Re: Safety FAQ-discharge classification

Tesla List wrote:
> >From rwstephens-at-ptbo.igs-dot-netMon Aug 19 23:01:31 1996
> Date: Mon, 19 Aug 1996 18:19:20 -0500
> From: "Robert W. Stephens" <rwstephens-at-ptbo.igs-dot-net>
> To: tesla-at-pupman-dot-com
> Subject: Re: Safety FAQ-discharge classification

<Big Snip>

> I've meant to answer your post sooner but my life has been upside
> down for the past three weeks having just survived a very 'should
> have been dead' type head-on pickup truck to pickup truck, highway
> speed,  highway collision three weeks ago.
Sorry to hear about your accident - GLAD to hear you survived and are


> A phenomenon which I have yet to hear anyone talk about, and which is
> especially evident in the streamer discharge to a grounded target in
> small tabletop systems is where a portion of the thin streamer seems
> to be twice as bright as the rest (majority) of the streamer.  This
> effect is often near dead center in the length of the streamer, but
> sometimes is much closer to the target end.  I've got video showing
> this effect near the target with my MTC unit with 10-12 foot scale
> arcs.  My observations so far conclude that this brightened area is actually an overlap area where the streamer is split into two streamers,
> following a nearly identical path shape, but separate and parallel to
> each other over this brightening distance.  Have you or anyone else seen this
> effect and figured out a reasonable explanation?
> Extremely grateful that someone upstairs clearly wants me to continue
> coiling, and pledging not to disappoint them, rwstephens

Glad you brought this up! I've seen the same thing in streamers to
ground, and on some occasions (more rarely) in streamers to air. On
those to ground it almost seems that the individual streamers coming
from the ground and toroid are faint, but then seem to reinforce each
other somewhere in between once the spark jumps the full distance. Now,
one would think that once a discharge path was formed, the current flow
would be the same all along the path, resulting in a relatively uniform
degree of spark intensity all along the path. Why the marked difference
in spark intensity at different portions of the spark?? Your question
really made me think about what might be going on rather than simply
admiring the pretty sparks!

A hypothetical explanation:
Assume that the grounded target is slowly brought closer to the toroid,
and that the target is much "sharper" than the smooth radius of the
toroid. Once streamers begin to break out from the target, the area
closest to the target becomes fairly heavily ionized, and the "roots" of
the streamers coming off the target will tend to be larger in diameter
than the tendrils at the end. The coronal discharge ultimately
terminates as a blue/violet cone-shaped "haze" with the wide end
terminating at the toroid. If one could measure the individual currents
in all of the discharge branches (and the haze), the sum of all the
smaller currents in all of the branches should equal the total current
flowing through the "root". 

The diameter of the discharge "root" tends to be larger in diameter than
the branches. The brightness of a spark discharge is a function of the
amount of current flowing AND the diameter of arc channel (i.e., current
density). In the case of a simple brush discharge, it appears that, for
a good portion of the discharge, the average current density in the
channel is about the same level: most of the discharge seems to be about
the same relative intensity until we get to the very tips of the
discharge branches. 

Now, assume we move the target closer until the very tip of one of the
many branches now fully spans the gap, and the much heavier discharge
current from the charged capacitance of the toroid/coil flows through
the resulting ionized path to ground. The electrostatic energy is
suddenly discharged to ground in a quick oscillatory discharge (at VHF
frequencies). Since the diameter of the ionized path is much smaller at
the tip of the discharge than at the root, the current density should be
significantly higher at the branch tip than at the root. Higher current
density will result in greater localized ohmic heating of the arc
channel wherever the ion path is narrower. During the rapid discharge,
the narrowest portions of the path should appear to be much "brighter".
We would also see "jumps" in brightness wherever the pre-spark streamer
had branched out (became thinner) from the root discharge. This would
cause very sudden changes in spark brightness along the path, even
though the current level was the same at all points along the path
during the heavy toroid-ground discharge.

If we place a small "bump" on the toroid (per Richard Hull), and repeat
the experiment, streamers will now originate from BOTH the toroid and
the grounded object. Once a complete path is formed, the narrowest
portion would be somewhere in the middle, resulting in a much brighter
middle region. As the current flows through the thicker paths back
toward the roots, the spark intensity would be reduced, leaving markedy
"brighter" segments in the middle of the arc path. 

As long as the discharge repetition rate is relatively low, most of the
secondary energy will be rapidly dissipated in a single (VHF) spark
discharge, and the ionized path will not have a chance to widen much
before all of the electrostatic energy is dissipated. Along the lines of
Malcolm's observations, with lower coupling coefficients, the streamers
tend to  be more "spindly", with more branching/forking. More prevalent
smaller forks mean more lower current smaller diameter paths at the
ends. These would tend to be significantly brighter once the large
discharge current flowed, making the phenomenon more observable under
these circumstances....  Then again, I could be wrong!  

Comments, flames, etc are welcomed!!

-- Bert --