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Re: Attn: Rich, Bert & Terry- Growing large arcs?
Scott and all,
My comments are interspersed below...
Tesla List wrote:
>
> Original Poster: Scott Stephens <Scott2-at-mediaone-dot-net>
>
> I remember numerous posts about the physics of streamer generation and
> formation recently and I was hoping for one (or all) of you experts to
> comment on the following.
> I believe plasma physics is more challenging than rocket science. So don't
> feel I expect you to write a book, just give me a specific case in the
> context of your favorite Tesla oil! And if my reasoning process is flawed,
> please comment!
<SNIP>
Plasma physics is a tremendously challenging area! I'm not aware of
anyone on this list that would come even close to being a true expert in
the field - however, we do have quite a few thinkerers and tinkerers
playing on the peripherals of the subject! And almost all of us are
certainly looking to grow larger sparks with minimal power or minimal
coil-size. The quest for only a small portion of the various questions
you pose could take the better part of a lifetime of research (and,
indeed, has for some of the true "long spark" experts in the field).
It's a WHOLE lot easier (and certainly much more fun!) to simply MAKE
long sparks than to try to understand the fundamental physics behind HOW
they work...
<SNIP>
> Lets say a large transformer is connected to a large bridge rectifier and
> filter cap, and generates a large ionizing voltage, for the purpose of
> generating (what else?) a large spark. Lets assume the initiating electrode
> is positive, for minimum required energy.
>
> My purpose is, to understand why no one has put a thyratron bridge rectifier
> on the output of their TC, to generate enormous sparks. Or have they?
Well, a few significant reasons come to mind - cost, availability, and
tube voltage withstand capability. While moderate voltage thyratrons (15
kV) are readily available at comparatively low cost even at Hamfests,
higher voltage/high power ones are very scarce. OE Technologies lists a
surplus one that's good for 32 kV at 20 kA for $1500. And 32kV is about
1/10th the voltage output of even a moderately-sized coil. I have no
idea what a 70-100 kV device would cost ( but you could request pricing
for Triton's F-202 or F-187, EG&G's LS-3229, or Litton's 100 kV 4-gap
L-4993). I suspect that there are very few of us who could afford even
one, much less four, of these beasties.
It's also not clear that a bridge circuit (of thyratrons or HV tube
rectifiers) would accomplish what you're looking for, since a standard
coil is really "single-ended", with the base grounded. Might be able to
get FW-rectified output if a pair of HV toob rectifiers were driven off
the outputs of a twin coil or center-grounded bipolar system...
>A professor once described a giant spark, over 100' long. It was confined
to a
> straight channel with E-field shaping electrodes, and required an immense
> trasformer to continously source current to grow the arc. He said the arc
> grew as long as current was applied (at ever increasing potential).
>
> So how much current? What initiating voltage? What electrode diameter? What
> arc velocity and diameter? Whats the L, R, & C of the arc per distance &
> time? No doubt you've read of that experiment?
The concept sounds basically correct. There's no limit on streamer
length, or on arc length, other than the capability of the power source
to supply sufficient voltage to overcome the ever-increasing energy
losses. Under the proper conditions, streamer lenth will increase
indefinately as long as you continue to increase/maintain the source
voltage (and current) to keep ahead of the losses.
I'm not familiar with this particular experiment. However, there are
numerous demonstrations of similar long spark and arc formation in the
literature regarding lightning, impulse generators, and in-air arc
creation during faults or current interruption in the electric power
industry.
One way that might be physically realizable for the advance coiler who
also wanted to live [die?] extremely dangerously(!) would be to
AC-couple the base the resonator in a 2-coil system to ground using a
comparatively high-valued HV capacitor charged to a high positive DC
potential. This would permit the top of the secondary to have a high
superimposed DC voltage in addition to the normal high RF voltage.
This combination would increase the overall peak positive voltage
available at the output, and if high enough (>1/2 the peak RF output
voltage envelope), could eliminate any negative voltage excursions with
respect to ground. And the base capacitance would provide an additional
reserve of energy to aid in formation and extension of streamers once
formed.
BTW, I believe that both Bill Wysock and Greg Leyh have done this type
of thing on occasion, but I don't remember the DC potentials and base
capacitance involved. I also seem to remember that Bill's system created
a massive EMP pulse during a ground strike that was also quite
destructive to nearby electronics... :^)
Safe speculatin' to you...
-- Bert --