Transient Space Charge Limited Picosecond Pulses Using Ultra Short Laser Pulses

The generation of short and powerful electron bunches and electromagnetic pulses (EMP) is a major requirement for many applications such as ultrawide band (UWB) radar sensors [1] , generation of picosecond X-ray pulses, ultra-fast electron diffraction and microscopy and as pulsed high brightness picosecond electron guns for SEM. Here we investigate a technique to generate high voltage ultra-short pulses using vacuum photo diodes for application in UWB radar sensors. The maximum current flux which can be extracted is determined by the space charge limit [2] . Here we develop a self similar analytic model for calculating the peak current and duration of the space charge limited pulses for the case of ultrashort (on the order of a picosecond or less) laser excitation of a photocathode. In order to compare to a real physical system, the pulse widths are calculated for the case of a 3 mm diode gap with various potentials ranging from 1kV to 5kV applied to the anode. In order to verify the self similar analytic solutions derived, numerical modelling of the transient pulse generation was also carried out using a 1D non-relativistic electrostatic Particle in Cell (PIC) code. The results show the possibility of generating EMP pulses of 100 ps or less in duration with peak currents of several amps leading to voltage pulses of hundreds of volts into 50 ohm loads with photocathode areas of the order of 1 cm 2 . The possibility to generate such pulses using high repetition rate (order of 10 kHz - 1 MHz) laser systems will be discussed in detail.