Best cap size for a sync gap

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Original poster: "Lau, Gary" <gary.lau-at-hp-dot-com> 

I finally "got around to it" to an experiment that's been nagging me for a 
very long time.  I wanted to determine the optimum cap size for a coil, in 
terms of maximizing the power pulled from the NST.  Tank frequency issues 
come later.

With a newly constructed sync RSG and a dummy load consisting of three 500W 
halogen lamps in series in place of the primary inductor, I scoped the gap 
voltage.  As is typical with sync gaps, I adjusted the phase such that the 
bang occurs somewhat after the peak charging voltage.  The later I phased 
it, the brighter the lamps and the higher the bang voltage. If the timing 
is too late, the gap stops firing altogether, so I brought it just to the 
brink of this point.

To measure the voltage, I use Terry's fiber optic probe, which sadly, I 
have yet to accurately calibrate.  But for the purposes of finding the best 
cap size, even a qualitative measurement is adequate.

I use an unmodified 15/60 NST, cranked up to 144VAC.  I had available two 
.02uF caps, and one .01uF cap.  With these I measured the bang voltages 
using .02, .03, .04, and .05uF.  The peak-to-peak bang voltages measured 
were 688, 618, 552, and 482 mV respectively, as directly indicated on the 
scope.  FWIW, if I scope just the unloaded NST secondary -at-120VAC input, I 
get 598mV p-p.

If I calculate the relative bang size with a simple scale-less formula of 
C*V*V (mV*uF*uF), I get
.02   9,467
.03  11,478
.04  12,188
.05  11,616

 >From this I conclude that using a .04uF cap with my 15/60 NST will result 
in the highest power throughput.

Has anyone else performed such an experiment?  Just trying to understand 
why my result is so at odds with the widely suggested value of .028uF for 
the same power supply.  Hmmm, wonder what I'd have gotten if I had tested 
at 120VAC?

Regards, Gary Lau
MA, USA