Fast capacitive probe for short and high intensity electromagnetic pulses diagnostic

Fast and high intensity electromagnetic pulses are constantly request especially for FEL of new generation and other scientific and industrial machines. Nowadays fast current or voltage pulses of sub ten picoseconds can be recorded in real time by fast digitizing oscilloscopes. To get good results it is necessary realize fast probes. Fast capacitive probes are conceived like a transmission line and it is indispensable to construct it with electrodes of suitable dimensions and form. The instrument we realized has got the central electrode folded in order to present a skin dimension close to external electrode sides and a particular configuration for containing the integrating resistor to avoid electromagnetic interferences. It is suitable for measuring fast voltage and current pulses propagating in coaxial structures of known characteristic impedance. Analysing the behaviour of the probe for pulses propagating in a 50 Ω coaxial structure the voltage amplificationresulted of (3.6 ±0,1)x10-4 and as a consequence the current attenuation factor of 56±1 A/V. The rise time response was very interesting. It was less than 350 ps, value limited by oscilloscope bandwave.