Plasma diagnostics of a direct‐current arcjet diamond reactor. II. Optical emission spectroscopy

Diagnostic analyses were performed on a dc arcjet diamond reactor using optical emission spectroscopy. Rotational and vibrational excitation temperatures were determined from C₂ and CH emission spectra and observed to differ widely at all locations in the plasma except near the substrate. Temperatures were observed to converge at the substrate, indicating thermal equilibration of the plasma. Rotational excitation temperatures for C₂ and vibrational excitation temperatures for CH appear to closely track the plasma gas temperature, while vibrational excitation temperatures for C₂ and rotational excitation temperatures for CH are anomalously high. A detailed analysis was conducted to identify the dominant excitation processes responsible for producing emission. Results show that (1) excitation to the excited state for both C₂ and CH is most likely due to thermal excitation near the substrate, (2) in the plasma bulk, emission by CH is produced primarily by chemiluminescent reactions while emission by C₂ is produced by electron impact excitation, and (3) optical emission spectroscopy may be used as a nonintrusive probe of the plasma gas temperature.