It's interesting that nearly all "fast recovery" HV diodes spec Trr at
50 ns. That goes for single-junction 1KV diodes and higher-voltage stacks. And the 50 ns is either specified as typ or as max.I guess there's some physics there, the carrier drift time across a kilovolt of "i" region or something.
I guess I'll have to make my own, a string of lots of lower-voltage diodes. There are duals and triples and even a few quads in surface-mount packages. But almost all of the 200-300 volt diodes also spec Trr at 50 ns!