From: Paul
Date: Thu, 10 Oct 2002 08:03:48 +0100
Subject: Re: [TSSP] Top Voltage
Hi Boris, > How on earth he could get breakout of the toroid alone > of that size (and it is a quite smooth one) with > "only" about 200 kV peak? I had in mind that the various factors that have been mentioned might combine to drop the breakout field quite a way below 26kV/cm, say down to 12 or 13 kV/cm. But with the sphere in place, and Terry reporting 190kV, that puts as back to the original observation: the sphere is not breaking out until much higher than software predicts. Terry wrote: > The center of the small ball is 1.9 inches above the > top side of the toroid. I remodelled this low sphere to predict a breakout voltage of 120kV, based on 26kV/cm. The 190kV breakout translates to a breakdown field of 42 kV/cm. So Terry's DC voltage measurements across a base blocking cap don't appear to be showing any sign of breakout earlier than the observed leaders. Maybe the software's wrong. It reproduces tabulated sphere-sphere breakdown voltages reasonably well, and the surface field strengths from which these are calculated are a free by-product of the cap calculations. If the field was wrong, so would the caps be too. Maybe I've got the average surface charge correct (so the cap is right) but the minima and maxima are a way out? The software searches for the location of highest surface field and goes off that. Perhaps that could be out by a factor of 2 and still give a reasonably good average field. Is the breakout threshold much higher than we expect when we're only dealing with a very few RF half cycles repeated every 100mS? Boris wrote: > If so,Terry improved OLTC secondary Q (or it was > improved by itself) and can do somewhat better by > changing k factor to see how rise time effect > breakout. Yes, or maybe put the small sphere on a CW coil as the only top terminal. The breakout voltages could then be determined (eg by measuring base current) and we could easily test if this changes as a function of burst length and burst rate. Note that with breakout voltages this far out from the expected values, there's no immediate pressure to setup a topvolts probe. A measure of Ibase is more than adequate to estimate Vtop. -- Paul Nicholson, --
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.