on a wide variety of atoms and molecules is studied. Room-temperature positrons confined in a Penning trap are allowed to interact with molecules in the form of low-pressure gases so that the interaction is restricted to binary encounters between a positron and a molecule. Data are presented for the ␥-ray spectra resulting from positrons annihilating in such interactions. The Doppler broadening of these spectra is a measure of the momentum distribution of the annihilating electron-positron pairs. Consequently, these spectra provide information about the electron and positron wave functions. Systematic studies of annihilation line shapes are discussed for noble gases, a variety of inorganic molecules, alkanes, alkenes, aromatics, and perfluorinated and partially fluorinated hydrocarbons. In the case of molecules, the measurements are used to determine the probability of positrons annihilating at specific locations in the molecule. For example, in the case of partially fluorinated hydrocarbons, we have been able to determine the relative probability of annihilation on the fluorine atoms and on the C-H bonds. Insights that these studies provide in understanding the interaction of low-energy positrons with atoms and molecules are discussed. ͓S1050-2947͑97͒05405-X͔

Measurements are presented for the annihilation rates of thermalized positrons in a variety of substances, including noble gases, simple inorganic molecules, hydrocarbons, substituted hydrocarbons, and aromatics. The measurements were performed using trapped clouds of room-temperature positrons, into which substances under test were introduced as low-pressure gases, so that only two-body interactions were involved. These data are compared with other values in the literature, and a compilation of annihilation rates is presented. The measurements illustrate the importance of both chemical composition and the vibrational modes of excitation of the molecules in determining the annihilation rates. The anomalously high annihilation rates observed for large molecules provide evidence for the existence of long-lived resonances. The nature of these resonances is not yet understood, and the data presented are expected to provide useful constraints for the development of theoretical models.

Progress in the ability to accumulate and cool positrons and antiprotons is enabling new scientific and technological opportunities. The driver for this work is plasma physics research—developing new ways to create and manipulate antimatter plasmas. An overview is presented of recent results and near-term goals and challenges. In atomic physics, new experiments on the resonant capture of positrons by molecules provide the first direct evidence that positrons bind to “ordinary” matter (i.e., atoms and molecules). The formation of low-energy antihydrogen was observed recently by injecting low-energy antiprotons into a cold positron plasma. This opens up a range of new scientific opportunities, including precision tests of fundamental symmetries such as invariance under charge conjugation, parity, and time reversal, and study of the chemistry of matter and antimatter. The first laboratory study of electron-positron plasmas has been conducted by passing an electron beam through a positron plasma. The next major step in these studies will be the simultaneous confinement of electron and positron plasmas. Although very challenging, such experiments would permit studies of the nonlinear behavior predicted for this unique and interesting plasma system. The use of trap-based positron beams to study transport in fusion plasmas and to characterize materials is reviewed. More challenging experiments are described, such as the creation of a Bose-condensed gas of positronium atoms. Finally, the future of positron trapping and beam formation is discussed, including the development of a novel multicell trap to increase by orders of magnitude the number of positrons trapped, portable antimatter traps, and cold antimatter beams (e.g., with energy spreads ⩽1 meV) for precision studies of positron-matter interactions.