I got noise out of 5V TTL encoder signals with clamp-around ferrites around the motor power leads. The encoder input was picking up the servomotor power.

The big problem here is that you can't do much about the source. A plasma arc started with audio-frequency AC (10KHz?) is going to emit RF. Not clear what the waveform looks like. If it's a square wave, which it probably is, there will be higher harmonics. Fortunately, it's only AC only during start; the cutter shifts to DC once the arc has been struck. Otherwise those things would be a huge interference headache whenever they were on.