Localized detached glow above a titanium hollow cathode

An argon hollow cathode was investigated as plasma source and sputtering source. The hollow cathode consists of a Ti-cylinder embedded in ceramic. There are various modes of operation: we have observed a corona discharge in argon and the typical hollow cathode discharge mode with strong titanium sputtering. In the latter case a sharply bounded intensive yellow-reddish glowing zone was observed at the "muzzle" of the Ti cathode. Under certain conditions this zone was observed to float freely between cathode and anode ring.