From: Bert Hickman
Date: Mon, 21 May 2001 07:44:13 -0500
Subject: Re: [TSSP] Genetic optimisation
Paul, Terry, and all,
Avalanche breakdown is an essential "first step" in the sequence of corona,
streamer, and leader formation. The E-field necessary to achieve avalanche
breakdown in air at Tesla Coil frequencies (10e4 - 10e6) is apparently
similar to that at DC - approximately 30 kV/cm.
However, under RF excitation, the local E-field around the HV terminal can
be significantly enhanced through the interaction of the terminal and any
surrounding space charge regions. Even if the RF voltage peaks are
initially insufficient to initiate avalanche breakout, they may generate
waves of positive and negative ions around the output terminal. Once the
surrounding space charge regions are created, they will increase the
"effective" E-field seen at the HV terminal, thereby reducing the terminal
voltage required to initiate breakdown. After a number of RF cycles, field
enhancement becomes sufficient to initiate breakdown at a lower peak
terminal voltage than for the DC case.
The effect of space charges on RF breakdown is presented in Loeb
("Fundamental Processes of Electrical Discharge, McGraw-Hill/Chapman &
Hall, 1939), reflecting the work of L. E. Reukema. Reukema used sphere gaps
and relatively short gaps (0.25 - 2.5 cm). He 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.
Yuri P. Raizer also arrives at a similar conclusion but from a somewhat
different slant ("Gas Discharge Physics, Springer-Verlag, Corrected Edition
- 1997) - he estimates the breakdown field in room temperature for
relatively long gaps (>6 cm) is about 26 kV/cm.
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.
-- Bert --
--
Bert Hickman
Stoneridge Engineering
Email: bert.hickman@aquila.net
Web Site: http://www.teslamania.com
Paul wrote:
>
> Terrell W. Fritz wrote:
>
> > ...
> > Voltage stress plot:
> > http://hot-streamer.com/temp/MattD2.gif
>
> Very nice. Could you produce a variation on E-Tesla which, when
> *given* a V-profile generates the gradient plot as above, and then
> explores it to find the spot with the highest gradient, which we
> take to be the weakest link - the 'Robinson spot' perhaps :). Then
> by scaling the gradient of the Robinson spot up to the breakdown
> value of air, say 10kV/cm, we get the maximum top volts before
> breakout.
>
> > However, that is assuming the breakdown voltage is
> > the same at DC, 60Hz, and 350kHz.
>
> Indeed. You would be calculating the *static* breakdown voltage
> with no account of the rate of change, but its a start. Comparison
> with the tabulated results of Marc's proposed project would be
> interesting, and might reveal the extent to which dV/dt is a factor.
>
> > ... The question then is when does air initially breakdown at
> > say 300kHz?
>
> Pity Jim Lux isn't on this list - I'm sure he could provide some
> essential background to this discussion.
>
> Cheers,
> --
> Paul Nicholson,
> Manchester, UK.
> --
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.