Spark Gaps on a Tesla Coil

Question

I understand the point of an LC circuit in a Tesla Coil: add an increasing amount of energy to the circuit with each cycle in resonance, but how does the Primary Coil complete this? Specifically, I was wondering about the Spark Gap. I understand that the Spark Gap, once it fires, will complete the Primary LC circuit so it can resonate. But does resonance occur after the spark gap has fired, or while it's firing? Because with resonance, electricity would have to flow back and forth. So is that what's happening in the spark?

Answer 1

Brief answer: Resonance (oscillations) of the primary circuit only occur while the spark gap is conducting. However, even at zero current, the spark still remains ionized and conducting for tens of microseconds. That means it won't quench, even though the high-freq oscillations repeatedly pass through zero amperes.

But, as the oscillations slowly decay, and the average current decreases, eventually the spark will quench. When the spark disappears, the oscillations in the primary circuit sudddenly halt.

Also note that when the primary coil halts, the secondary coil's oscillations still continue for a much longer period.

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Overall sequence:

1. The HV power supply (DC or AC 60Hz) rapidly charges the capacitor.
2. The spark gap fires, connecting the capacitor to the primary coil.
3. The coil/capacitor oscillates at high frequency (the spark stays lit.)
4. Over several cycles the secondary coil begins oscillating as well.
5. The primary coil/capacitor rapidly loses energy (EM energy is moving to the secondary coil.) The tighter the coupling between coils, the faster this occurs.
6. Over several high-freq cycles, the average primary current falls to zero, the average secondary coil simultaneously rises to maximum, the spark gap then quenches, and the primary coil/capacitor stops oscillating.
7. The secondary coil now continues to ring, and its oscillations slowly die away as energy is lost: to wire-heating, to heating of sparks and corona, and to a very small amount of radio waves.

The critical adjustments are:

1. Matching the LC frequency of the primary coil and capacitor to the secondary coil's natural frequency.
2. Adjusting the coupling between primary and secondary, in order to transfer the LC oscillations into the secondary much faster than the RLC decay time of the primary, but not overly rapidly (since then the oscillations may then transfer back again before the spark quenches, leaving the secondary with low voltage.)
3. Adjusting the spark gap so it quenches just as the primary coil oscillations have fully transferred to the secondary. Cooling of the gap may be required (by large solid electrodes, by multiple small electrodes in series, by a rotary gap, or a combination of all three.)

Interesting triva: if instead of a spark gap we used a switch, then when the switch is suddenly closed, the capacitor and primary coil oscillate. But then the oscillations all "slosh into" the secondary coil, and primary-circuit oscillations are halted. Next, they "slosh back" again, and the primary oscillates, while the secondary coil stops. Then, forward again from primary to secondary, then back again, then forward, over and over. This "slow slosh" is an example of "Line Splitting" or "Coupled Pendulums" or coupled oscillators, and it has a low frequency determined by the amount of coupling between the two coils, as well as the pri/sec resonant frequency. Ideally we want our switch (or spark gap) to open just as the RF voltage on the secondary coil reaches maximum, and before the RF voltage on the primary coil starts rising again during the first "reverse slosh."

When driving the system with raw AC (neon sign transformer,) all of the above must be adjusted to occur in a few hundred microseconds: a little less than 1/4 of one AC cycle. That way the AC supply has time to fully charge the main capacitor, and then the total energy in the capacitor can be deposited into the secondary coil before the next half-cycle of 60Hz begins. This maximizes the output wattage of the system, where the secondary puts out enormous high-freq surges at 120Hz.

For more info, see figs 2.12 and 2.13 here.

Also, here's a classic 1991 paper from AJP journal, with oscilloscope photos of the primary-secondary energy transfer.

Answer 2

When the spark gap fires (via dielectric breakdown), the resistance of the spark gap changes from very high to very low. This effectively couples the tank capacitor to the primary coil, and causes a spike of current. When the tank capacitor is empty and the primary coil is charged, the collapsing magnetic field will couple energy to the secondary and produce a spike of current. This spike of current flows through the still-ionised spark gap back into the tank capacitor, repeating the process until enough power has dissipated to allow the arc to collapse.

The ionization of the air is a macro-level event that takes several milliseconds to dissipate, while the primary side resonance is on the order of several microseconds.