Topload and Arcing

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Danny Stone by way of Terry Fritz <twftesla-at-qwest-dot-net>" <dansto-at-pacbell-dot-net>

I am in the process of designing a coil, using the wonderful software 
available online.
The coil is 6 inches in diameter, and the estimated spark length is about 
40 inches. Using an aspect ratio of 1:4, the coil length is 24 inches. 
Naturally, I'm adding some space at the top and bottom of the coil form, 
for a total form length of about 30 inches.
You probably can guess my question: With the topload (a toroid) sitting on 
top of this form, how can I avoid arcs to the primary, since the distance 
is less than the estimated spark length? The obvious solutions are 
insulation (of the primary) or providing a grounded point to attract arcs. 
Neither are highly desirable for my application.
I've wondered why some of the coils I've seen on the web have two toroids, 
and I recall reading somewhere on the list that this configuration can 
prevent (or alleviate) arcing to the primary. Is this correct? If so, how 
does this work? Is this primarily why the stacked toroids are used?
Finally, how do I calculate the total capacitance of the stacked toroids? 
Do I treat them as two separate toroids, calculating their free-space 
capacitance (from the equation) and summing them as two parallel capacitors?

Thanks,
Danny