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 extinguishing 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 established, 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 conductive 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 ¬ 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.