From: Bert Hickman
Date: Mon, 07 Oct 2002 06:45:29 -0500
Subject: Re: [TSSP] TOPV breakout threashoul
Marco.Denicolai@tellabs.com wrote:
>
> Hello all,
>
> > -----Original Message-----
> > From: paul@abelian.demon.co.uk [mailto:paul@abelian.demon.co.uk]
> > Sent: 5. lokakuuta 2002 12:29
> > To: tssp@abelian.demon.co.uk
> > Subject: Re: [TSSP] TOPV breakout threashoul
>
> >You could look
> > to see if the obvious breakout (clearly visible leaders) occurs
> > at substantially higher voltage than we predict based on 26kV/cm,
> > and you could look for signs of weak breakout occuring before
> > that ie the faint cloud reported by Malcolm. It would be very
> > nice if we could reproduce this and we might find that it occurs
> > at roughly the expected voltage, rather than the 2 to 5 times
> > higher at which the leaders are forming in our sample of one.
>
> Last week I had a chance of quick reviewing some references about spark
> formation. Those included some investigation about how it is affected
> by:
>
> - AC application (different freq., DC component, rise time, etc.)
> - successive pulses
> - electrode shape
>
> In a nutshell, all of the above factors influenced the breakdown
> threshold so much (e.g. 50%) that you really couldn't state a kV amount
> simply according to the curvature of the electrodes.
>
> That's why I'm asking, where this 26kV/cm is from?
>
> Regards
>SNIP>
Hi Marco,
I'm probably the guilty party. You're correct - a single number fails to
adequately describe the complexity of the situation, especially for very
fast rising pulses or where there's significant field enhancement
through interactions with preexisting space charge.
The question that was originally posed last May in the TSSP List was
what was known about the minimum E-field field necessary to cause
initial breakout in air at STP, and was the RF breakout voltage lower
than the generally accepted 30 kV/cm value for DC. After doing a bit of
searching of the literature, I posted the following message on 5/21/01:
"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."
Best regards,
-- Bert --
--
Bert Hickman
Stoneridge Engineering
"Electromagically" (TM) Shrunken Coins!
http://www.teslamania.com
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.