From: Paul
Date: Wed, 01 May 2002 22:35:47 +0100
Subject: Re: [TSSP] Secondary voltage stress factor
Bert wrote: > I would expect that the initiating conditions for surface > discharges could exist at E-fields of 10 kV/cm, perhaps less... That's low enough to be within reach of some practical coils then. For a coil like Thor, which could reach over 400kV if the bang were 20kV, the secondary is long enough to give an average gradient of around 2.5kV/cm, and even when multiplied by a stress factor this would still fall well short of a 10kV/cm threshold. Terry's small coil could reach 250kV with a 10kV bang, an average of around 3.8kV/cm, and the stress factor for this toroided coil is 1.5, so we might expect to find an occasional peak of 5.7kV/cm. Chris Swinson's flat coil 350kV over 55cm radius gives an average of 6.4kV/cm, and with a stress factor of around 2.2 could reach 14kV/cm occassionaly. (the above mentioned stress factors include the effect of primary induction, which is not quite what we're constructing in the table, eg the 'table' value for the above coil is 2.05 rather than 2.21). Forget the base current measurements. I'll settle for anecdotal reports of primary bang voltage at which given coils begin racing arcs. > http://www.sandia.gov/media/images/jpg/Z02.jpg And the energy we see dissipated in the picture is just the leakage! A few slight mods and it would make a wicked can crusher :). > http://www.geocities.com/CapeCanaveral/Lab/8063/lichtenb.htm Yeah, they're the ones. Nice site. Boris wrote: > I'm not very optimistic in determining its start by > simulation nor possibility for discovering all the > critical factors that enable the phenomenon. Nor am I, for the reasons you give. I appreciate the reminder that the dV/dx on the conductor is not the same thing as the grad(V) on the dielectric surface. And the scale factor problem that you describe is a serious obstacle. Treating the thin coating by the method of equivalent surface charges is I think not viable. It is tricky enough trying to account for the thicker dielectric of the coil form. Numerous unknown factors must be involved in determining the longitudinal breakdown voltage at the coating surface, and I see no hope of predicting it from any set of first principles. This is a case where we must collect data from actual coils and see if it is consistent enough to establish a predictive recipe. What we're after is an estimate of that surface breakdown voltage, in terms of the coil dV/dx that coincides with it. Given such a recipe, we can predict, for a given coil, whether racing arcs are likely. -- Paul Nicholson, --
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.