From: Paul
Date: Wed, 16 Oct 2002 17:30:27 +0100
Subject: Re: [TSSP] Top Voltage
Hi All,
I've made up some code to output E-field profiles through the
field around the coil. It also does equipotential charts.
Plotting the normal component of the E-field as we rise above
the top of the OLTC sphere terminal, we get
http://www.abelian.demon.co.uk/tmp/tf1210ab.field.gif
The red trace shows the sphere in the low position, down close
to the toroid, and the green shows the sphere raised high.
Both spheres have been set to a potential which gives 30kV/cm
at the surface.
In both cases the initial drop of the field is the same, both
act much like an isolated sphere when examined close to the
surface.
The fields become noticeably different beyond about 1cm above
the surface.
By about 8cm above the surface, the E-field is asymptotic
towards the background field from the toroid beneath, which for
the low sphere (red) is higher because the it's not as far above
the toroid.
I was hoping there might be a big difference near the surface,
in the first cm or two, but appears not.
Something qualitatively odd with these breakout voltages:
We'd expect that as the sphere is raised above the toroid,
it becomes more exposed and we would expect a lower breakout
voltage. Down near the toroid, the sphere is better shielded,
and in accordance with the usual effect of shielding, the
sphere breakout voltage would rise.
This is borne out by the model voltages, 175kV for the low sphere
and 101kV for the high.
Terry's measurements on the face of it, seem to be the other
way around:
tf1210a: 12th Oct, medium sphere just above toroid,
http://hot-streamer.com/temp/OLTC10-12-04.gif
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
Observed: 50% breakout @ Vfire=270, equates to 204kV
Observed: 100% breakout @ Vfire=300, equates to 226kV
Requiring what seems like an increase from 188kV to 204kV.
However, the former is described as 4 breakouts in 64 shots while
the latter is 50% breakout.
Perhaps if the high sphere was set to a Vfire which gave a 4/64ths
breakout, then it would be below 250 rather than above 250.
Therefore I think we must try to set a standard for saying that
breakout has occured, in order to make relative comparisons.
Perhaps we should raise Vfire in each case to the point where
a DC component is detectable in the base current. As in the
recent tests with a base blocking cap.
Chances are that when the coil is at 50% breakout, it is already
well above the breakout threshold. Perhaps we should fire at
very slow BPS, raising Vfire each time until the first bang
that leaves a residual DC on the base blocking cap. Then stop
at that point and record Vfire. Hmm, if the voltage across
the base cap was fed to a scope trace, the residual DC would be
clear, and the peak Ibase can also be read off at the same time,
by taking the peak volts across the cap and calculating from that.
--
Paul Nicholson,
--
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.