  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.
