# Spark minimum current

I know from the title may sounds wired. But it makes sense to me at least for my restricted knowledge of high voltages. I'm always working with low or house voltage as well as this question may refer more to physics.

Let me try to explain: There is a threshold voltage (break down - described by a formula) to break through an air gap. Reach this voltage and electrons will start to flow.

Here my concerns start: after enough current has flown in the gap air get heated and become plasma that is something completely different and has high conductivity. The air gap has become a conductor. As any resistor some energy will be wasted in heat to compensate the cooling of the plasma and keep the "channel" open.

When current is too low, too little heat is produced and plasma resistance begins to increase. This will (at constant current driving) increase resistance and start heating again plasma -> this should (if current sufficient) balance the system at a constant amount of plasma.

When current is too high heat will start to increase expanding the plasma that will balance the system at a point where extra heat will be same as the heat that plasma exchanges with the air.

So if those considerations are right then, after the initial breakthrough, current may be used to modulate plasma "size".

From my guess this is partially confirmed as once a friend was showing me a "music modulated" spark in his garage (that was producing sound) and he was modulating current; whether the only phenomena that would be able to produce sound waves was plasma.

But if so, there can be found a minimum current to keep the plasma channel and therefore the lowest current point at which a spark can be maintained alive.

Is this description accurate? If so, how can I determine the lowest current necessary to maintain a spark over an air gap?

• The current will depend on the supplied voltage and the resistance of the air/plasma which will change extremely rapidly and probably far too fast for any circuit to respond to. I doubt that any steady state is ever reached, making fine tuning rather difficult. In a vacuum however - that is a different story, see "field electron discharge".
– user98663
Commented Sep 16, 2016 at 7:22
• Related : Poulsen arc, which was used as an RF oscillator in the 1920s. Driven from a suitable (reactive) impedance, the arc sustains, but via oscillations, rather than at a steady current.
– user16324
Commented Sep 16, 2016 at 9:16
• TIG welders maintain a spark over a 1-2mm range with 5-10V, but I wouldn't say the current is low! The required DC current to maintain the arc should go down with a reduced spark distance, but it would probably be an unfeasably short distance for a reasonably low current. Commented Sep 20, 2016 at 21:57