The influence of a negatively biased ring electrode on the occurrence and dynamics of a fireball created on a central disk in a DP machine plasma

The double plasma (DP) machine is a very useful device for the study of plasma potential structure, like space charge sheaths or double layers (DL). For a certain domain of background pressure and plasma density, we have obtained a very stable fireball surrounded by a DL by applying a positive potential on a disk electrode (DE) immersed in plasma. The experiments have been carried out in the target chamber of the Innsbruck DP machine. The plasma density was 3.7x108 cm-3 and the argon background pressure 5x10-3 mbar. The DE was made of tantalum with 1 cm diameter. A ring electrode (RE) with 3 cm outer diameter and about 1 cm inner diameter surrounded it. The current-voltage characteristic of the DE shows many upward and downward sudden jumps of the current, accompanied by hysteresis. These jumps are associated with the occurrence and the dynamics of the double layer structure formed after a self-organized process. After first upward jump a very stable double layer occurs. The potential corresponding to this jump depends strongly on the negative bias applied on the RE. The current domain of stability decreases with increasing the negative bias applied on the RE. After exceeding the stability range, the DL becomes unstable, triggering non-linear oscillations similar as reported by Song, D'Angelo and Merlino. The frequency of these oscillations depends strongly on the applied negative biased of RE. (authors)