Observation of a H- Beam Ion Instability
  We report the results of observations of H- beam instabilities at the Fermilab Linac. By intentionally creating "high" background pressure with different gases in the 750 keV transport line we observed coherent transverse beam oscillations. The minimal pulse length required to observe oscillations and the frequency of oscillations are functions of pressure and mass of the background gas. The oscillations are present in both transverse planes and very quickly reach saturation in amplitude growth. The observed characteristics of beam oscillations are in quantitative agreement with "fast beam-ion instability" described by Raubenheimer and Zimmermann. Effects described here are occur far from the normal operating range of the Fermilab Linac but may be important for many future high intensity accelerators. 
Milorad Popovic and Todd Sullivan