OLTC II Initial calculations

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Original poster: "Stephen Conner by way of Terry Fritz <teslalist-at-qwest-dot-net>" <steve-at-scopeboy-dot-com>

Hello list,

I've been doing calculations for my next OLTC, which I name Steve's Folly. 
This is the first acceptable design I got (with some help from FANTC)

Primaries:		Two single turn, 15" diameter, 3" copper strip.
Tank cap:		10uF 1000V MMC
Switch:		Two IGBT Bricks (Powerex CM600HA-24H 1200V 600A)

Secondary:		10" x 20", 1500 turns
Toroid:		6" x 20"

Resonant freq:		65.3kHz

Bang energy:		10J
Peak primary current:	4100A (2050A/brick)
Top voltage:		625kV (if lossless: 450kV if 50% efficient)

Output -at- 120bps:	58"

I have a question for the more experienced tesla-teers on the list. I heard 
there is a relation between the top voltage and the secondary length i.e. 
if the secondary is too short for the voltage it will flash over and be 
ruined. What is that relation, and do you think I am the wrong side of it? 
I'm a bit uneasy given that if you apply the 1.1kV/mm rule, the sparking 
distance for 625kV is 27" which is longer than the secondary! Plus, I don't 
want to be striking the primary/control box too often. I could easily make 
the secondary taller/thinner if I have to.

Another thing, do you think the resonant frequency is too low? I heard that 
streamers don't grow so well at low frequencies. Again, I could probably 
raise it a bit without running out of peak current.

Any input much appreciated,

Steve C.