From: Paul
Date: Sat, 11 May 2002 21:21:42 +0100
Subject: Re: [TSSP] Topload breakout potentials
[TT42] John wrote: > I have the coil tuned so that the sparks barely break out at a full > variac setting. This gives just about 2 simultaneous streamers, > maybe 3. If I tune (tap) the primary for a higher frequency, then > breakout occurs much sooner, but there are more streamers, > but they are all shorter. It may be that the coil is somewhat > mistuned this way (?), but it gives longest sparks this way. Yes, I understand. > This may be why you had to set the tap point at 12 turns or so > in the model, but I'm using 14.5 turns in reality. Yes, I think so. That fits with the model. In tune at around 12 turns we get the waveforms http://www.abelian.demon.co.uk/tssp/tmp/jftt42.wave.gif in which you hit around 590kV with a topload surface gradient of 30kV/cm, so early breakout with lots of short streamers. (which in practice means that the topvolts will get clamped at a lower voltage, say 26/0.051 = 510kV, by the streamer loading). At 14.5 turns, we get http://www.abelian.demon.co.uk/tssp/tmp/jftt42a.wave.gif reaching around 380kV to give 19kV/cm on the toroid. That's below the 26kV/cm threshold so the topload is struggling to break out, and I guess that makes for only a few, but long, streamers. BTW, don't take much notice of the primary waveforms, etc. They're valid up to the breakout point, but beyond that we haven't taken account of either the loss of energy or the changing boundary conditions due to the streamer loading. All we can reliably use is the pre-breakout part. Does your top toroid break out from around 24 degrees below the horizontal? Have you tried this coil with a bigger topload? Would be nice if there was a particular size that brought the secondary down into tune with the 14.5 turn tap, and at the same time increased the ROC to drop the surface gradient down to say 20kV/cm. > I don't know if my tune setting is perhaps > partially compensating for a too small toroid? Yes, I think we're seeing that. I wrote: > For Bart's coil which we're also looking at on another thread, a > 15.3 kV firing evolves 460 kV on the topload. The max surface > gradient for Bart's topload is 0.026 V/cm/V at an elevation of +20 > deg, which gives a field strength of 0.026 * 460 = 12kV/cm. Bart's coil should be having trouble breaking out. Bart wrote: > The gap firing voltage was 15.3kVp. I didn't go beyond this value. > Breakout was occurring... Is it a smooth toroid? Did you have to add a breakout point? At this voltage do you get a few long streamers or plenty of short ones? I'll set to work on Marc's coil now... -- Paul Nicholson, --
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.