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Re: Real data from real experiments.

Hi Chuck,
           This might not seem terribly helpful but in choosing a cap
you are looking at a balance between bang energy and gap losses. For 
example, if your transformer can charge a _1 uF_ cap to the full peak 
voltage the transformer can supply, you have 10x the bang energy of a 
0.1uF cap which can only get charged to the same voltage, BUT the gap 
losses are going to be 10x that figure as well (for a given k) since 
the primary inductance has to be dropped 10x to maintain tune 
(remember gap loss scales with I to a first approximation). You run 
into the law of diminishing returns. It would be desirable for 
example, if you are running 200BPS with a 1uF cap and that is the 
maximum size your transformer can fully charge at that break rate, to 
halve the cap size and double the break rate because, although gap 
losses are roughly the same, the extra repetition will maintain 
stronger spark ionization. At some point (I've heard around 600BPS) 
there is no further gain in upping break rate. In summary, a good 
rule of thumb then is to choose the maximum capacitance that can be 
fully charged at around 500 - 600 BPS or less if your break rate 
doesn't go that high. A sense of balance also helps. To get the best 
use from a large cap you need to run at a low frequency - the lower 
the better so you can maximize primary surge impedance.
     If you plan on running at half the break rate and your supply can 
fully charge twice the capacitance at that break rate, double the 
capacitance. Using the highest k your system can stand also reduces 
gap losses since the gap has to conduct for a shorter time to effect 
energy transfer. 
     I'm sorry - I don't have specific figures for pigs as I don't
use them at this stage. Perhaps Richard Quick, Hull or some one else 
who is experienced in pig coiling can help with the max sized cap you
can stand. Also, the choke used with the pig can also assist in 
boosting cap charge beyond or below raw pig capability depending on
its inductance - for a given break rate and capacitor, a small 
inductor will boost charge and a large inductance will hinder it. 
     Greg Leyh wrote a great piece on this in his TCBA article which 
I don't have handy. All sounds a bit nebulous but I hope it helps.

Malcolm


>     The transformer is a 10 KVA pole pig at 14,400 VAC.  I use a
> stationary/ rotary combination, up to 8 stationary gaps at .025" each
> and the rotary is 12 rotating contacts on a 12" inch diameter, run
> normally at 2500 RPM. Hope that helps.
> 
> Chuck

PS - I designed my new resonator on the following basis :
Min frequency 50kHz with a total secondary capacitance around 70pF
and a height permitting peak voltages of a bit over a megavolt. To 
achieve 1MV with that capacitance I need to transfer 35 Joules per 
bang. With a topload of, say, 10pF (still permits a large radius of 
curvature because of mutual capacitance between topload and resonator)
I need to transfer around 15J which is probably as much as I'll ever 
need with the supplies I'm planning to use. The extra capability is 
there though so this is the last resonator I need to build.