The capacitors are charged off a 0-300V power supply using a DC resonant charging circuit, to get them to about 600V. Then the IGBT is fired dumping the charge into that piece of copper pipe.
The current measurement is a bit suspect. Look how it seems to begin with a sudden jump instead of rising smoothly from zero. Also see how the phase of it leads the collector voltage. That suggests the current shunt is inductive. We'll be building a proper tubular shunt soon, and cranking the voltage up to 1000V, so stay tuned...