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.