Interesting; definitely worth making sure if it's just radio noise.
Yes' I'm pretty sure. In the test just done I put a battery-powered GMC directly behind the cell, and it showed nothing above background, while the pancake detector in front of the cell showed about 5x background. So the first thing I need to check is grounding problems in the DAQ system.
For this test I added 150 ml of deionized water to the cell. The arc in water is still bright blue, with none of the yellowish color seen in tests with HCl. What is interesting is that the waveform at the electrodes was substantially changed by the addition of water. Most of the time it looked like transient2 below. But occasionally something different happened - see transient3a.
I took a close look at that one with the scope's analysis tools. The anode voltage initially went up to 524 volts with the characteristic curve seen in transient2. Then it abruptly dropped to negative 242 volts in just 36 nsec. Based on the traces in transient2b and c, that's a slope of -21kV/usec, extremely fast. It would be very hard to do this with a semiconductor switch, and it's not surprising that it can generate lots of RF. By integrating the area under the rising part of the curve and estimating the inductance of the coil, it might be possible to calculate the energy involved in this event.
There's still a tendency of the electrodes to stick together, not surprising given that the clacker resembles a spot welder. Maybe some electrolytic pre-treatment would build up a layer of oxide enough to inhibit this tendency. So that will be my next step on Monday.