Good work, Steve. I tried it with my 12kV NST and I'm also getting bubbles, a significant amount. The cell is a couple of tapered restaurant sauce cups. I have a 5/16" spark gap with a DIY nano Farad cap. 300 W on the primary, but, with the diode, it's perhaps only half the rated value. edit: And I'm using well water.
With this many Watts, I'm wondering about steam?