In a message dated 99-05-08 06:17:37 EDT, you write:
<< Malcolm already pointed the two possible
> causes that I imagine that can be at play: distorted voltage profile
> due to the proximity of the primary, what eventually degenerates
> to sparking between the coils, or excitation of high-order resonsnce
> modes. Where does these racing sparks appear?
> Antonio Carlos M. de Queiroz >>
I've seen racing sparks occur just about anywhere along my secondary
coils. Often too-tight coupling causes racing sparks near the
bottom of the secondary at around the height of the shallow inverted
cone primary. Other times, the racing sparks occur at 20 % from
the bottom, 30 %, 50 %, 70 %, 90 %, or anywhere in between. I've
also noticed that sometimes, the sparks jump just a short distance
of an inch or two, other times, the sparks seem to jump 4" or so.
They may be traveling along the surface in a sense. Sometimes the
sparks loop out from the secondary, and look like the looping
promenenses that loop out from the Sun's surface.
I've also noticed that they have a very strong tendency to form at
points on the secondary that face sharp points on the primary, which
strongly suggests field distortions and corona as a cause. I always
cover my primary tap point now with a piece of poly to reduce this
facing stress on the secondary.
I did one experiment in which I used a long cylinder of poly around
the entire secondary, but the sparks simply glided along the surface
until they struck the toroid, so it was no help.
I've noticed that mistuning tends to cause the racing sparks higher
along the secondary, but overcoupling (but in tune) tends to cause
the racing sparks near the bottom of the secondary. In some cases,
the mistuning causes the racing sparks very near the top of the coil,
which actually shows where the HV peak based on tuning is occuring.
My 6.5" by 23", #28 wire secondary has numerous burned points all
along it's length from years of abusive tests, but it still gives good
performance. In fact two turns are burned entirely in two, but this
doesn't noticeably affect the performance.
Lou Balint did some magnifier tests in which he placed the secondary
in oil, but at k = 0.6 or so driver coupling, the sparks went through the
oil and destroyed the secondary and pvc envelope tank.