High-current space-charge-limited pulses using ultrashort laser pulses

The maximum current that can be delivered by a 1-D vacuum diode is limited due to the space-charge reduction of the accelerating field. For a steady-state operation, this is given by the Child-Langmuir space-charge limit. However, for pulsed diode sources, the instantaneous current can be much higher than this limit, as long as the pulse length is much less than the transit time across the diode gap. This enables the generation of high current pulses with pulse durations of the order of tens of picoseconds using photocathodes driven by ultrashort laser pulses. The generation of such short and powerful electromagnetic pulses is important for numerous applications such as ultrawideband radar or as photocathode sources for accelerators. In this work, the limiting current pulse is investigated in the case of a very short, square-top initial pulse and it is found that these pulses propagate in a self-similar manner, remaining as a square-top charge cloud in space throughout their acceleration and propagation. The resultant current and pulse duration turn out to be dependent on only three parameters, which are the applied voltage, the vacuum transit time, and the fraction of the saturation charge density in the initial charge cloud. The resultant scaling of the resultant peak current and pulse duration are calculated numerically as a function of starting sheet-charge density, allowing the calculation of the resultant pulses for any ultrashort, pulse-driven vacuum photodiode design.