Progress in creating low-energy positron plasmas and beams

A summary is presented of recent research to create positron plasmas in new regimes of density, temperature, and particle number. The operation of a new, compact positron accumulator is discussed. It has a number of improvements including enhanced vacuum capabilities and an easily modified electrode structure. Using a 90 mCi ${}^{\mathrm{22}}\mathrm{Na}$ source and neon moderator, a plasma of ${\text{3×10}}^8$ positrons, with a diameter of 6 mm (FWHM) and a density of ${\text{2×10}}^7 {\mathrm{cm}}^{\mathrm{-3}},$ has been accumulated in 8 minutes. This is a factor of 50% more positrons and an order of magnitude increase in plasma density over the performance of the previous accumulator. Plans for a separate, high magnetic field (i.e., 5 Tesla), low-temperature (<10 Kelvin) trap are described. This trap is expected to permit the creation and long-term storage of cryogenic plasmas with more than an order of magnitude larger particle number and more than two orders of magnitude in plasma density. A method is described that uses positron accumulation techniques to create a cold, bright positron beam (e.g., <20 meV FWHM), tunable from ∼0.1 eV upward. Results are described of studies of positron scattering from atoms and molecules in a new range of energies (e.g., <1 eV) using this cold positron beam. Other applications of trapped cold positron plasmas and beams are briefly discussed.