Mode Transition and Related Discharge Phenomena of a Tube Plasma Source Operating in Low-Pressure Pure Nitrogen Atmosphere

A low-frequency (20-80 kHz) tube plasma source was designed for plasma beam generation at low pressure. In the atmosphere of pure nitrogen, the plasma source exhibited two distinct discharge states. The discharge sustained at low current (G mode) was a capacitive discharge and transited into a resistive discharge self-pulsing mode (SP mode) when the current exceeded a critical level. The SP-mode discharge was identified to be a mixing discharge consisting of continuous discharge and periodic sparks or pulsing arcs. Powered with 10-110 W, the SP-mode discharge created a plasma plume with high electron density (0.9-5.1×10¹¹ cm^-3) and relatively low electron temperature (1.62-0.55 eV). The vibrational and rotational temperatures of the SP-mode plasma in the tube were determined to be ~5000 K and 350-700 K by fitting the optical emission spectra, respectively, distinguishing from those of the G-mode discharge.