Modelling of picosecond timing signals from fast vacuum photodiodes

Very high bandwidth vacuum photodiodes with 10 s of picoseconds rise time are utilised in Cherenkov detectors for inertial confinement fusion diagnostics. Experimental measurements of the pulse shape and time resolution of a fast Photek PD010 photodiode tube, undertaken using very fast pulsed laser illumination at the ORION facility, Atomic Weapons Establishment, Aldermaston UK, were compared with a computer model to understand detector performance with the aim of improving the time resolution. The physical processes defining the performance of the photodiode detector were modelled using CST Studio Suite software. This package combines very high frequency electromagnetic field modelling, particle tracking, space charge effects and secondary electron emission simulation, and has enabled a realistic simulation of detector behaviour and performance predictions to be made. We present a comparison between experimental measurements of time resolution and pulse shape over a range of device operating voltages, with simulations obtained with CST particle tracking software using the device CAD design file to accurately model the PD010 device. These results demonstrate the remarkable correlation that can be achieved between experiment and simulation, even at picosecond time-scales.