[TCML] Solid state efficiency, was: mini Tesla coil specs

[Bert Hickman]

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 at sonic.net wrote:
> 
> From: Ken or Doris Herrick <kchdlh at sonic.net>
> To: Tesla Coil Mailing List <tesla at pupman.com>
> 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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