Just a quick update on this one. I've done some testing very specifically on the EMI induced in a shielded wire taped adjacent and in very close proximity to the plasma cable. This test cable is a CAT 6 Ethernet cable, where each pair is foil shielded and the whole cable is braid shielded.
Also note that I didn't actually conduct everything in the following order, but I thought it made more sense to present it this way.
I got some interesting results (and way, way more noise than anticipated).
No wires connected to the scope (except the power cord)
Thanks dad for suggesting this one after a mammoth amount of head scratching
At a (extremely short) peak of ~-40v, I was already surprised by how intense this noise is. Note the scale of the chart is 10v/div and 250ns/div. This pulse however only lasted for less than half a microsecond.
The cable's shield wasn't grounded and instead connected to the signal line of the scope. The scope's ground was earthed.
Peaks are well above 100v and noise is at least 3 microseconds
Grounding the shielding substantially improved the noise. Apologies for a lack of photo, I will repeat the experiments tonight and upload another image of the shielded cable for comparison.
The shielding reduced the peaks to ~30v, but this also changed dramatically depending on whether the torch was grounded when the arc was initiated. I believe that preliminary a set of small ceramic capacitors to ground should suitably protect the drivers, but I think I'll be testing it with a stand along step stick and cheapo arduino first!