Electron plasma for antiproton cooling in the ATHENA experiment

The first phase of the ATHENA (Antihydrogen Apparatus) experiment is devoted to the study of cold antihydrogen production. The apparatus includes an antiproton capture trap designed to trap and to cool antiprotons coming from the CERN Antiproton Decelerator (AD). The antiproton cooling is achieved by means of the collisional interaction with a cold cloud of trapped electrons. The electron plasma is loaded in the trap before the antiproton capture by means of a small-size heated filament and cooled to sub-eV temperatures by cyclotron radiation. We report some measurements devoted to the characterization of the electron plasma. The ATHENA apparatus design does not allow the use of complex diagnostics; therefore the plasma properties are obtained using electrostatic wall probes, radio- frequency diagnostics, and dumping the electrons onto a charge collector. A simple experimental method to obtain an estimate of the electron plasma radius is discussed. (9 refs).