An Optical Emission Spectroscopic Analysis of Vibrational Excitations in Electron Beam Driven N₂ Plasmas

A time-resolved characterization of the vibrational excitations in an electron beam generated N₂ plasma is conducted via optical emission spectroscopy. The electron beam is initiated by the Naval Research Laboratory’s Febetron pulsed-power generator which is configured to produce a 100 kV and 4.5 kA peak voltage and current pulse with a 100 ns pulse time width. The beam is directed into an N₂ gas fill chamber where beam-impact and thermal heating causes excitation and partial ionization of the gas into a low-temperature plasma. Spectral measurements of the light emitted from this plasma are conducted by means of a 0.5 m focal length Czerny-Turner spectrometer in the near-UV to visible wavelength range. Spectra are imaged with a 2D time-gated intensified CCD that allows tracking of the temporal evolution of vibrational states over the lifetime of the beam pulsed plasma. Analysis of these spectra is focused on measurements of the second positive (SPS) and first negative systems (FNS) of N₂. Low pressure (100 mTorr) and high pressure (1 Torr) regimes are explored and comparisons made between vibrational population distributions of each system. Modeling of the vibrational spectra is conducted which allows spectral fitting and measurements of the vibrational temperatures of the plasma at various times during the pulse.

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