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Re: theory(?) for long sparks


From: 	Benson_Barry%PAX5-at-mr.nawcad.navy.mil[SMTP:Benson_Barry%PAX5-at-mr.nawcad.navy.mil]
Sent: 	Wednesday, November 26, 1997 4:02 PM
To: 	tesla-at-pupman-dot-com
Subject: 	RE: theory(?) for long sparks


Hi Jim, All,
     When I finished designing and building the 2 Megavolt Marx generator at 
the Lightning Laboratory I did some voltage and current comparison 
measurements.  The voltage rose, leveled off like a square wave, and then 
collapsed at which time the current started to rise.  This took only about 2 
microseconds for a 10 foot point plane arc (2.2 MV) with a current peak of 
about 14 kiloAmperes.  The erected Marx capacitance was around 3 nanoFarads. 
 The terminal capacitance of a Tesla coil is several orders of magnitude 
less than this.  Is it possible that the source charge feeding the streamer 
effects the propagation velocity?

                    Barry

 ----------
From: "tesla"-at-pupman-dot-com-at-PMDF-at-PAXMB1
To: Benson Barry; "tesla"-at-poodle.pupman-dot-com-at-PMDF-at-PAXMB1
Subject: theory(?) for long sparks
Date: Tuesday, November 25, 1997 11:49PM

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From:     Jim Lux[SMTP:jimlux-at-earthlink-dot-net]
Sent:     Tuesday, November 25, 1997 8:27 AM
To:  Tesla List
Subject:  theory(?) for long sparks

I have just gotten my copy of "spark discharge" by Bazelyan and Raizer (CRC
press). I have been inspired by the picture of the 100 meter+ spark
produced by only 5 MV, and have given some thought to what it takes to make
long sparks.

It appears that a spark propagates at around 2-3 cm per microsecond. If the
voltage pulse producing the spark only lasts 10 microseconds, the maximum
spark length that can be developed will be in the area of 20-30 cm.

What is necessary for the spark to start is that the field at the starting
point be greater than the breakdown for air (i.e. around 30 kV/cm). Then,
for the spark to propagate, the voltage has to increase enough to cover the
drop along the spark, and, the source has to supply sufficent current to
"fill" the spark channel with charge. (I am doing horrible injustice to the
pretty complex physics here, sorry).

So, then, if you want to produce a 100 meter spark, (i.e. 1E4 cm), the
voltage has to be there for at least 5000 microseconds, and probably
longer. Further, the voltage has to be high enough to overcome the drop in
a 100 meter long spark channel, i.e. several MV.

A tesla coil running at 100 kHz (for example), has a half period of only 5
microseconds, well short of the desirable 5 milliseconds. It is possible
that the fine structure (i.e. the 100 kHz carrier) isn't the significant
thing, but rather the overall envelope (i.e. the time til the first
"notch"), which would be more consistent with the observed 1-3 meter length
sparks from a medium sized tesla coil.

Ideas anyone?


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