Spark Gaps

 * Original msg to: Jdf149-at-psu.edu
 * Carbons sent to: usa-tesla-at-usa-dot-net

> I haven't heard anything on this list in a bit.  

The list has been spasmodic at best as of late.

> Can someone explain to me, the physics of a rotary spark gap? 

It is a forced break in the tank circuit. A proper rotary gap can
control break rate (BPS = breaks per second = PPS pulses per
second), and dwell time. It allows the coiler to better control
Tesla tank circuits when using externally current limited

> If what I read is correct, the more breaks per second the
> better? 

To a point, and assuming you have a power supply large enough to
fully charge the caps between each fire of the gap.


              Spark Gap Technology

I recently explained the definition of "Q", and the requirements
and functions of high Q grounding systems in Tesla coils. Another
area that needs attention is spark gap technologies.

Spark gaps are the "brain" of the Tesla Coil. They are high the
voltage switches that allow the tank circuit capacitance to
charge and discharge. As performance of the spark gap switch is
improved, peak powers in the tank circuit grow without requiring
additional input power. When a good coiler sets up and fires a
system, the first thing he looks at is his ground. The second
thing he looks at is his spark gap system.

Before I cover the main points on spark gaps, I want to talk for
a moment about their more modern replacements, the vacuum tube,
and the solid state transistor (FET etc.). Both modern day
replacements can be made to function in Tesla type oscillators 
in several modes. A single resonating coil may be base fed RF
current from solid state and tube drivers, or primary coils may
be driven with amplifier circuits. Class C amplifiers are
preferred. Both of these modes work well within the power
handling abilities of the switch (tube or solid state device),
but when it comes to handling raw power, nothing delivers the
megawatts like the old fashion spark gap. The spark gap gives
the biggest bang for the buck.

No discussion of spark gaps 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. The most
common method used is expending the arc out over a series of
gaps. Gaps of this type are know as "series static gaps".
"Static" in this use refers to the fact that the gap is not
actively quenched. 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.

The second type of quenching technique involves using an air
blast. A high speed air stream is introduced into one or more
gaps. The air stream does not alter the magnetic lines of force
that cause a dielectric breakdown in the gap, so gap distance
remains unchanged. But once an arc is established, the air stream
removes hot ions from between electrodes and physically disrupts
the established arc. The gap is swept clean of hot ions, the arc
breaks, and the capacitors are allowed to recharge.

A third type of quenching used is the magnetically quenched gap.
A strong magnetic field is placed between the electrodes. Since
this field alters the field formed by the high voltage prior to
breakdown of the dielectric in the gap, it may affect the break-
down voltage of a given set of electrodes. Once the gap breaks
down however, the field shape changes. The high current flowing
through the gap generates a field shape associated with the
current. By placing a strong magnetic field in right angles to
the current flow, the arc is disrupted. This disruption tears at
the magnetic lines of force formed by the high current channel
flowing through the gap. The arc is twisted, and broken, without
having to remove ions. 

Another type of spark gap called the "quench gap" is used on
coils designed for CW output. This gap was discussed in a
previous post and will not be covered here.

The next stage employed in spark gap technologies is placing a
rotary gap in the circuit. The rotary gap is a mechanical spark
gap usually consisting of 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. By carefully matching the output of the supply transformer
to the value of capacitance in the tank circuit, then running  
a properly set up synchronous gap, it is possible to have the gap
fire only at the voltage peaks of the 60 cycle input current.

This technique allows the tank circuit to fire only on the
maximum voltage peaks and delivers the pulse from a fully charged
capacitor each time the gap fires. If properly engineered,
synchronous spark gap systems will deliver the largest EMFs to
the secondary coil. 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, and I have
found that breaks rates around 450-480 bps give the best
discharge. 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 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 static gaps will
be overwhelmed. More sophisticated gaps are required to replace
the static series gaps. 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.

Somewhere in here I need to cover the Q of spark gaps. Not all
spark gaps have the same Q. I have found that using large series
static gaps with lots of electrodes; the Q of the gap system
decreases as the quench time decreases! Try to avoid static gap
designs with more than 6 - 8 electrodes in series.

As my power levels went up, and my spark gap Qs went down, I
experimented with options to regain performance. I found that by
running static gaps in a combination of series/parallel gave me
good quench times and I regained some lost Q from the arc having
to make so many series jumps. The idea was to split the arc down
into two or three equal paths, reducing the current traveling
each set of series gaps. In this fashion I was able to achieve
excellent quench times with a small rotary running around 5 kVA.

The lesson learned was too many gaps in series kills the Q of a
spark gap. By adding gaps in parallel, and reducing the number of
gaps in series, some Q was regained while power levels increased.
This is a valuable hint in spark gap designs.

Another factor that should be brought into this discussion is the
effects of cooling the electrodes. To start with, I have never
run even a simple static gap without some airflow. My first few
really good static gaps were constructed inside of PVC pipe
sections with a 5" muffin fan on top. The fan did not supply
sufficient air to disrupt the arc, but did assist in removing hot
ions, and cooling the electrodes down. This allows for longer run
times. As my work progressed I realized that reducing the
electrode temperature, while not actually quenching the gap,
reduces the amount of metal ions introduced into the arc, and
makes the gap easier to quench with an airblast or magnets.

I am going to cut this off here. I feel I have covered most of
the basics, and thrown a few ideas out into the cyberspace. I
would be more than happy to expand on spark gap technologies at
any time should somebody have any specific questions, comments,
problems, or corrections. Remember, armchair debate is no
substitute for actually going out an experimenting with a few
live systems, and I am always hoping someone will tell me a
better way to do it!

One final safety note. Spark gaps are loud, and emit a lot
of hard UV radiation. Wear hearing protection as required, and 
never stare at an operating spark gap without welding goggles.
To examine the arc on large coils, a sun observation filter
on a small telescope will tell you if your gaps are quenching.

Richard Quick

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