Today I received some 0.4mm (0.26ga) Kanthal A1 wire for further testing. Out of the box, the first impression was that it's quite stiff compared to 28ga Ni200 wire, but it can still be bent with relative ease (+ some effort).
With cathodic plasma electrolysis operation using just tap water, which was previously shown with Ni wire to cause oxidation, it quickly develops a dark layer, likely from Al2O3. As it does, initially a gray-brown substance gets dissolved in the aqueous solution, possibly some aluminium compound. Applied voltage was 800V, but under operation and locally hot water, it decreased in the 240-300V range (due to the power supply limiting current).
After the initial induction period where the tip becomes black, it seems more difficult than with the 0.3mm Ni wire to start a reaction, but also to melt it quickly when it begins. It does not seem to become incandescent easily either. This change seems proportionally larger than the difference in wire gauge. By bringing the wire closer to the top of the water surface the reaction does become stronger and sometimes white flashes also appear (my idea is that this is due to the high-melting point surface Al2O3 oxide layer).
A possible factor decreasing performance could be the wire stiffness coupled with high electrical resistivity, which could make electrical conduction difficult under my current configuration. After wrapping the wire more tightly around the tungsten rod I used so far (which I also cleaned up a bit with sandpaper), it seemed to improve a bit, but still did not become incandescent as well as with the 0.3 mm Ni wire.
The idea behind getting Kanthal wire was that it could have helped somewhat in initiating a plasma reaction due to the spontaneously formed surface oxide layer (potentially inducing a sort of dielectric barrier discharge-kind of operation), but it's possible that it's having the opposite effect; alternatively it could be that too much energy is being wasted into joule heating as this is after all a material intended for resistors. A second motivation was that Al2O3 seemed to show particularly bright results in different tests.
The area surrounding the cathode tip still seems quite hot after operation on the other hand, although this is just a subjective impression. So, apparent wire temperature might not necessarily be a good indicator of how well the reaction is progressing if not perhaps for theoretical reasons. It certainly does not look as good, though.
In 1-2 weeks I should receive some 28ga titanium wire, also chosen for similar reasons (spontaneously formed, passivating surface oxide layer). Additionally, the melting point is somewhat higher than most common materials. Titanium dioxide however has a lower melting point than aluminium oxide.