RE: Quenching question.

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Luke" <Bluu-at-cox-dot-net> 

Thanx for the thoughts.

I have posted a paper on my web page about what I have learned with neg
resistance and the basics of the arc in the spark gap.
Just click the links on the left tesla ..., learning, neg resistance.

I would appreciate any comments.
I may not have all the details figured out or written in there but feel
I have a somewhat very basic understanding of it now.  Getting
comfortable with the knowledge and having it really sink in my take some
time though.

I would like to know if I at least have things stated correctly there.

Thanx to everyone that has helped pound that stuff in my head and
pointing to other sources.

Luke Galyan
Bluu-at-cox-dot-net
http://members.cox-dot-net/bluu

-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Sunday, March 07, 2004 8:06 PM
To: tesla-at-pupman-dot-com
Subject: Re: Quenching question.

Original poster: Bert Hickman <bert.hickman-at-aquila-dot-net>

Luke,

Quenching occurs if the dielectric strength of the gap has recovered
sufficiently to prevent reignition of the arc after the last current
zero
crossing. This implies a combination of the following:

1. No portion of the electrodes can remain incandescent. This is often a

problem, since an incandescent cathode spot is normally created whenever
we
form an arc. If the cathode spot cannot be cooled quickly, it remains a
profuse source of thermal electrons. The free electrons are then
accelerated by E-field in the gap, where they can retrigger spark
breakdown, and arc reignition. Using more massive electrodes, external
cooling, and selected refractory metals or excellent thermal conductors
(copper, molybdenum, tungsten) helps. It's also important to prevent
buildup of oxidation byproducts since these can also become sources of
thermal electrons.

2. Hot air (which has a lower density and lower breakdown voltage) and
conductive ions must be physically removed or cooled sufficiently to
prevent reignition. Forced air cooling helps as does using a more
thermally
conductive atmosphere (such as hydrogen gas).

3. The voltage stress can be reduced so that is no longer sufficient to
cause reignition of the gap. Although this eventually happens naturally
as
bang energy in the TC is dissipated, the challenge is to make it happen
at
the first or second primary "notch". Using a lower voltage stress per
gap
(i.e., using more gaps), physically widening the gap (rotary gap), and
using a lower operating frequency (i.e., lower dv/dt) can improve this.

The "old timers" of spark radio combined many of these qualities in the
"quenching gap". They used relatively massive copper electrodes, a large

number of relatively small gaps, and an alcohol atmosphere (that changed

under use to a mostly hydrogen atmosphere, and later on a pure hydrogen
atmosphere). This approach also prevented further electrode oxidation.

Now to your question:
It is possible to have quenching that's "too good". As you probably
already
know, when the main gap fires, energy transfers from the primary to the
secondary over a number of cycles at the operating frequency of the
coil.
The higher the mutual coupling between the primary and secondary, the
fewer
cycles required. This energy transfer process is called "ring-up" - for
example, ringup in a coil with a coupling coefficient (k) of 0.18 will
take
~3 complete RF cycles to complete. If the spark gap can somehow be
forced
to stop conducting before ringup has completed, then only a portion of
the
primary circuit's energy will transfer to the secondary, and performance

will be reduced.

Although spark gaps operating in air at STP will likely not be able to
quench prematurely, it may be possible to do so with multiple gaps in a
pressurized hydrogen atmosphere. And premature quenching can certainly
occur in electronically switched gaps, such as improperly driven IGBT's
in
an Off Line Tesla Coil, or in a coil using a single trigger pulse and a
fast hydrogen thyratron switch. This has in fact been demonstrated in
systems by Terry Fritz (OLTC) and Richard Hull (H2 thyratron coil).

Best regards,

-- Bert --
-- 
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We specialize in UNIQUE items! Coins shrunk by Ultrastrong Fields,
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Tesla list wrote:

 >Original poster: "Luke" <Bluu-at-cox-dot-net>
 >After reading a bit more about arcs (still taking some of it in)
quenching
 >occurs when the arc is cooled to the point where the air between the
 >electrodes can no longer stay in a plasma state.  This cooling can be
done
 >by lowering the current through the arc or by external cooling like
 >blowing cool air or gas across the gap.
 >This got me curious.
 >Imagine TC built with a gap that used external cooling of the gap to
aid
 >in quenching.
 >This cooling is variable and can be turned up or down to achieve
optimum
 >output.
 >For this question I don't care how the cooling is done or if there is a

 >known way to get the amount of cooling needed.
 >Just imagine that it can be done to any level desired.
 >Obviously if the cooling were turned down the gap would heat up and not

 >quench fast enough to allow the TC to operate efficiently.
 >As the cooling were turned up and the gap starts to quench faster the
TC
 >will perform better.
 >As the cooling is turned up and the quenching happens faster the time
from
 >arc to quench gets shorter.
 >So when the gap is left to conduct too long it hinders the TC
performance
 >and when the time of conduction is shortened the TC performance
increases.
 >My question is this:
 >If a spark gap were made that could quench almost instantly after the
arc
 >is established could it be too fast to allow the TC to give good
performance?
 >In other words if the time the gap conducted were brought to a very
short
 >time could it hinder performance of the TC?
 >It seems that if the time were too short, current would just start to
flow
 >from the capacitor to the primary coil then the gap could be made
 >to  quench when only 1/16 of the power from the capacitor has emptied
into
 >the primary.  This would prevent the primary / secondary coils from
seeing
 >very much of the energy in the cap.
 >So do we want a gap that quenches real fast or do we want a gap that
 >quenches at just about the right amount of time?  We seem to go for a
gap
 >that quenches very rapidly.  Could that be only because we can't get
one
 >to quench fast enough yet, let alone one that quenches too fast?
 >Thanx
 >Luke Galyan
 ><mailto:Bluu-at-cox-dot-net>Bluu-at-cox-dot-net
 >http://members.cox-dot-net/bluu
 >
 >.