Fast Capacitve Probe for Electromagnetic pulses diagnostic

Generation of electromagnetic pulses of fast and high intensity is constantly in progress. The real time diagnostic of short current or voltage pulses is essential for good application with sophisticate and modern devices. Streak camera systems are able to detect sub ps pulses of particle or X-ray beams, but losing the possibility to use the beams during the detection. Besides, real time measurements of electromagnetic pulses offer a better control of the device functioning. Nowadays fast current or voltage pulses of sub ten picoseconds can be recorded in real time by means of modern fast digitizing oscilloscopes, but to get good results it is necessary to realize fast probes. Fast capacitive probes, which we will present in this work, are simple capacitors conceived like a transmission line. For this reason it is indispensable to construct them with electrodes of suitable dimensions and forms, e.g., i: the probe has to got a folded central electrode in order to present the internal skin dimensions close to external ones, ii: it has to got a particular configuration able to contain and to shield the integrating resistor avoiding electromagnetic irradiations. Such a probe is suitable for measuring fast voltage and current pulses propagating in coaxial structures of known characteristic impedance. Analysing the behaviour of the probe in a 50 Ω coaxial structure, the voltage amplification resulted to be 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 320 ps, which was limited by oscilloscope bandwave. In vacuum electrical devices the capacitor probe can measure potential values of the order of 100 kV.