Re: Topload and Arcing

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

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Esondrmn-at-aol-dot-com>

In a message dated 8/13/01 7:42:52 PM Pacific Daylight Time, tesla-at-pupman-dot-com 
writes:

<< 
 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
  >>
Danny,

I use strike rails on the primaries of both of my coils.  Connected to the 
main RF ground and with a two inch or so opening in the circle so it is not a 
solid loop - as in shorted turn.  My larger coil (6" diameter) uses stacked 
toroids for just that reason.  The smaller toroid on bottom provides shading 
for the top windings of the coil and the larger elevated toroid is where the 
sparks leave from.  You can see this at my webpage:
http://members.aol-dot-com/esondrmn/index.html
I believe the equivalent capacitance is about 60 % of the calculated value of 
each toroid added together.

Ed Sonderman