The plasma tube is best driven by the square wave as every time the plasma tube/Phanotron is lit, the most power is generated. The square wave essentially is turning it on and off at max power when lit as well.
Driving with a square wave means maximum power output. Sine waves would just lower the power output, and any other waveform would not provide any benefit. Some really fancy waveforms could actually cause issue depending on how wild the settings got.
Spikes would effectively lower the power output as well, as the spikes would be a the maximum power level, and the nominal power of the rest of the waveform would be sub-optimal.
If your spikes exceeded the max power of the plasma system, then you would be creating a situation like amplitude wobbles where you overdrive the system and possibly burn it out.
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