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Ion acceleration in the evacuated drift tube geometry shown in Figure 1 has been experimentally studied to establish the acceleration mechanism. The ion energy spectrum has been determined by mass spectroscopy, time of flight and nuclear activation techniques. The primary results are that the peak ion energy, ??i, can be written as ??i ?? 3Z??e (1) where Z is the charge state of the ion, and ??e is the beam electron kinetic energy. Also, the ion acceleration event occurs when the electron beam current I, exceeds the space charge limiting current, Il, where Il is given by Il = (??2/3-1)3/2(mc3/e)/[1+2 ln(R/rb)](l-fe) with y ?? (1-??2)-??. ?? is the ratio of the electron speed to that of light in a vacuum, m and e are the electron mass and charge, respectively, fe is the ratio of ion to electron densities, R is the drift-tube radius, and rb is the electron-beam radius. These results are consistent with an electrostatic collective ion acceleration mechanism.