Plasma potential locking

Summary form only given. A positively biased electrode (anode) in a plasma is assumed to have no, or little, global impact if the electrode is small enough, i.e., used as a diagnostic probe. For a sufficiently large anode the plasma is globally impacted via plasma potential locking where the anode potential controls the bulk plasma potential. Prior experiments1 and theoretical descriptions2 are extended here through simulation of a low pressure Ar plasma to better understand this behavior and especially the transition region between the two limiting cases.In the small anode case an electron sheath is formed at the anode and the amount of current extracted is small enough to have little global impact. However, as the anode-to-wall area ratio approaches 1.5 me / m i (or 0.6% for Ar), the plasma responds globally to the presence of the anode by increasing the bulk plasma potential to balance the increased electron current lost through the anode. Further increasing the anode size, at an area ratio of approximately 2.6 me / m i (or 1.0% for Ar), the anode-plasma interface becomes an ion sheath and the bulk plasma potential is locked to the anode potential (plus an amount according to the ion sheath rise). The regime between these two limits, ranging over only 0.4% of the wall area, exhibits interesting behavior where combinations of electronand ion-sheath characteristics co-exist. This and other behaviors will be described.