From: Bert Hickman
Date: Tue, 22 May 2001 07:38:47 -0500
Subject: Re: [TSSP] Genetic optimisation
Paul, I only wish I had more time to devote towards supporting the excellent work you're doing! My responses are interspersed below... Paul wrote: > > Hi Bert, > > Somehow you always seem to drop in an informative post. > > >... [Reukema] found a progressive lowering of breakdown voltage, > > by as much as 17% (about 25 kV/cm), as the frequency was increased > > between 20 kHz and 60 kHz. However, no further breakdown voltage > > reductions were seen between 60 kHz and 425 kHz. > > That sounds promising. Does the 425 kHz limit imply things change > above that frequency, or is that just where his measurements stopped? Although Reukema limited his tests to 425 kHz, the space charge enhancement mechanism appears to reach a limit at about 60 kHz. Although experimentally verified to remain relatively constant at about 26 kV/cm from 60 kHz through 425 kHz, it probably applies to higher frequencies in the range applicable to Tesla Coils. Different mechanisms come into play when we extend this to microwave and optical breakdown in air. > > > ...[Raizer] estimates the breakdown field in room temperature for > > relatively long gaps (>6 cm) is about 26 kV/cm. > > Are we OK to extrapolate this figure to gaps of order a metre or > so? > Apparently so. Initiation of breakdown is a threshold avalanche process ultimately determined by the relation of electron creation versus their removal. In an electronegative gas such as air (predominated by the oxygen component), electrons are rapidly removed by the formation of negative ions. This removes them from further participation in avalanche multiplication. Per Raizer, avalanche multiplication in air at STP (the onset of breakdown) has a lower E-field limit of about 26 kV/cm once we are in the "long spark" regime. A long spark (>6 cm) is characterized by multiple avalanches in an evolutionary sequence: streamer flash(es) --> leader propagation (fed by groups of streamers) --> spark (if leader bridges the gap). If the local E-field never reaches the 26 kV/cm threshold, the initial electron avalanche that begins the process cannot occur irrespective of frequency or gap length. > > So... barring precise measurements, a good starting point for > > estimating E-field for terminal breakout at Tesla Coil frequencies > > might be about 25 - 26 kV/cm. Yes, assuming a smooth HV terminal... > > That sounds very reasonable. Perhaps Terry could use this value to > calculate the breakdown voltage of a test setup and compare it with > the peak top voltage measured in an actual run? If they came out in > the same ballpark then we would have the basis of a two pronged > strategy - Marc's gathering of experimental results, plus Terry's > field calculations - which between them might be able to bracket > the actual values to a range narrow enough to permit meaningful > modeling. > > With a bit of luck we might expect a reasonable match for smooth > toroids, with progressively lower measured breakdowns for rougher > surfaces, at which point Terry will invent a fiddle factor to > adjust for the toroid material. > > While we're on the subject of genetic methods, a few background > links, > > http://www.rennard.org/alife/english/gavintrgb.html > http://cmp.ameslab.gov/~jrmorris/ga/ > http://ai.bpa.arizona.edu/~mramsey/ga.html > http://www.hao.ucar.edu/public/research/si/pikaia/tutorial.html > > Cheers, > -- > Paul Nicholson, > Manchester, UK. > -- Best regards, -- Bert -- -- Bert Hickman Stoneridge Engineering Email: bert.hickman@aquila.net Web Site: http://www.teslamania.com
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.