Observation of non-thermal metastable ion velocity distributions in a miniaturized multi-dipole confined plasma device

Abstract

Hot cathode discharges are common plasma sources for fundamental plasma physics studies and other applications due to their capability to produce quiescent plasma. This work presents experimental observations of presheath-associated non-thermal metastable ion velocity distributions in a miniaturized multi-dipole confined plasma device measured by laser-induced fluorescence. The intensity of this non-Maxwellian component is related to the collisions of these metastable ions with background particles. Additionally, the flow velocity of this component is lower than the Bohm velocity; thus, its energy is lower than the presheath potential drop kTe/2. This implies that these non-thermal metastable ion velocity distributions are formed via presheath acceleration and are associated with the source asymmetries of the miniaturized device. The strength decreases as the neutral pressure increases once the neutral pressure is adjusted, suggesting that the presheath length is the critical condition that determines whether these components can be observed.