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Re: [TCML] Solid state efficiency, was: mini Tesla coil specs
Dex and Ken,
The physics of spark propagation is markedly different for positive
versus negative sparks (in a divergent E-field, such as around a TC
topload). All other things being the same, positive sparks propagate
more "efficiently" in air. Once initial breakout occurs, a
positive-going high voltage pulse will travel further than a similar
negative-going pulse. In a diverging E-field, a positive spark will
bridge a gap at a lower voltage than a negative spark. This is still
true, even though negative corona will "break out" at a lower voltage
than positive corona. These "polarity effects" are well known by
professional high voltage workers and engineers.
Ken is indeed correct - there is an "optimal" voltage risetime that
leads to maximum propagation "efficiency". One noted researcher, Yuri P.
Raizer, has developed a relationship for the optimal voltage risetime
for a positive spark to travel a distance of L meters ("Gas Discharge
Physics", page 362):
T(optimal risetime) = 50*L (in microseconds)
Unfortunately, although the above relationship appears to work quite
well for monopolar impulses from Marx Generators, it's not at all clear
how (or even if) the above relationship can be adopted to the complex
waveforms of Tesla Coils. Using either the RF waveform or envelope leads
to relatively low operating frequencies for typical coupling coefficients.
We also know that the longest TC sparks are not obtained during single
single events (bangs), but instead via bang-to-bang growth. Newer sparks
build on the heated channels of their predecessors when the break rate
is sufficiently high (>70-80 BPS). This suggests that we might try
combining polarity effect and bang-to-bang growth by polarizing the
system so that the highest voltage peak after ring-up is always of
positive polarity. The positive peaks will provide the longest "reach"
during propagation. This should be simple to implement through suitable
coupling coefficient and phasing for SSTC, DRSSTC, or DC-resonant SGTC
systems, and should cause optimal spark propagation for a given input
power, frequency, and break rate.
BTW, an excellent book (also by Raizer), "Spark Discharge", 1991, CRC
Press, ISBN 0849328683 can currently be obtained for around $38 or so on
Amazon and other large book sellers. This book was originally in the
$130 range. It is technical, but quite readable considering the
complexity of the subject. Any serious spark researcher should have this
title in their library.
Bert
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Dex Dexter wrote:
Spark propagation for a positive or a negative spark?
I think I read there were some differences ,and tesla coil
sparks are generated by bipolar waves.
Dex
--- kchdlh@xxxxxxxxx wrote:
From: Ken or Doris Herrick <kchdlh@xxxxxxxxx>
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Subject: Re: [TCML] Solid state efficiency, was: mini Tesla coil specs
Date: Thu, 12 Nov 2009 08:57:08 -0800
A research paper I have a copy of has found spark
propagation in air, measured over 1 inch of distance, to require about
50 ns of time. That extrapolates to about 20 inches of travel per
microsecond. So with a high rate of voltage-rise, more charge can be
applied to the electrode than can bleed off through the spark during its
first several inches of travel.
Ken Herrick
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