HEAVY ION BEAM PROBE MEASUREMENTS OF RADIAL POTENTIAL PROFILES IN THE MODIFIED PENNING DISCHARGE

A heavy ion beam probe is used to examine the radial potential pro-file of a plasma in the Modified Penning Discharge [1]. The plasma has strong (z kV) electrostatic potentials near the anode ring which can be a large fraction of the injected ion beam energy. A primary (singly ionized) thallium ion beam is injected through the plasma in the midplane of a double Penning anode ring. After passage through the plasma, primary and sec-ondary (doubly ionized) ions are detected with either a set of flat probes or an electrostatic energy analyzer. 'A calculation of the primary orbit through the plasma is performed to obtain an approximation to the measured pri-mary beam trajectory. As the real radial potential profile is unknown, an adjustable model is used in the computer program. The adjustable poten-tial profile is varied until the best agreement between measured and calcu-lated trajectories is obtained. Secondary orbits are also predicted using the best-fitting adjustable profile. The calculations indicate that secondary beams originating at more than one point in the plasma can be simultaneously ob-served at the electrostatic analyzer with certain primary beam initial con-ditions. Such multiple secondary beams are observed near the predicted conditions