Beam-Ion Instability in the Fermilab Linac

Beam-ion instabilities were observed in the 750 keV transfer line of the Fermilab linac when various gases were injected and the pressure was raised to between 1 × 10−5 and 1 × 10−4 Torr. The collective resonant frequencies recorded, of the order of 1 MHz, are in qualitative agree-ment with the linear theory of fast beam-ion instabilitypro-posed by Raubenheimer and Zimmermann [1], but fail to follow the A−1/2 dependency, where A is the ion mass number, and have the tendency to increase with pressure. The observed growth times along the beam are in rough agreement with those provided by the theory, although the latter tend to be somewhat larger.