From: Paul
Date: Sun, 13 Oct 2002 17:05:40 +0100
Subject: Re: [TSSP] Top Voltage
Hi All,
I've had a look at the modeling of Terry's coils in the five
cases looked at (three spheres raised high with the current
probe, 13th Sept) and the recent two measurement on the middle
sphere at lower height.
First, I found an error in the input file for the recent measurement
(9-12 Oct, tf1210.in). I'd got the small sphere radius in there,
not the middle sphere. Doh!
With that fixed, we predict a breakout potential of 152kV for
26kV/cm threshold field, and 175kV for a 30kV/cm field. We're
estimating 190kV-200kV measured. That gets us into the right
ballpark for that one at least.
Looking back at the three earlier results, there's some doubt
about the ceiling height. I modeled the coils with a 2.0 metre,
(79") roof height and wall radius, which is a little short.
This will have artificially increased the surface field on the
small spheres, thus lowering the predicted breakout voltage below
the observed.
With the correct ceiling heights, the modeled Fres are all
about 2% on the high side of tcma values. I've stuck in
an artificial lumped capacitance of 2.8pF which brings Fres
into line with tcma, but won't mess up the top field. I don't
know where this extra C comes from in practice. I've put
the walls rather arbitrarily at 2.5 metres (98"), so it may
be that. The toroid is now 23.5" rather than 24" so that has
increased the modeled Fres a little too.
I've set the ceiling height to 84" which I think is the correct
value.
After recomputing everything, we have
tf1309s: 13th Sep, small sphere, rod, current probe,
http://hot-streamer.com/temp/PaulArc/020913/Tek00001.gif
http://hot-streamer.com/temp/PaulArc/020913/TEK00000.CSV
Predicted breakout: @ Vfire=92, equates to 67 kV
Observed: @ Vfire=119, equates to 87kV
tf1309m: 13th Sep, medium sphere, rod, current probe,
http://hot-streamer.com/temp/PaulArc/020913/Tek00002.gif
http://hot-streamer.com/temp/PaulArc/020913/TEK00001.CSV
Predicted breakout: @ Vfire=116, equates to 85 kV
Observed: @ Vfire=191, equates to 139 kV
tf1309b: 13th Sep, big sphere, rod, current probe,
http://hot-streamer.com/temp/PaulArc/020913/Tek00003.gif
http://hot-streamer.com/temp/PaulArc/020913/TEK00002.CSV
Predicted breakout: @ Vfire=146, equates to 107 kV
Observed: Vfire=226, equates to 165 kV
The above are a little better than shown before, because now the
ceiling is a little higher, the topload a little less wide, and
the frequencies have been adjusted to a closer match.
tf1210a: 12th Oct, medium sphere just above toroid,
http://hot-streamer.com/temp/OLTC10-12-04.gif
Predicted breakout: @ Vfire=230V, equates to 175kV,
Observed: 4 in 64 shots @ Vfire=250, equates to 188kV
tf1210b: 12th Oct, medium sphere raised to 7.25" above toroid,
http://hot-streamer.com/temp/OLTC10-12-11.gif
http://hot-streamer.com/temp/OLTC10-12-11.CSV
Predicted breakout: @ Vfire=134, equates to 101kV
Observed: 50% breakout @ Vfire=270, equates to 204kV
Observed: 100% breakout @ Vfire=300, equates to 226kV
Seems we're in the right ballpark for the sphere close to the
toroid, but when the sphere is exposed well above, the measured
breakout doesn't fall to our expected value, and we end up about
a factor of two short in our predictions.
I think the result for tf1210a is pretty good, so something
is going astray as the sphere is raised. Terry can you confirm
the rod is 3/8" diameter. Perhaps it is a ceiling effect
again. The high spheres are some 30" below the ceiling, so the
ceiling will have more effect on breakout than the ground, etc,
will. Maybe Terry's lab needs a tinfoil ceiling, which is
effectively what the model is using. Perhaps when the sphere
is lower down close to the toroid, the surface field is less
dependent on the ceiling and walls and so becomes more predictable.
Hopefully this will become clearer when data comes in for other
systems.
Looking at Terry's other readings, below breakout, we have
100 Firing Volts
PK FREQ kHz (Error +/-) Q FACTOR (Error +/-) LEVEL
1 37.154 (0.01%,4Hz) 93.74 (0.99%, 0.9) 0.0dB
150 Firing volts
PK FREQ kHz (Error +/-) Q FACTOR (Error +/-) LEVEL
1 37.162 (0.01%,4Hz) 92.77 (1.17%, 1.1) 0.0dB
200 Firing Volts
PK FREQ kHz (Error +/-) Q FACTOR (Error +/-) LEVEL
1 37.160 (0.01%,4Hz) 92.13 (1.17%, 1.1) 0.0dB
Notice that the Q is already falling at 150 volts, if anything
I'd expect it to rise due to reduced primary loss factor. Does
this indicate early breakout? Maybe not, but I'm left wondering
why the Q is going down as the voltage is raised.
I've a lot more work to do to refine these figures. Time domain
calcs are still running, so more later I hope.
--
Paul Nicholson,
--
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.