There's even more info about these types of speakers  along with the long history here https://massless.info .  There's plasma and ionic types.

There was a full range Plasma loudspeaker system marketed at retail in the late 1970's (Plasmatronics). It was an advanced unit sound-quality wise and carried a premium price tag. It used a Helium Gas plasma, and you had to periodically replenish the tanks of Helium (an inert gas).

Problem was the plasma attacked the lungs of those in the room as it played, and the developer (name was Hill)  had health issues as a result of his involvement in plasma research (his day job was in the Military). Until I knew more, I would go very carefully with this project. Apparently not all Plasma audio designs are dangerous, but then again, some are. Find out more before you leap, is all I'm suggesting.

The good news is a Plasma driver is extremely low distortion and can cleanly output Ultrasonics. Low Frequency reproduction is another matter. 

The Ultrasonic output carries both advantages and disadvantages. Most audio equipment assumes US is inaudible and therefore very little of it is low distortion. Although our ears cannot directly discern steady-state (sine wave) ultrasonics, we can perceive transient ultrasonics and in some cases research subjects have experienced nausea when presented with ultrasonic audio signals (well above 20 KHz). So, the performance of a working, safe Plasma driver may not result in audio pleasure. Just sayin'.

Note: I was in the industry at the time. Not all of what I wrote above is public knowledge.  However the audibility of Ultrasonic Audio signals has been proven, so feel free to fail to debate it. Instead do your research in the field of Neurophysiology where you will find numerous research papers on the subject.

This is cool. It is a bit like IML digitweeter from 1981... :-) With a high pass filter in front and better (tungsten) electrodes,   it could be a nice tweeter. 
like the IML on this site. http://www.plasmatweeter.de/