Re: Spark length vs. coupling results

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

Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>

Hi Steve,

I ran a model of my OLTC for different coupling:

http://hot-streamer-dot-com/temp/030606-01.gif

The numbers work out as:

http://hot-streamer-dot-com/temp/030606-02.gif

It looks like very high coupling around 0.6 might be "nice".  There is 
actually a lot of energy left for higher coupling to use. I am not sure 
there is any practical way of getting that high, but this does seem to 
suggest that coupling should be as high as you can practically make it.

Cheers,

         Terry

At 01:14 AM 6/4/2003 +0100, you wrote:
>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.
>