Re: Quenching question.

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

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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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
>
>.