The results are presented of a series of experiments investigating the mechanisms leading to the collective acceleration of protons occurring in electron beam propagation through evacuated drift tubes. Detailed measurements have been made of the electron and ion beam propagation and of the ion energy spectra. The results show that the acceleration cannot occur as a result of a beam head or deep potential well mechanism. It seems likely that the acceleration is due to the self‐excitation of a wave on the beam, possibly the electron‐ion two‐stream instability.