SISG: OLTC-ish coil design: Help?

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Original poster: "J. Aaron Holmes" <jaholmes@xxxxxxxxxxxxxxxxxx>

Hi, all!

I'm crunching numbers on a SISG-based coil I'm getting
ready to build, and wondering how to make best use of
the parts on hand.  Specifically:  I've got a 2400V
200VA (yes, little!) potential transformer and a 14"
sphere for a topload.  Additionally, I've set the
somewhat arbitrary goal of putting 200W into the coil.
 If I use three SISG sections, my firing voltage will
be 2700V, so at 120BPS I'll need about 450nF in the
primary cap.  That's a big cap, and drags the primary
frequency down under 100kHz.  This has a bunch of
seemingly-negative side effects; it suggests that I
either drastically reduce the number of turns in the
primary down to, e.g., *one* (like in a OLTC), thereby
drastically *increasing* my peak primary current (bad
for the little IGBTs, methinks), --OR-- I could put
2000+ turns on the secondary (gulp), thereby making it
pretty lossy but lowering its frequency to the point
where my primary can actually have a few turns in it
(and keeping the primary current from running away).

Any other ways out of this?  As the coil will be
low-powered (200W), I want it to be physically small,
and hence I don't want to go berserk on adding
topload.  The 14" sphere ought to look nice atop a
6"x24" secondary.

Alternatively, I could go for a smaller cap and shoot
for a higher break rate on the SISGs, but Terry
indicated that he didn't get much bang for the buck
(or would that be *buck* for the *bang* in this case?)
when he pushed things above 120BPS.  Hmmm...

If I fairly arbitrarily choose 2000 turns of #30 wire
on a 6"x24" secondary with the 14" sphere topload, the
frequency works out to about 90kHz.  To match that in
the primary circuit, I'd need little more than four
turns (!) which yields little more than 6uH.  That, in
turn, suggests a peak primary current of almost 700A!

Maybe I should just concede defeat on the 200W and
shoot for 150W to be safe ;-)  I don't want the
performance to "suck", but I also don't want to be
red-zoning the little IGBTs.  I suppose if I can
duplicate Terry's success and get >24" streamers out
of the thing with well under 200W, I ought to be
pretty darn happy! ;-)

Regards,
Aaron, N7OE