Experimental observations of collectivehyphen;effect acceleration with electron beams of various ngr;/ggr; (ngr; is the Budker parameter and ggr; is the usual relativistic factor) values injected into neutral lowhyphen;pressure gases show, for beams with ngr;/ggr; near unity, that the peak accelerated ion energy exceeds the energy usually obtained from the deep stationary potentialhyphen;well stage. It is suggested that high ngr;/ggr; electron beams, with a large transverse kinetichyphen;energy content, undergo a longer transition period from the stationary to the moving phase and translate with a lower velocity than that expected solely on the basis of spacehyphen;charge neutralization processes. Ion trapping during and following the transition stage results in final ion energies much greater than 3Ee, whereEeis the electron beam kinetic energy.
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