I do have plans for reducing the fanout - actually having either 1 driver chip for each IGBT or possibly one driver chip for every two IGBTs. However I have reduced the priority of doing that move as it is starting to appear that the problem I currently has nothing to do with gate drive.
My priority is now how to get a clean turnoff. Originally I was blaming poor design of the gate drive, but I have seen it work fine with a resistive load rather than the flashtube so I will now be looking at ways to 'tame' the flashtube.
I have now added a 'freewheeling' diode between the colectors and the positive supply (of the capacitor). This has greatly reduced the positive overshoot to about only 80 volts above the capacitor voltage. I quite naively hoped this would solve the problem so I tried firing the tube at 700 volts. Worked a few times, but only just the few. After about the tenth discharge one of the IGBTs blew - full short circuit between all three terminals.
It was suggetsed that it could be something as simple as the inductance of the tube so I tried to make an experimnent to see whether that was the case. I built a resistor out of 200 (!) small resistors, giving it a shape as similar as possible to that of the xenon tube . The idea was that the inductance would be very similar and the resistance would be equal to that of the tube at the time of turnoff. Unfortunately the resistor was no match for the current I tried to put through it and just went up in a puff of smoke and a loud bang. Turns out I was a bit over optimistic assuming that keeping the pulse short enough would allow the resistors to survive - and I also made a mistake setting the pulse length to 200uS instead of 20uS as I had meant to.
I've now decided to take an opposite (and counter intuitive) approach. I will be instead adding inductance in series with the tube. One thing I think this might do is provide a high frequency blocking between the tube and the IGBTs. ALthough this will not necessarily solve the problem I am hoping it will allow me to figure out better where the oscillations are originating. I will be able to check the voltage on each side of the inductor. I should see much larger oscillations on one side than the other. If it is on the IGBT side then I will know there must be some very significant collector to gate feedback. If on the other side it would confirm the suspicion that the tube is causing the oscillation - due to its negative resistance or some other weird plasma physics thing.