Re: Streamer formation on the scope...

["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>

Terry wrote:
> http://hot-streamer-dot-com/temp/SFarc01.gif 
> The bandwidth of the toroid probe is 40MHz but the level is
> uncalibrated here.

We can see the reactive current charging the fixed C of the discharge
terminal/wire.  If we estimate the capacitance of that structure, we
can apply a rough calibration to the waveforms.

Most interesting that the breakout is much more pronounced on the
negative going half-cycles.  Does that correspond to -ve topvolts - 
depends which way round you've put in the probe.

In view of this, with a high-k DC system, it may begin to matter
which way the secondary is wound wrt the primary. 

I'm glad to see a consistent breakout threshold on each half-cycle,
that's just beautiful!

> http://hot-streamer-dot-com/temp/SFarc02.gif

> ...there are many spikes, but really there is only one during each
> bang.

Hopefully we can get some single shot events captured.

> http://hot-streamer-dot-com/temp/SFarc03.gif

A spike well suited to integration.  Looks like the probe bandwidth
is sufficient to cover the bulk of the energy passing by.

> http://hot-streamer-dot-com/temp/SFarc04.gif
> Note the small frequency at about t=25nS... 

Nice.  A probe artifact, or a genuine component of the discharge
current?  You'll have to rig something up to generate an artificial
current spike of similar duration to see if that excites ringing in
the probe.  I'm happy to bet that this is a streamer resonance, and
the frequency of it relates to the electrical length of the streamer
in a transmission line fashion.

> I turn up the juice I start to see these super spikes:
> http://hot-streamer-dot-com/temp/SFarc05.gif

That ringing is quite persistant, and there seems to be several
components to it.  We'd better characterise that probe. Can you pass
a square wave current through the probe and capture the waveform?

>  Here is a nice one.
> http://hot-streamer-dot-com/temp/SFarc06.gif

That's a beauty.
If those ringings increase their period consistently from one half-
cycle to the next, it would likely be due to the extension of the
streamers.

> http://hot-streamer-dot-com/temp/SFarc07.gif
> http://hot-streamer-dot-com/temp/SFarc08.gif

> Seems to have about a 120nS harmonic or 8.3MHz.  There is
> another 50MHz ring in there too.

Yes.  If the distributed reactance of the streamer dominates over its
loss resistance, we should expect to see a spectrum of modes.  That
would be very nice if so, because we could apply a transmission line
model to the streamers.  The streamer becomes a lossy line to nowhere
whose input impedance is seen by the topload.

On the whole, the bushy nature of the breakout from this tip seems
to be reflected in the streamer current consisting of a very large
number of small transients of the kind in SFarc08.gif.  The transients
seem to come in noisy bursts mainly on the negative (we think?) 
voltage half-cyles.  Would be nice to see how this differs from that
of a long single streamer.  I think we need to try to work with the
latter if possible, so that we can try to see how the streamer 
evolves (and maybe those HF ringings will allow us to do that). 

I think this is really marvelous and your casual tests 
demonstrate that there's absolutely loads of interesting stuff to 
discover in here.  I hope you can calibrate the probe and characterise
its frequency response (got a noise source by any chance?) and give
us some data files.  The difficulty will be to choose a timebase that
allows reasonable accurate integration of the area under the fast
pulses, but allows us to capture most of the firing event. Wish we
had 10^6 points rather than 10k.  Could be bordering on special-
purpose hardware.  Hopefully a bigger breakout in a single streamer
will have fewer and slower fine transients so that we can fit it
all into a 10k data buffer.

Want to try to simulate a step-wise extension of a streamer?  
Charge up a length of coax with DC, current probe on the input,
and capture the transient when you connect an extra length onto the
end.  Does it look qualitatively like the one in SFarc08.gif?
I'll bet it has a lot more ringing due to the low loss.  Maybe you
should try with (resistive) vehicle HT lead instead of coax. Anyway,
we have a lovely RLC characteristic in those spikes.
--
Paul Nicholson
--