Scaling Studies of Collective Ion Acceleration with Intense Relativistic Electron Beams

Abstract

Recent experimental observations1 of collective ion acceleration produced when an intense relativistic electron beam was iniected into a low pressure neutral gas have firmly established a threshold beam current for the process which was predicted by Olson2; for this geometry ion acceleration does not occur unless the electron beam current (I) exceeds the space-charge limiting current (Il). In this paper we report additional observations of ion acceleration obtained in two series of experiments. In the first series, the guide tube and diode were configured to provide a large ratio of injected beam current to space charge limiting current, and drift tube endplate effects were examined. When the separation between the anode and tube endplate become less than the drift tube diameter the efficiency of the collective acceleration process is strongly reduced. In the second series of experiments the scaling of accelerated ion energy with electron beam kinetic energy was investigated using two electron beam machines which differ in stored energy by a factor of approximately 20. Using the larger machine proton energies in excess of 16.5 MeV have been attained. In addition, we also present an expression for the optimum pressure for ion acceleration in the drifting beam neutral gas geometry.