From: Paul
Date: Sun, 05 Nov 2000 22:36:42 +0000
Subject: Re: [TSSP] NSVPI - Latter Results
Malcolm Wwrote: > ...If you buy the notion suggested by a crude analysis of that > behaviour that gap loss is predominantly V.I rather than I^2.R > and that Vconducting is approaching constancy, you can see why. Yes, I see, it's the small constant Vgap. I suppose by definition the gap loss must be V.I, so long as V changes sign promptly when I does, ie we can ignore the self inductance and capacitance of the gap. Also by definition I^2.R applies, so it's not a case of one rather than the other - its just that we have to accept that R is a function R(I). A couple more questions, Recently Barry posted: > ...J.C.Martin in the book "J.C.Martin on Pulsed power" > ... he gives the formula for spark gap impedance as > ((1.7 * L)/ r) + Z > where L is the inductance of the spark channel, > r is the length of the resistive phase, and > Z is the impedance on Ohms of the circuit driving the spark gap. > ... he gives his famous formula for the resistive phase of spark > formation as > (88/(Z**(1/3) * F**(4/3))) * (rho/rho-zero)**(1/2) > where the time is in nanoseconds, > Z is the impedance driving the plasma channel in Ohms, > F is the field along the chammel in units of 10 KV per centimeter, > rho is the density of gas used, and > rho_zero is the density of air at STP. > Rho will change with bang rate. Let me guess that the resistive phase is a short period of constant resistance during which the arc gets established. It looks to be on the order of nanoseconds and is presumably relevant to fast pulse discharges in accelerator and fusion supplies. I take it we can ignore this resistive phase, since tesla gaps conduct for many tens of microseconds? I guess it ought to be relatively straightforward, given a digital scope, to capture and store the V/I curves for a range of gap lengths and widths. I take it they would come out looking a bit like two diodes back to back in parallel? Are the V/I curves a function of frequency too, or would one set of curves do for all? If such curves exist, are they 'square' enough so that a gap can be reliably characterised by a 'forward voltage drop' which is just a function of the gap dimensions and independent of current? It seems to me that such a set of curves would provide the essential raw material from which an effective loss resistance (ie an integrated loss per cycle divided by the mean square current) could be established for a given gap. > My expts also suggest (and appear to be well supported by the > experiences of others) that as one moves to the high L/C - high V > regime, quenching type #2 (avoiding power ARCS) becomes more and > more of a bugbear. John Freau is one excellent experimenter whose > experiments with high L/C primaries have seen his efficiencies climb > markedly. Thanks, was wondering about that. Is there a primary L/C record holder I wonder? I've been thinking about rigging up a high energy single shot primary, using a voltage multiplier to accumulate about 60kV across the tank. Cheers, -- Paul Nicholson, Manchester, UK. --
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.