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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.

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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.

--- 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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