Firetube formation through sheath-plasma instability in expanding RF plasma

Abstract

Formations of complex structures observed during the instability instigated expanding capacitive mode of RF discharge in a linear vacuum vessel, in the absence of an externally applied magnetic field or any other additional constraint applied. Systematic measurements of the plasma parameters, using multiple probe diagnostics, carried out to investigate the parametric dependence of sheath development reveal visible cylindrical tube like multiple double-layer formations around a mesh-type cylindrical plasma source. Such firetube formations provide many interesting and vital information on expanding radio frequency (RF) plasma sheath, charge particle trapping and the associated sheath instability with the RF discharge. The sheath structures, glowing spot formations, space charge formations, complex structure and firetube formations in the expanding plasma are some of the features discussed here in relation to such an expanding RF discharge. Low-frequency sheath instabilities are also observed in connection to the firetube formations using a coaxial mesh-type cylindrical plasma source powered at an operating frequency of 13.56 MHz. The ion acoustic frequency-dominated instability is identified as a sheath-plasma instability associated with the RF plasma.