Spark length vs. coupling results

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

Original poster: "Stephen Conner by way of Terry Fritz <teslalist-at-qwest-dot-net>" <steve-at-scopeboy-dot-com>

Dear list,

After the recent discussion on efficiency of pulsed SSTCs etc, I decided to 
do those experiments that Jimmy Hynes suggested.


.oO METHOD Oo.
I ran the mini OLTC three times, once with the secondary in normal 
position, once with it raised by around 1", and again raised by about 2". 
In each case I measured the output with an antenna made from a metal cup on 
an insulating base 18" from the topload, and connected to a x10 scope 
probe. I adjusted the DC link voltage so that the peak-peak antenna voltage 
(and hence hopefully the topload voltage) was the same on each run. I 
adjusted the quenching for first notch in each case, and measured the 
maximum spark length between a breakout point and a grounded point. All 
tests were conducted at the same rep rate of around 40bps. More details of 
apparatus and method on request.

.oO RESULTS Oo.
Here are the scope traces I got. The top trace is antenna voltage 
(500V/div) and the bottom one is primary capacitor voltage (100V/div)

For high coupling http://www.scopeboy-dot-com/tesla/scopehighk.jpg
DC Link voltage was 100V and maximum spark length 41mm (1.6")

For medium coupling http://www.scopeboy-dot-com/tesla/scopemediumk.jpg
DC Link voltage was 115V and maximum spark length 40mm (1.6")

For low coupling http://www.scopeboy-dot-com/tesla/scopelowk.jpg
DC Link voltage was 133V and maximum spark length 40mm (1.6")

Accuracy/repeatability of spark length measurements +/-2mm (+/- 0.08")

.oO DISCUSSION Oo.
First of all it's obvious that the spark lengths are the same to within the 
measurement uncertainty. However, looking at the scope traces, the envelope 
of antenna voltage does not show the waveform one would expect. There is an 
obvious dip, and the peak of the secondary envelope does not match the 
notch in the primary envelope. This may be due to corona losses dragging 
the topload voltage down. It is observed that moving a grounded point close 
to the breakout causes the dip to get bigger as streamer corona forms. Or 
alternatively it may be due to energy from the secondary sucking back 
through the anti-parallel diodes after quenching.

Secondly, as the coupling is decreased, it takes a good deal more primary 
capacitor voltage to produce the same topload voltage. With high coupling, 
around 150V is required, but with low coupling it has to charge to more 
than 200V, i.e. at least double the energy. Other experiments I conducted 
suggest that the Mini OLTC system is around 55% efficient (From wall plug 
to topload :) ) with high coupling, so with low coupling it would be a 
miserable 20%.

.oO CONCLUSIONS Oo.
Rise time of the topload voltage envelope does not seem to affect the spark 
length between points to any significant degree. For all of the coupling 
factors investigated, the spark length was about the same as would be 
expected for a DC voltage of the same magnitude as the peak topload 
voltage. This suggests that impulse-related breakdown effects are not 
significant in this scenario.

On the other hand, low coupling makes the system much less efficient. More 
power is wasted as I^2R heating, corona, radiated RF, etc. during the 
ringup period, simply because it takes longer. Hence low coupling would 
reduce spark length for a given input power, and make components run 
hotter. So practical coils should still aim for the highest coupling 
possible, with due consideration to racing sparks or poor quenching.

.oO FURTHER WORK Oo.
Maybe the rise time of the envelope does not affect spark growth, but there 
is still a possibility that the rise time of each individual cycle, i.e. 
the resonant frequency of the coil, might. Someone else can investigate 
this one because I don't really feel like making three more mini OLTCs 
identical except for different gauges of wire.

Also, the impulse characteristics of a gap are dependent on the shape of 
the electrodes. Maybe the results without a breakout point would be quite 
different. The Mini OLTC won't do anything without a breakout point, but 
the next one sure will <evil cackle #4>

Finally, the rep rate used was too low for streamer growth. If we had been 
trying to grow a streamer between points using multiple bangs, the results 
might have been different.

any comments?

Steve C.