If you want to see light emission on the cathode, pushing above 30 V
might be useful. See:
http://mtaonline.net/~hheffner/SpotsPairs.pdf
At higher voltages, e.g. over 100 V, anode spots are amazing looking,
far more dense, and far brighter. At threshold voltages it looks like
they dance all over the place, like an intense scintillation counter,
but if you look carefully you see that specific active spots simply
flash periodically. I looked at some of the active spots on an
aluminum anode under a microscope, after removal and drying, and saw
what appeared to be a highly polished deeply concave surface.
One thing I noticed that was strange, is that the spots tend to occur
spread out in the center of the anode, away from the edges of foil
electrodes, just as SPAWAR's thermal images show the cathode hot
spots arrayed. It would seem to make more common sense that they
occur first at the electrode edges, where presumably the field
intensities and currents are larger.
It could be the spots tend to form at self resonant EM surface wave
oscillation nodal points on the foil.
Best regards,
Horace Heffner
http://www.mtaonline.net/~hheffner/
