Re: Streamer formation on the scope...

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

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

Hi Jim and Paul,

At 10:28 AM 9/6/2002 -0700, you wrote:
>
>Terry,
>
>Some transmission line questions...

Oh oh!!

>
>20-30 nSec is about the round trip time in 6 feet of cable (1 ft/nsec one 
>way, in space... 20 nsec = 10 feet round trip in free space, 6-7 feet with 
>velocity factors in 60-70% area). Capacitive loading always seems to make 
>the round trip time a bit longer.
>
>How long is the coax from the current probe to the scope?  Is the probe a 
>50ohm device feeding a 50ohm scope or is it a voltage source feeding a 1Meg 
>scope?  What sort of coax is it (the really low C, low microphonic scope 
>probe stuff, or something like RG174/RG58)

The source Z is a 100 ohm SM resistor that seemed to calm down the
transient response.  The scope is 1Meg 13pF in.  The cable is the finest
RG-58 the Radio Shack sells ;-))

>
>A quick way to disambiguate measurement cable artifacts from real stuff is 
>to change the cable length by about 30%.. (you don't want twice or half...)

I feed a 100mAp-p square wave in and got this:

http://hot-streamer-dot-com/temp/02090601.gif

The data file is at:

http://hot-streamer-dot-com/temp/02090601.CSV

The input is in blue as picked up by a 60MHz 10:1 probe.  The top is the
output.  The output looks nice and stable but feeding the 50ohm A33120a's
output right into a 1 ohm resistor through about 3 feet of coax causes a
big spike...  I might have screwed up the fiber probe since I did not have
as good of toys back then.  I may be able to bring the spike back to the
fiber signal.  I "thought" it should be a square wave, so I "made" it one
at the time...

The current sensor has a 1 ohm resistor in it so the calibration is
100mAmp=~2.5mV for yesterday's experiment.  The polarity seems correct.  I
worry when everyone tells me the spikes should have been positive, but the
probe's polarity seems right!

Note the delay as the light passes though the 10 meter fiber cable, the 6
foot coax, the scope probe's 3 foot cable...  About 70nS...

So we run into problems when trying to do high frequency measurements here.
 The fiber probes really were made for less than 2 MHz work.  They have
twisted pair wires in them and all that so 40MHz is probably going to be
really interesting trying to separate the real stuff from the rings in the
probes.  This is not exactly a microwave setup or anything ;-))

http://hot-streamer-dot-com/temp/P9060006.jpg


Then Paul writes...

>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.

I think I have it right...

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

Yes!!  The polarity was a big factor in the GMHEICSLR coil!!  That single
shot coil really needed for the first big cycle to be negative to get the
longest arc!!  It made a dramatic arc length difference if you reversed the
polarity.

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

It's all electronic so there are few unstable variables ;-))

>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.

See the above probe test data.

>
>> 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?

Done ;-))

>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?)
 
The fancy signal generator has a button for noise.  I think it does white
and pink...  Have to dig though it's menus...

>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.  

Good news!  The Tek7000 series has 32Meg depth instead of only 10k!  Bad
news...  They "start" at about $30,000 and stop at over $100,000...  Don't
hold your breath there pal ;-))

>--
>Paul Nicholson
>--
>

Cheers,

	Terry