Size DOES matter?

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Original poster: Steve Conner <steve@xxxxxxxxxxxx>

Hi all,

I was pondering the relative lack of spark length from my DRSSTC. There 
seemed to be no shortage of energy going into the thing. No parts were 
heating excessively, so I assumed most of it was going into the discharge. 
But the sparks were not so impressive. I was getting maybe 33" at any 
breakrate above 100hz, and John Freau's formula predicts 40" for 5.5 joule 
bangs at 100Hz.

What's more, they didn't grow that much with breakrate. I well remember 
Richie Burnett writing about a classic coil that made maybe 14" sparks in 
single shot mode but grew dramatically to 66" at 200Hz breakrate. (Richie 
was not born yesterday so I assume he checked that the tank capacitor 
charging voltage was the same in both cases.)

Mine did 22" with a single bang and still only about 33" at 200Hz with 1kW 
power input. (by way of comparison my classic coil did 40" with 1kW input 
at 400Hz, and 17" with single bangs of very roughly the same energy.)

So I got to thinking, maybe size does matter? Maybe the ultimate limit to 
spark growth is not how much power/energy your oscillator can deliver, but 
the size of the resonator? It's often been said that three times your 
secondary length is the maximum spark length you should attempt. Well maybe 
there is a law of physics that says you just can't get more than that no 
matter how much power you supply!

So how could that be? Well for a start it's not too hard to see how toroid 
size could influence spark length. Streamers need a certain field strength 
to grow. And, the maximum field strength at the surface of any toroid is 
limited to the breakdown strength of air. So it seems reasonable that a 
smaller toroid might not "throw" the field (and hence possibly the 
streamers) out to such a long distance: simply because it has less surface 
area to accommodate field lines.

The resonator length may also have an effect. If we admit the argument 
above, then the toroid should be brought almost to its breakdown voltage 
for most efficient streamer throwing. If you try to drive a large toroid 
with a small resonator then the resonator will flash over end-to-end before 
the toroid reaches breakout. Hence (even with a breakout point) the spark 
length will be limited by flashover rather than power availability.

To prevent flashover of an undersized resonator you need to reduce the 
output voltage by fitting a breakout point and/or turning the peak power 
down (this is achieved by loosening the coupling on a classic coil, or 
turning down the DC link voltage- and increasing burst length to restore 
bang energy- on a DRSSTC) The reduced output voltage will probably lead to 
smaller sparks.


Does anyone have any comments as to whether any of this makes sense ;)

Steve Conner