Observations of collective ion acceleration with drifting intense relativistic electron beams

In this paper we report several observations related to the acceleration of positive ions when an intense relativistic electron beam is injected into a low-pressure neutral gas. In addition to varying the type and pressure of the neutral gas, we have varied a number of geometrical parameters including the radius of both the beam and the guide tube, the length of the guide tube (endplate effects), and dielectric constant of the guide tube. We find that ion acceleration occurs only if the peak electron-beam current I0 exceeds the space-charge-limiting current, I/subl/0, for the cavity. Maximum average ion energies are obtained when I0/I/subl/0approximately-greater-than3: the ion (proton) energy spectrum is rather broad with the spectrum peak occurring at 3/2E/sube/, and the maximum energy cutoff at approximately 3E/sube/, where E/sube/ is the electron-beam kinetic energy