Electron-rich luminous sheaths and fireballs

Luminous sheaths and fireballs evolve if a positively biased electrode is inserted into a thin background plasma with a sufficiently high neutral pressure. These phenomena show a wide variety of instabilities and nonlinearities and have been seen either on plane solid or gridded electrodes appearing as sharply bounded localized structures in front of the electrode. A wide variety of strong low-frequency and high-frequency oscillations were detected during the appearance of a fireball and in, case of pulsating fireballs, also in during pulse pauses when the fireball recedes to an ionization sheath. Furthermore, inverted fireballs were produced, which are trapped inside a hollow, highly transparent spherical grid and separated from the surrounding plasma by a nearly current free double layer. This fireball configuration leads to electron oscillations at frequencies of around 100 MHz. Also luminous ionization sheaths were investigated, including transient phenomena during transition from ion-rich to electron-rich sheaths. Time-resolved current-voltage characteristics of the electrode are presented. The time scale for the formation of an electron-rich sheath is determined by the ion dynamics and amounts to about an ion plasma period. When the ions are expelled from the sheath a high-frequency sheath-plasma instability grows with more than 200 MHz.