Plasma fireballs, their creation and behavior*

Fireballs were first observed as localized discharge phenomena on positively biased small electrodes in plasmas. Many similar forms of bounded glow phenomena in low-pressure gas discharges are now also termed fireballs. The strong glow of a fireball is due to energetic electrons impacting with neutrals. The sharp boundary of the glow is evidence for a localized electric field such as a sheath or double layer. Fireballs mostly appear as spheres or cylinders attached to the electrode, but new forms have been observed with various geometries and/or in magnetic fields. We have undertaken many investigations of the creation and behavior of fireballs, in particular the instabilities that can be detected in the low frequency range, due to ion fluctuations, as well as in various high frequency ranges, due to electrons. The particle dynamics within the plasma space charge formations and the exchange between them have been investigated. Recently we have concentrated on the interaction between several separately created fireballs, showing also the influence of the fireballs on the background plasma parameters. The current oscillations show specific frequencies for the dynamic states of the fireballs, where periodic expulsion and backflow of ions occur near the surrounding double layer.