Re: DC secondary components

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

Original poster: "Paul Nicholson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <paul-at-abelian.demon.co.uk>

Jim Lux wrote:
> Corona discharge isn't a particularly high current phenomenon
> ( a few mA),
> ...Without giving it much thought, I would expect the DC average
> current to be in the microamps,

Hmm, indeed. The corona only a small fraction then of the topload's
displacement current...so the DC component of the corona will be
another small fraction again, so yes, it might be only a few tens of
uA. That *is* a tricky one. 

You can see in the fourier spectrum in
http://www.abelian.demon.co.uk/tssp/tfss270501/
a small low-frequency blip from slight rectification of the beat
envelope, which is how we would expect see the DC component.  But in
this case, the signal was a fairly low voltage using a solid state
gap and thus no corona, so it must have come from non-linearity in
the recording.  Just shows that any attempt to detect this must be
very highly linear to succeed.

> One approach might be to run the secondary current through an
> electrolytic cell and measure the amount of metal added or removed

A cunning solution!

Or perhaps a minute deflection of a compass needle close to the 
secondary.  Anyone got an aurora detector?

> Most of the spark theory says that the charge for a free air
> streamer moves back into the source as the spark collapses,
> leaving essentially neutral (net) air behind (it might be ionized,
> but there's an equal number of + and - ions).

in which case we would see no DC leakage at the base.  Any that we
do see must then be due to charge leaking permanently away from the
topload - beyond recall, so to speak, by the next half-cycle. 
I guess we'd only see these currents, if they exist at all, when
the topload is close to arcing to something.  A reliable detector
might make a neat warning device or automatic cut-off.
 
dwp wrote:
[DC build-up on the topload] 
> ...has been widely reported, for varying designs, that there is.
> It appears to be on the order of a few KV, rather than the
> 100s of KV associated with the nominal output, however the
> DC does seem to be there.

You'd better tell us just exactly where this has been 'widely 
reported' because it's obviously wrong.  A few kV of DC on the
topload would give a constant (few kV)/(few tens of ohms) amps
through the secondary, and the wire would quickly melt. And if
that's not good enough for you, consider: a dc base current
component cannot be bigger than the ac base current, since it
must come (somehow) from rectification of just a fraction of it.
Thus, even if the entire coil base current was somehow rectified
at the topload, the top voltage DC would only be a few tens of amps
times a few tens of ohms.  Those are two real good reasons why you
should seriously question any claim of DC on the topload.  Static
charges evolving on the surface of dielectrics, or on floating
conductors placed near to the coil or topload, are quite another
matter, and these may be one source of faulty observations.

[Applying a DC pedestal to the coil base]
Jim wrote:
> an intriguing idea.  Especially if you were to make the voltage a
> significant fraction of the topload voltage during breakout, 

I was wondering if it might help preserve the remnants of streamer
channels between bangs, thus enhancing subsequent growth.

> You'd want a good RF bypass cap across the power supply

A non-trivial component, I imagine.

> You'd want a small TC (low voltage) for this so you could really
> make a difference. A few kV out of 500 kV on a big coil might not
> make a difference.  10-20 kV out of 75-100 kV though...

Hope someone rises to the challenge, to see what a DC supported
breakout looks like.

--
Paul Nicholson
--