From: "Malcolm Watts"
Date: Tue, 4 May 2004 09:10:07 +1200
Subject: Re: [o-o] [TSSP] Modeling with ANSOFT
Hi Paul, all,
Given the difficulties of measuring the topvolts
without disturbing the system perhaps it might be worth examining a
slightly different approach. I think any probing *must* go through
the centre of the secondary. Suppose the probe has an equivalent
capacitance to the toroid and stands in for it instead. Then the
problem is developing a useful ROC at the top. A hemisphere with the
same diameter as the secondary sitting directly atop the secondary
suggests itself. The probe then might make up the difference in
capacitance between this hemisphere and the normal toroidal (or
whatever type it is) terminal.
?
Malcolm
On 30 Apr 2004, at 19:31, Paul wrote:
> Marco,
>
> > I have still a lot of thinking/reading/measuring
> > before I can tell you anything valuable. It takes time...
>
> Have you anything with which to measure the operating
> topvolts of the coil, to the extent of capturing a
> calibrated waveform into a scope?
>
> To be useful, the measured signal would have to report
> specifically the toroid voltage and not just sample the
> E-field flux from the vicinity of the topload.
>
> This signal, combined with that from a coil-top current
> probe, would allow us to calculate the total streamer
> charge, as a function of time, through the breakout
> event.
>
> We have Q = CV for the topload and its breakout load.
>
> So the current into the topload from the coil is
>
> dQ/dt = CdV/dt + VdC/dt
>
> The measured signals give us dQ/dt, V, and dV/dt, so
> that we determine dC/dt and the 'breakout charge' VdC/dt
> as functions of time.
>
> If we just sample the flux, we get a signal roughly
> proportional to Q, which isn't very helpful.
>
> This is the point on which we are a bit stalled at the
> moment. Sometime ago we discussed the prospects of
> a shielded capacitive divider, perhaps running down inside
> the secondary.
>
> Sorry I'm not familiar with the commercial field modelling
> packages, so can't really help with that. But I think it
> would be easy if you just model the system with charged rings,
> discs, toroids, spheres, etc, and set them to appropriate
> voltages (eg Terry's voltage distributions).
>
> > I would like to verify the postulate of 5 kV/cm required
> > to elongate up to, say, 1.5 m and 1 kV/cm required to elongate
> > further.
>
> These average field strengths look reasonable. It shouldn't
> be too hard to verify them. You could take a topvolts
> measurement and then use a closed formula for the field
> around a sphere or toroid - ignoring the coil altogether.
>
> I think Antonio and Godfrey were passing a few suitable
> formulas back and forth a while back, or were they just for
> surface fields? I should think you could just pretend the
> topload is a sphere as far as the average E-field gradient
> is concerned - once you get a little way from the topload it
> doesn't matter much what shape it is.
>
> It would be interesting to see how your 5kV/cm to 1kV/cm varies
> with risetime, bps, etc.
>
> But I think you will need to measure topvolts at some point.
> --
> Paul Nicholson
> --
>
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.