Re: the cure for racing sparks

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

Original poster: "Paul Nicholson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <paul-at-abelian.demon.co.uk>

Assuming a well made, undamaged secondary to begin with, here's my
tentative list of racing arc causes:

#1 Topload breakout voltage higher than secondary breakdown
   voltage. (V) Cure: add breakout point or reduce ROC of topload.
   Example: Steve's coil with the sphere.

#2 Coupling too high: (V) Rise time is only over 1 or 2 cycles,
   rather than 3 or 4.  Reduced streamer capacity means
   insufficient restraint to topvolts rise. Cure: reduce coupling
   a little.

#3 Primary too close to secondary: (RVC) E-field gradient in space
   between pri and sec is too high (say > 12kV/cm). 
   Cure: raise secondary or redesign primary.

#4 Gross tuning error: (RV) Primary is exciting 3/4 wave mode to 
   produce a voltage max around half height. Cure: sort out the
   primary tuning error or cap fault.

#5 Ground circuit open: (RVC) The base of the coil is not tied to
   earth potential. The base finds it's own potential wrt ground
   according to the balance of external C on the coil.
   Cure: make sure the coil base connects to whatever collects the
   coil's E field (ground, counterpoise coil, toroid, whatever).

#6 HF ringing from primary: (VC) Higher modes of the primary
   excited at each gap firing, injecting HF into the secondary.
   Cure: modify primary spectrum.

#7 Primary induction is too concentrated on a small region of
   secondary: (VC) Excites higher modes of the secondary and wastes
   energy.  Cure: redesign primary for more uniform coverage of the
   secondary.

#8 Discharges to earth from the topload redistributes the coil's
   remaining stored energy into a 1/2 wave mode with a volts max
   around a third to half height. (RV) Cure: stay with streamers
   to air, or use wider toroid.

#9 Insufficient control of E-field around coil top: (RC) Allowing
   breakout from the top turns. Cure: lower topload, or reduce ROC
   of topload, or add breakout point.

(R) means radial sparks coming out of the coil, either curving
    round to meet topload, or curving out and down to
    primary/ground.
(V) means vertical sparks along the coil, either long arcs, or fine
    rapidly changing streamers running all over the coil, or near
    the ends.
(C) corona forming on the surface of the coil.

Some are more speculative than others, and it would be nice to find
examples of as many as possible.  I suppose that any coil will
ultimately be limited by case #1, and will have a maximum tolerable
ROC of topload (say 6" for Steve's coil?) above which the coil
will break down before the topload breaks out.
--
Paul Nicholson,
Manchester, UK.
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