From: "Barton B. Anderson"
Date: Sun, 12 May 2002 16:29:09 +0000
Subject: Re: [TSSP] Topload breakout potentials
Hi Paul, Paul wrote: > For Bart's coil, > > Bang energy = 0.5 * 61nF * 15.3kV^2 = 7.1J > > Cee = 47pF, so peak topvolts is at most > > sqrt( 2 * 7.1J/47pF) = 550 kV. > > The model is only registering around 460 kV so I guess I haven't > quite got the primary in tune. Are you able to measure the > secondary Fres with everything in-situ, with primary gap open? Yes, but I'll have to wait until tomorrow. I was using my equipment at work. I'll bring it home tomorrow and measure. > Have I got the toroid size correct: 30" outer diameter and a 9" > diameter tube? Yes. The 30" is the outer diameter, not diameter at centers. > The surface gradient is 0.026 V/cm/V so could achieve 0.026 * 550 = > 14.3 kV/cm. That seems to fall well short of the 26kV/cm we expect, > yet you're getting substantial breakout. The only quantity used > from the tssp model is the Cee=47pF, which we know isn't going to be > very far out - a couple of percent at most. You have a smooth > toroid so we can't blame surface rugosities. Maybe I'm working out > the surface gradient incorrectly. The method I'm using is based > very directly on the charge distribution computed for the toroid as > a step in the capacitance determination. I tried another different > method too, which gave similar values. I agree it's odd. There are some differences I believe. I'm running in STR mode. John and Marc may be running in LTR mode. The fact that I'm using a hefty transformer allows me to charge a cap pretty darn fast. I wonder if it's charging beyond the xfmr voltage? I typically breakout around 12mH on the ballast, which is about 40% on the ballast variac. > >From your description, I'd say that the coil started out being > well in tune at 13 turns, and by tuning low to turn 14 you got fewer > and longer streamers, which is similar to John's and Marc's coils. > So that bit makes sense. You detuned the primary further by > raising the secondary Fres and this further concentrated the > breakout into a single long streamer. Again that makes sense. > > Unless something odd is happening in Bart's charging circuit to > give a bang something approaching four times the 7.1J estimated, then > we may have to conclude that for some reason toploads are breaking > out at less than 14kV/cm rather than up around 26kV/cm. I doubt it. 14kV/cm just seems to low for the surface to ionize. I suspect I am seeing a higher voltage at the cap upon conduction. I suspect I can check this with a simple static gap? Maybe using identical 1" brass sphere's, I could run it a few seconds and see if it will run at 120bps in this mode. Setting the sphere gap to 0.5" should require an arc voltage of ~28kV to arc across. Assuming I leave the ballast variac at it's SRSG position (same current), I could begin decreasing distance until I get breakover. The arc voltage should be able to be back calc'd from the distance. Take care, Bart
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.