Diode characteristics in two electron-beam generators, IT and ACE, are examined, with particular reference to the role of prepulse conditions. The advantages and limitations of field-enhanced edge emitters are discussed. In both low- and high-current-density applications, a stable impedance–time history is shown to be associated with a band of prepulse voltage which is a function of both its time variation and driving impedance. Above this band rapid impedance collapse occurs, and below it long current switch-on times are experienced. This behavior is postulated to be dependent upon cathode plasma formation and growth during prepulse and early main pulse phases. The results of a preliminary investigation into the controlled use of prepulse for high-current-density beam production from field-enhanced edges are examined. The characteristics of hollow cylindrical cathodes operating in pinched mode are shown to depend on prepulse conditions. In particular, the transfer of finite energy (of order tens of millijoules) to the diode during the prepulse phase is shown to be desirable for rapid establishment of the required diode impedance in the main pulse phase.