Hi,
Thats a very good point! I haven't taken apart any commercial high power 'strobes' which would pulse at up to 10Hz and probably have a life in terms of thousands of hours, but they would have to use a half decent cap. Where-as, as you pointed out, disposable camera flash circuits only need to work for 30 flashes.. Even though I would have thought manufacturers would use as cheap cap as possible, in my *limited* experience with playing with disposible camera circuits is, the xenon tube goes first, then the transformer - I haven't tested the HV cap to death. So I'm out of my depth here :/
That said, considering your coil has a fair ammount of inductance, current doesn't rise instantly anyway, and at the high voltage of 90-100V, roughly 8-15x the usual voltage of 'low ESR' applications would make the ESR less of an issue. And just because a cap isn't listed specifically in the 'low ESR' catagory, doesn't mean to say it doesn't have a very low ESR
I would gather a lst of candidates from your supplier, with the right voltage/capacitance, and see if you can get the ESR for every one, and compare them with a cap specifically for the job (the rubycon FW ?). I'm pretty sure the higher the rated voltage of the cap, generally the lower the ESR for a given capacitance.
One more point, the cap you're using is designed for a 280-300V discharge (disposible camera's never charge to mre than 300V), but you're using it at 1/3rd of that voltage. My impression is, even if its a 'cheap cap' with limited life, using it at a much lower rating, combined with the fact its driving an inductive load (limited current rise time on discharge) means it should last a lot longer than its camera application. Sure, its probably not a good brand like rubycon, an may not be ideal for it, but seems like its not a bad idea - plus easily obtainable for those who wish to clone the circuit.
As for possible failures when a cap is at the end of its life in this app? I'm afraid i don't have the experience, just what I've been taught, and what I've read
Large current spikes pushing the limitations of the caps 'rated riple current' (again, related to ESR) over heats the cap - however you're not discharging this 50 times a second.. such a low duty cycle as 'two shots a minute' should be very gentle on the cap. But I would up the rated voltage to something at least 20% more than the maximum charge voltage you intend to use. For 90V, 120V+. Over voltage can break down the dielectric, and if the cap survives, drop its rated voltage even futher.
Sorry I cannot be of more help, this response is pretty much just me 'brain storming' and thinking out loud, I have no had any experience of long-term use of caps in discharge circuits, but I'm *sure* someone in here has!