Looking to this base ring design, I could not avoid thinking of continuing the 90 degrees groove up to the top of the anode. This provides a Guide or railway to the plasma sheat and could be used in many ways. If they rise vertically ok we will have a concentrated plasma in the points. Looking to images of a used anode, we can see the grooves that the discharge makes trying to get into the center hole.
Now, we could make this vertical grooves slightly Helical, (Rifled) This will induce a circular motion to the plasma, similar to the effect of the coil and improving the filament formation allowing to manage the plasmoid size.