From: Paul
Date: Fri, 17 May 2002 14:11:16 +0100
Subject: Re: [TSSP] Topload breakout potentials
Bart wrote:
> I performed an arc voltage test tonight to find the cap voltage...
> ...dial caliper for accuracy of gap...
> ...started at 0.5" and turned the voltage up to 180Vac. At 0.35", the
> gap arced...
> With a sphere diameter of 1.25" spaced at 0.35", the arc voltage
> should be around 22.3kV using the equations from the North report
Brilliant stuff - that's a great test and it certainly moves us on a
bit. Looks like you were burning the midnight oil there!
I've put in the 22.3kV in place of the 15.3kV. That moves things nicely
in the right direction, I was beginning to run out of ideas.
As a cross check on the software, I used your sphere gap to test the
simulator's modeling of the surface fields. I set it to compute the
surface field in the gap between the two balls and obtained a specific
gradient of 1.398 V/cm/V, which times your 22.3kV gives a nice 31.2kV/cm
at the surface. That's pretty much the value we'd expect to see at the
breakdown threshold, so I guess tssp agrees with North :).
Anyway, with this extra bit of primary voltage the model peaks at
24.5kV/cm at your -12% tuning. The surface field at MCTV is 38kV/cm
so I'm wondering if the initial 24.5kV is enough to start a small amount
of breakout, which adds a little to the Ctop, which brings the secondary
down a little into better tune, therefore more Vtop, more breakout, etc,
and so on, eventually multiplying to give you copious breakout with the
coil effectively in tune. I guess this process might take place over a
few consecutive bangs, or perhaps within one beat?
Do you have difficulty quenching? At 12% detune the primary waveform
doesn't form a very good notch - it's still at 50% of the max amplitude,
thus retaining 1/4 of the bang energy in the primary. If some
corresponding detuning of the secondary/topload as described above
wasn't also occuring, then perhaps you would have trouble putting out
the gap arc. Is it the case that some of the art of tuning the TC is
finding just the right amount of primary detune so that the breakout
drops the sec Fres enough for the majority of energy to transfer? That
certainly makes tuning a much more interesting operation! I suppose in
that case, tuning for deepest primary current notch would be the thing
to do, since that implies max energy transfer is achieved during the
beat. It surely can't be coincidence that four of the five coils looked
at so far use a fair amount of detuning.
If we take the tuning setups shown on the web page to be the those that
give optimum performance for each coil, then we can use the detune
percentage to calculate the effective capacitance of the streamer load
(assuming the streamers load the secondary down to match the primary),
and from that we can estimate the total streamer charge, which in turn
we can compare that with the 'surplus charge' value calculated on the
basis of how much larger the MCTV is above the breakout voltage.
Let's just try some ballpark figures for ba0:
The secondary+topload has a total shunt capacitance of 40.7pF and to
detune by 12% requires 51.1pF, so we suppose the streamers are providing
the extra 10.4pF.
Now if the streamers are clamping the surface field at 26kV/cm, that
takes a topvolts of 26/0.043 = 605kV. Thus our 10.4pF of streamers
holds up a charge 10.4pF * 605kV = 6.3uC.
Now the MCTV would allow the topvolts to reach 880kV, which exceeds the
breakout potential of 605kV by 275kV, and thus the coil is capable of
supplying a surplus charge over and above that required to reach break-
out of 275kV * Ctop = 275kV * (30.5pF+10.4pF) = 11.2uC, (The 30.5pF is
the topload capacitance).
Thus if the tuning is 'ideal' for the coil, the streamers are holding up
around 6.3uC, but the coil is theoretically capable of delivering
11.2uC. It's nice to see these two numbers landing in the same
ballpark,
and that the first is less than the second.
For now, the latest coil models are uploaded to
http://www.abelian.demon.co.uk/tssp/cmod/
and I'll see about adding some extra rows showing the streamer cap and
charge estimated from detuning.
--
Paul Nicholson,
--
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.