Right, the trigger pulse doesn't determine the flash duration. And yes there is a second FET here controlling the pulse width. I would like to keep the PIC software in control of the exact pattern of flashes it blinks in. An ideal pattern I am initially aiming for is weak-weak-strong. The cap is basically sized to be close to the max rated flash energy. Also the little transformer needs about 6 sec to charge it currently from a full flash so quicker ones are not possible. Not only is there no room in the enclosure for multiple caps, each would require its own switching element anyways. It could then be done with an SCR in that configuration but other than that there's little gained and it's way outside the desirable spec.
Still, it's a mystery just what is happening. I tried to check the board over and over and found nothing amiss. This is the transistor:
https://www.toshiba.com/taec/components/Datasheet/2SK3562.pdf
and it's driven with 5Vgs which is around 7.5 amps Ids at room temp. (I'm going to replace that with one that does 15 amp @ 5vgs later) So even if the tube took that much, it should be nowhere near done with its pulse in 3 usec so it should have been cut short but wasn't. I wondered if the current could have been high enough to create an IR drop on the ground trace to somehow keep it on, but nothing in the board layout would make this likely that I could see. I wondered if the pulse could have shaken up the PIC. It has a 0.1uF bypass cap next to Vdd/Vss and its ground was not placed between the flashtube ground and flashcap -. Actually I'm wondering now because when I think about it, if the PIC reset itself it would restart its timing interval which is currently all it does, thus I would not have noticed it reset. I will check on that.
Circuit works fine otherwise. I had it popping out 10 flashes/min for awhile, transformer got hot, everything else pretty cool. So we can rule out having shorted out that FET, a single pulse would have toasted it.
Normally I would troubleshoot this hands-on much more, but without a HV probe or two for the scope I can't do a lot. Nor am I really comfortable doing a lot of handling on a hacked together circuit with 450v and enough energy and current capacity to kill me many times over. So it's going to require more though than random tinkering.
Any ideas? I mean, it should cut the flash short, I don't see how it the transistor could fail to do so.