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Re: Unexplained arcing current
Hi Gary,
> Original Poster: Gary Lau 24-May-1999 1913 <lau-at-hdecad.ENET.dec-dot-com>
>
> Then I disconnected my 450pF bypass caps. With the variac all the way
> up, the NST could no longer jump the 3/8" gap. I narrowed the gap just
> slightly to the point where it arced. Now the ampmeter reads only 5
> Amps, and the resistors just get warm. Clearly the bypass caps are key
> to this mystery but I would have guessed that their 225pF in-series value
> is so distant from the 10.8nF required for mains resonance, that they
> wouldn't have mattered. Yes, I've double checked the bypass cap value.
This was the reason why I suggested converting your CR filter into a RCR
filter. To keep the I^2R losses the same (for comparison), you would need
to replace the two 1.6kohm resistors with 4 800 ohm resistors {(800R-C-
800R) x 2}. I was just taking a wild guess that you where encountering
some kind of mains resonance. Adding an R in each leg of the NST should
help dampen out any oszillations between the NST and your bypass caps.
> With the comparatively low energy arcing that is occuring in these
> experiments, I can see the nature of the arc geometry much better (safer)
> than in normal operation. One very surprising thing is that rather than
> there being the intended ring of arcing coming from the copper half-torus
> which represents the minimum arc distance between the electrodes, 95% of
> the arc appears to be concentrated in a very thin channel which is
> precisely coaxial with the holes (~3/8" dia) through the brass
> electrodes. I can't see where this channel terminates, but it is
> continuous and appears to extend well into the electrodes. I'm now
> wondering about the wisdom of sucking air through the arc, as opposed to
> blowing. The sucking may result in a lower air pressure in the channel,
> which will result in a lower breakdown voltage per length, and a longer
> arc with higher resistance. Perhaps the arc channel connects to the
> electrodes through a broad area inside the electrodes where the air
> pressure is at a minimum...
Can you remove the brass fitting? Maybe you can see where the channel
ends, by looking at the discoloration. On the other hand, as you are sucking
air, etc in, this might lead to false conclusions. Could you rebuild your PVC
setup using plexiglass tubing? It wonīt take the heat very long, but at least
you could see exactly where the arc terminates. As it is just a test bed, you
wouldnīt need to make it look neat. The sucking action of the air flow might
be "thinning out" your spark at the gap (i.e: concentrating the hot part of the
plasma somewhere, which isnīt really what you want it to do). I think it makes
sense that the arc is coaxial to your holes. If you reverse the action (blowing
instead of sucking) this might change, because you now get turbulent areas,
where the two air flows (left and right) meet. This might even help keep the
electrodes cooler, because you are no longer drawing the plasma into, but
rather away from them now. The sucking action might also lead to an "extended"
spark (simliar to the action of a Jacobīs ladder). Using the blow-through
technique (and the resulting turbulences), might aid in gap quenching.
Just some thoughts. My experiences (very few) with single gap pressure
/ vacuum quenching have never been very sucessful. My simple static gap
with zero air flow has always resulted in longer sparks for whatever reasons
(and I canīt say I fully understand the exact action of the gap, yet)
Coiler greets from germany,
Reinhard