Sheath oscillations during directional motion of fire tube formation in expanding RF plasma

Low-frequency sheath oscillations, initiated by the sheath-plasma instability, are observed in connection with the complex structure formations in expanding radio frequency (RF) plasma generation. The temporal variations of the floating potential during the double layer formations, measured using electric and magnetic probe diagnostics, suggest the significant role of sheath-plasma instability in charged particle dynamics in the present operational regime. The burst oscillations of the potential occur due to the oscillations of the electric field associated with the high-gradient plasma source geometry, the sheath field. The initially localized discharge evolves into multiple luminous annular plasma structures with the progressive RF power. Although plasma discharges using additional electrodes immersed in plasma have been investigated extensively in DC plasmas, present experiments are carried out in the absence of any additional plasma source as well as any externally applied magnetic field. In line with our earlier reports, [Chakraborty et al., Phys. Plasmas 25, 033518 (2018); Paul et al., Phys. Plasmas 26, 023516 (2019)], the present work emphasizes the complex sheath structure formations, discharge transition from unstable to stable state, followed by the sheath oscillations through sheath-plasma instability during the expanding radio frequency discharge features that differentiate it in many ways from the earlier experimental investigations. Published under license by AIP Publishing.