Re: theory(?) for long sparks

To: "'Tesla List'" <tesla-at-poodle.pupman-dot-com>
Subject: Re: theory(?) for long sparks
From: Tesla List <tesla-at-stic-dot-net>
Date: Fri, 28 Nov 1997 17:26:45 -0600
Approved: tesla-at-stic-dot-net

From: 	Greg Leyh[SMTP:lod-at-pacbell-dot-net]
Sent: 	Friday, November 28, 1997 4:06 AM
To: 	Tesla List
Subject: 	Re: theory(?) for long sparks

Jim Lux wrote:


> 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.

Me too... This has been my bedtime reading for the last week.

> 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?

TC's may be able to effectively do better than the 2-3 cm per
microsecond
limitation, owing to the ion persistance between gap firings.  Case in
point:
my DC coil will produce 3-4' streamers below 100PPS, ~10' streamers at
200PPS
and 20-25' streamers at 350PPS.  The primary cap voltage is the same at
all
gap speeds.


-GL