4 Inch tesla coil - Inquiries

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Hydrogen18" <hydrogen18@xxxxxxxxxxxxx>

I've got a 4 inch tesla coil that I've pulled out of storage and set 
about improving and trying to make into a more logical setup. Specs 
are as follows

Secondary - 4 inch Schedule 80 pipe, 1500 turns 28 awg, 18 inches long

Primary - Coil of 1/4 inch copper pipe, about 13 turns worth. laid 
out flat and zip tied to plexiglass. 1/4 spacing between turns. about 
2 foot overall diameter.

Spark gap - Static style, 1 inch spherical brass drawer pulls mounted 
to 1/2 inch copper pipe. This can be adjusted really easily. A cheap 
blower supposedly running at 25000 rpm blows alot of air in an 
uncontrolled fashion across these. They get pretty dang hot, but the 
arc seems to be quenched pretty well, although I have no first hand 
experience with spark gaps outside of my own design.

Topload - 12 inch toroidish thing with a section of 4 inch dryer duct 
stacked around it. Seems to discourage primary strikes pretty well, 
although it was designed for such

Tank Cap - 942C20P15K's(2000 VDC 0.15MFD) in strings of 8, up to 3 
strings of these in paralell(plenty more strings ready to be 
assembled). Mounted in a wooden box but the capacitors themselves are 
in 180 degree sections of PVC pipe for insulation. Connection is made 
at all junctions to the strings by means of 1/4-20 brass bolt.

Power supply - 4 MOTS producing around 9500 volts RMC AC, current is 
limited by 10 lbs 10 AWG wrapped around a 3 inch piece of Schedule 80 
with an adjustable iron core(I have also left the shunts in the MOTS, 
as I see no need to drive them as hard as some people do; mine are 
not under oil but epoxy potted). Driven by a 30 Amp 240 VAC outlet, I 
do not feel this is the limitation in my setup.

So far my recent attempts have yielded point to point strikes of 28 
inches or more; longer arcs accounting for the bend. That was with 2 
capacitor strings. Attempting to use 3 capacitor strings seems to 
make finding a tuning point impossible, all I suceeded in doing was 
getting the primary so hot I burned myself(this was in just a second 
or so of having the spark gap firing). I am dumb tuning this coil, 
just starting at turn one and moving out until I get a large 
response, and then making smaller movements from there. I've made the 
following conclusions:

- Steel anywhere in the primary circuit basically destroys 
performance and likes to get really hot.
- Tuning seems to be very pecuilar on a lower coupling coil(I had 3 
inch coil in the past with 'high' coupling between the primary and 
secondary, you could make turns of adjustment to the primary without 
noticeable output in spark).
- The charging current available to the coil doesnt seem to affect 
things much, other than the spark gap getting louder.
- This spark gap is better than the last, because I don't have to 
wait for it to cool down between runs. It takes a very long time run 
time for the initial breakdown voltage to change due to tempature of 
the electrodes. They are hotter than a car brake rotor at the parts 
you can observe, but I suspect where it matters they are remaining 
cool. They also dont require a resurfacing every 30 seconds of runtime.

And I've got these concerns:

- My primary is very poor. The spacing is nonuniform. I think tuning 
is changing because the turns are not held very securely.
- Should I add another gap to my static spark gap? Are there any 
telltale signs of an overloaded static power gap? I know that the 
signs of a 'power arc'(the deep orange-reddish arc that does not 
quench) or rapid electrode erosion are two to look for, and I have neither.
- I have a safety gap on my transformer, so I'm not concerned about 
opening my spark gap up too much. But is there a primary side side 
RMS VAC that I shouid aim to have the spark gap breakdown at?

My questions are from here:

- Should I bother improving the primary? I'm not getting turn to turn 
arcing, it's the inconsistency of the shape that's my concern.
- What's the maximum size capacitor for this coil I can reasonably 
expect to run?(I know my limitation is the spark gap, and probably 
will be for all my coiling years given the easy availability of large 
dangerous HV transformers)
- Why is my primary getting so darn hot? Is off resonance driving of 
the coil the cause of this?
- Is there a better(intelligent, mathematical) method of tuning the 
coil I can employ?
- What kind of spark length performance should I expect from this coil?
- Any other obvious mistakes?