Electro-optic probes and test generators for 500 Kv nanosecond pulses

This thesis describes a research investigation into novel probes for the measurement of ultra-fast voltage pulses with a peak value up to 500 kV, and during the course of the work five probes were constructed for voltages of between 60 kV and 500 kV. Essential requirements of the probes are that they should be immune to high-levels of electromagnetic noise and also isolate the measuring equipment from the high voltage of the test circuit. Their designs were therefore based on an electro-optic (Pockels) Cell rather than on an electromagnetic device. The first item in the probe is a capacitive divider that attenuates the high-voltage under investigation to the level that can be fed to a Pockels Cell. Light from a laser is circularly polarized and passes through the Cell, with the attenuated voltage pulse causing the Cell crystal to change its molecular shape. This produces a change in the refractive index of the crystal and the emerging light signal becomes elliptically polarized. After conversion into electrical form, a waveform is displayed on an oscilloscope that is an accurate representation of the input voltage to the capacitor. To test the performance of a probe requires a generator capable of producing the required high voltage with a very short rise time, and a number of these were developed for use with the different probes. Careful comparisons of the performance with that of several commercially available probes showed unequivocally that the new probes were far superior. The thesis concludes by presenting ideas for future probe designs and suggests what form the ultimate probe might take. Much of the work reported in the thesis has already been presented at major international conferences or in prestigious academic journals.