Quenching

Quenching refers to the process of cooling, stretching, or spreading out the arc within a spark gap sufficiently to prevent the previously ionized path from reigniting upon reapplication of voltage. Although the term is most often applied to AC power arcs and RF spark gaps, it can also apply to high voltage DC switching. Quenching a high voltage arc normally requires that the arc current passes through zero (called a current zero) for some period of time. This can be done through normally occuring AC current zeros, during brief current zeros that occur upon completion of primary-secondary energy transfers, or (for DC arcs) by temporarily diverting current from the arc to create a synthetic current zero for the arc.

In spark gap Tesla Coils quenching is ideally performed when all of the available system energy has been transferred from the primary LC circuit to the secondary LC circuit. This is sometimes called quenching at the first "notch" (the first point of zero primary current and voltage). At this point virtually all of the initial energy that originally resided within the primary circuit tank capacitor has been transferred into the secondary LC circuit. By quenching at the first notch, energy now residing in the secondary LC circuit is prevented from transfering back to the primary circuit where it can contribute to output sparks. However, in order to properly quench, the spark gap must recover sufficient dielectric strength so that doesn't reignite from the voltage induced into the primary circuit (via transformer action) from the oscillating secondary circuit. Quenching at the first notch provides the highest level of energy to the secondary.

Quenching is important for coil performance since the primary spark gap tends to be the lossiest element in a well designed spark gap Tesla Coil. The system loses significant energy whenever the main spark gap is conducting, and energy lost within the main gap reduces the amount of energy available to generate secondary sparks. Poorer quenching results in lower operating efficiency and shorter sparks. The higher the coupling between the primary and secondary, the more difficult quenching becomes. Tightly coupled spark gap coils may quench at the second notch (i.e., when energy has fully transfered between the primary and secondary system over two full cycles), while poorly designed spark gaps may fail to quench well past the second notch. If gap electrodes (or an oxide layer on the electrodes) are permitted to become incandescent, thermionic emission will severely degrade the gap's quenching ability. Using ample forced air cooling, periodic cleaning, and employing electrodes with significant thermal mass will prevent this. Heavy loading on the secondary side (due to heavy streamer loading or a ground strike) will seemingly improve quenching since it also reduces stored energy from the secondary, thereby reducing the probability that the spark gap will reignite.

Various methods can be applied to quench an arc:

• Jets of air that literally 'blow out' the plasma
• Using a large number of small static gaps in series between massive electrodes to rapidly cool and extinguish the arcs. For example, the Multi-plate discharger of Max Wien, (though possibly invented by Nikola Tesla) was used in medium power spark sets which was known as the "whistling spark" for its distinctive signal
• An electromagnetic field at right angles to the gap (which spreads out and elongates the arc).
• Using a combination of rotating and fixed electrodes in a rotary gap in order to rapidly stretch, cool, and break the arc
• Using other gases, such as hydrogen (which rapidly cools the arc and electrodes), or sulfur hexafluoride (SF6) which has a much higher dielectric strength to assist in quenching.
• Using a dielectric fluid (such as mineral oil) to rapidly cool and extinguish the arc. This approach is commonly used in power utilities for circuit interrupters.