Electron beam propagation in the ion focused regime (IFR) with the experimental test accelerator (ETA)

The IFR is a well-known stable, low pressure (0.10 to 0.120 torr in air) propagation window. Secondary electrons created by collisions of beam electrons. with gas atoms are rapidly expelled by the strong radial electric field of the beam charge. The ions that remain inside the beam partially neutralize the electric field, allowing magnetic pinch forces to focus the beam. Experiments with the ETA beam have re-verified this stable window and are reported. Image forces from a close wall IFR propagation tank are also experimentally shown to center the beam and damp transverse oscillations. Results of experiments using 5 and 15 cm diam beam tubes are reported. For pT > 2 Torr-nsec (gas pressure x time into pulse) the beam charge becomes completely neutralized by the ions, allowing a buildup of plasma and resultant beam-plasma instabilities. The onset of these instabilities has been measured using rf pickup loops (0–2 GHz) and microwave detectors (6–40 GHz); and are also reported.