choosing the right capacitor, secondary and topload

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Original poster: Crystal Clear Solutions <webmaster@xxxxxxxxxxxxxxxx>

I have a 15kv/120ma NST, primary 120v/60hz
6" secondary 29.5" tall 22g wire
sych gap 120 bps
assume all protective circuits/safety gaps/filters are in place

I have read about Cres and Cltr.  What is the best Cap to choose for 
the largest sparks? I've gotten Cres = 0.01uf and Cres for a sych gap 
= 0.026uf and Cltr = 0.055uf.  Do these sound close?

Here is a second question.  If my Cp is fixed and I can retap my 
primary at any point to adjust for a new resonant frequency (Fres), 
how will varying the height/wire size of my secondary affect spark 
length?  Will increasing the Ls and increasing Fres yield bigger 
sparks?  Is there some trade-off?

Third question: If one assumes Cp fixed and I can retap primary at 
any point to adjust for new Fres, how will increasing the toroid size 
affect spark length?  From my reading increase the toroid size 
increases Cs so Fres will go down.  Lp will have to increase and this 
will drop the secondary output voltage.  However, increasing the 
toroid size, by increasing Cs will allow for a greater breakout 
voltage which should increase the spark size.  Where is the "sweet spot"?