Spark GAPS!!!!

Quoting Mattthew <cwolins-at-orion.it.luc.edu>:

> For months now I have seen memos, article, and scribbles 
> about what type of SPARKGAP to use.  I am now completely 
> confused. Can someone please send me a list. 

OK, I can try to loosen the knots.

Spark gaps are commonly classified into two major types. "static"
and "rotary". Static gaps typically don't move. Rotaries
typically employ a motor driven disk fitted with protruding metal

Static gaps can be further classified into different types:
quench, simple, vacuum, air blast, magnetic. There are other
hybrid types out there as well.

QUENCH: this static gap consists of many plates separated by a
tiny distance, typically a mica washer. When bolted together the
actual gaps are sealed which prevent air exchanges. After a short
break in period the oxygen in the sealed gap chamber is consumed
and the gaps run in a nitrogen atmosphere. The arc is completely
enclosed so these gaps are silent and present no UV hazard. These
gaps produce a much smoother wave train than other static gaps.
These gaps saw frequent use in early marine sets, and the best
units were built by Telefunken.

SIMPLE/SERIES: these are the gaps most often used by neon coilers
on their first coil. They can start out as simple as a couple of
bolts with the heads filed smooth. Off the shelf electrode/gap
assemblies are available from welding suppliers, and even non-
resistive automotive spark plugs can be employed. Homemade simple
static gaps (like the venerable cylinder static gap) are easily
constructed from hard copper water pipe, steel ball bearings,
zinc or brass slugs. These gaps make a bit of racket (noise) and
should be shielded to prevent UV exposure.

VACUUM: these static gaps are designed around a vacuum system,
typically a home or shop vac motor/impeller. The idea here is to 
build a box of some type, reduce the air pressure inside of the
box, and only allow air into the box through the gaps between the
electrodes. Typical vacuum gaps use sections of hard copper water
pipe for the electrodes. Vacuum gaps were recently pioneered by
Richard Hull of TCBOR fame and offer superb performance and very
long run times. 

AIR BLAST: these static gaps cool and quench using a high
velocity jet of compressed air to disrupt the arc between the
electrodes, cool, and disperse ions. These are extremely fast
quenching and make lots of noise.

MAGNETIC: these gaps use a powerful magnetic field between the
electrodes to quench the gap. A bit difficult to design and
engineer for the beginner, these gaps can offer a fairly quiet
and  efficient method of obtaining excellent performance from a
static gap.

Hybrid gaps typically employ combinations of quenching
strategies. I recently saw a rotary hybrid in the TCBA
publication NEWS. This gap was based on the Tesla turbine
and while the electrodes were rotating, it is really a 
static gap that takes advantage of the high velocity speed
created by the rotating disks to quench the arc.

Any of the above gaps will work well with a neon power supply.
Neons have internal current limiting which makes quenching
comparatively easy.

The next stage employed in spark gap technologies is placing a
rotary gap in the circuit. A rotary gap is all but required when
using power supplies that employ external current limiting such
as: pole pigs, potential transformers, and plate transformers.
The rotary gap is a mechanical spark gap usually consisting of a
revolving disk with electrodes mounted on the rim. The rotor is
spun and the electrodes move in relation to a set of stationary
electrodes nearby. As a moving electrode comes near a stationary
electrode, the gap fires. As is moves away the arc is stretched
and broken. The rotary gap offers the sophisticated coiler the
opportunity to control the pulse in the tank circuit. A properly
designed rotary gap can control the break rate (bps) and the
dwell time. 

Rotary gaps are run in two modes, synchronous and asynchronous. 
A synchronous gap runs at a fixed speed and is constructed so
that the gap fires in direct relation to the 60 cycle waveform of
the line feed to the capacitors. The point in the waveform where
the gaps are closest can be changed by rotating the synchronous
motor housing or by altering the disk position on the motor
shaft. They are however, the most finicky, and difficult to
engineer of any spark gap, and require sophisticated test
equipment to set up.

Asynchronous gaps are more common. They work quite well and are
much easier to run. Fixed or variable speed motors may be used,
though variable speed gaps give the builder the most experimental
leeway. Break rates need to be in excess of 400 bps. Since the
gap is firing more often than the 60 cycle waveform switches
polarity, more power can be fed into the tank circuit, as the
capacitors can be charged and discharged more rapidly. Though
this system will increase the amount of spark from the secondary,
sparks are generally not as long as with synchronous gaps.

At higher powers (over 5 kVA) even a rotary gap will not deliver
the quench times required for excellent performance unless it is
very large. If the arc in the spark gap hangs too long (NOT
quenched), it leaves the tank circuit electrically closed. With
the gap still firing energy will backflow from the secondary into
the primary and create continued oscillation in the tank circuit.
The secondary is then supplying energy to maintain the arc in the
spark gap. As power levels build, so does the pressure on the
spark gap. Engineering more sophisticated gap systems is the only
solution in large 1/4 wave coils and Magnifiers.

The easiest solution at 5 kVA is to add a simple static gap in
series with the rotary. By messing with the gap settings it is
not difficult to develop a gap system that fires smoothly and
quenches well. As power levels increase though, simple static
gaps will be overwhelmed. More sophisticated gaps are required to
replace the simple static. Magnetic or airblast gaps must be used
in conjunction with the rotary gap to remove the strain on the
rotary and get the quench times back down.

I guess no discussion of spark gaps to this depth is complete
without at least a rough definition of "quenching". This term is
commonly thrown around when talking about spark gaps. When I
began coiling, I saw the term frequently, but never could find a
good definition. 

Quenching refers, more than anything else, to the art of extin-
guishing an established arc in the gap. The term points to the
fact that it is much easier to start a gap firing than it is to
put one out. In Tesla coils, putting out the arc is imperative to
good tank circuit performance.

A cold, non-firing, spark gap is "clean". It contains no plasma,
or hot ions. On applying voltage to the gap, a tension is esta-
blished, and electromagnetic lines of force form. The physical
shape of the electrodes determines to a large degree the shape of
the field, or lines of force, and the resultant breakdown voltage
of the gap at any given distance. In other words, electrodes of
different shapes will break down at different voltages, even with
identical distances between them. 

Once the voltage punctures the air (or other dielectric gas)
the gap resistance drops. The breakdown ionizes the gas between
electrodes, and the arc begins to ablate and ionize the metal
electrodes themselves. This mixture of ions forms a highly cond-
uctive plasma between the gap electrodes. Without this highly
conductive channel through the gap, efficient tank circuit
oscillation would be impossible. But the plasma also shorts the
gap out. A gap choked with hot ions does not want to open and
allow the capacitors to recharge for the next pulse. The gap is
gets "dirty" with hot ionized gases, and must be quenched.

Quenching typically relies on one or more techniques discussed
previously. The most common method used by beginners on neon
powered coils is expending the arc out over a series of gaps. The
plasma is formed in several locations, and the voltage at each
gap is lowered as more electrodes are placed in series. Heat, hot
ions, and voltage are distributed. As the tank circuit loses
energy to the secondary coil, the voltage and current in the tank
circuit, and likewise across the series of gaps, drops to the
point where the arc is no longer self sustaining. The arc breaks,
and the capacitors are allowed to recharge for the next pulse.

Hope this helps!

Richard Quick
... If all else fails... Throw another megavolt across it!
___ Blue Wave/QWK v2.12