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Re: An Interesting Problem
---------- Forwarded message ----------
Date: Sat, 18 Oct 1997 09:16:00 -0500 (EST)
From: Benson_Barry%PAX5-at-mr.nawcad.navy.mil
To: tesla-at-pupman-dot-com
Subject: Re: An Interesting Problem
Hi John, Malcolm, All,
I found an article:
"THE IMPULSE BREAKDOWN TRAJECTORY IN AIR
BETWEEN ROD AND PLANE ELECTRODES"
by D. B. Watson and L. Ma
in the Vol. 4 No. 1, February 1997
issue of the
IEEE Transactions on Dielectrics and Electrical Insulation
pages 108 to 113
which may have some relevance to this discussion
in which the authors performed experiments to determine
why the spark would sometimes bend its trajectory
near the pointed electrode. I quote part of the
introduction (paragraphs 3, 4, 5) and the conclusion:
"A further observation from point-plane breakdown
investigations is that the point of impact on the plane
electrode lies close to the central axis when the applied voltage
is only high enough to produce a low probability of flashover,
but that when the voltage is raised such that the probability
of flashover increases the impact point on the plane electrode
moves away from the center[7, 8]. This has been explained in
terms of prebreakdown space charge build-up in the
interelectrode gap.
Electrical breakdown of air in a rod-plane electrode
system in certain respects is similar to that between point and
plane since in many situations it is preceded by corona and
streamer formation [4, 6, 9]. Streamers initiated in the
vicinity of the rod electrode can travel far into the air gap
before being quenched due to space-charge build up ahead
[10, 11] while some may have enough energy to completely
bridge the two electrodes [4, 6]. However, the bridging of the
electrodes by a streamer does not necessarily produce
electrical breakdown [4, 6]. Many streamers may arrive
at the plane electrode before a sufficiently energetic one
is followed by the breakdown discharge.
The space charge due to corona and unsuccessful
prebreakdown streamers is deposited in the air gap, and this
space charge modifies the field distribution [9, 11]. In
particular the radial field, which builds up while space-charge
accumulates, can cause successive streamers to deviate
further from the axis as the propagate toward the plane
electrode [6] so that the final streamer together with the
breakdown discharge often follow a path which is far from
the axis of symmetry [4, 6, 8]."
4. CONCLUSION
"The number of segments in the flashover trajectory
has been found to vary with the impulse overvoltage. This
result is compatible with a prebreakdown space-charge
model. Single-segment sparks are associated with breakdown
before signifigant space-charge build up. However, if sufficient
space-charge accumulates before breakdown it creates a
strong radial field. The breakdown streamer is then driven
away from the axis until it enters regions where the radial
field is too low to sustain further radial propagation. This
results in a flashover trajectory having two or more segments."
I think that this "space-charge model" is relevant
because the Tesla coil streamers form and reform at a
sufficiently high enough frequency that the uncombined
space-charge left overs from a previous streamer or
streamers will effect the next streamer as in the DC case
above but in an alternating way with the waveform. Any
potential nonsymmetries in the secondary ringdown
waveform can cause space-charge of a particular polarity
(mostly positive due to lesser mobility) to accumulate in the
air about the present streamer with the effect that the
succeding streamers may either become neutralized or
repelled by it.
Barry
----------
From: "tesla"-at-pupman-dot-com-at-PMDF-at-PAXMB1
To: Benson Barry; "tesla"-at-pupman-dot-com-at-PMDF-at-PAXMB1
Subject: Re: An Interesting Problem
Date: Wednesday, October 15, 1997 7:19PM
<<File Attachment: 00000000.TXT>>
---------- Forwarded message ----------
Date: Wed, 15 Oct 1997 11:59:09 -0400 (EDT)
From: FutureT-at-aol-dot-com
To: tesla-at-pupman-dot-com
Subject: Re: An Interesting Problem
In a message dated 97-10-15 06:14:43 EDT, you write:
<< snip>The question: if the gap can clearly fire at this setting, what
>is stopping it in its tracks when the thing does break out on
>occasions? Any answers?
>
>Malcolm >>
Malcolm,
Maybe the effect is related to the way that a spark can alternate
between opposite sides of the toroid at higher powers, in this case,
due to lower power, it alternates between one side and nothingness,
all I can think of are ion effects. I wonder if a gentle breeze from a
fan blowing onto the toroid would give any clues?
John Freau