CN spectroscopy and physico–chemistry in the boundary layer of a C/SiC tile in a low pressure nitrogen/carbon dioxide plasma flow

Abstract

The boundary layer of a C/SiC tile in a low pressure nitrogen/carbon dioxide plasma flow is investigated by numerous measurement techniques. CN emission is especially studied through its red and violet systems. Radial and axial profiles of CN relative density are obtained and absolute values are estimated by calibration, thanks to a tungsten lamp. Rotational and vibrational temperatures are determined and compared with wall, heavy particle, and electron temperatures. High vibronic bands should be taken into account in order to determine accurate vibrational temperatures. Δv=0 sequence of the violet system is examined in detail and differences between calculations made using existing sets of spectroscopic constants and experimental spectra are pointed out. Inadequacy of very accurate Fourier transform spectroscopy (FTS) sets of constants in high temperature calculations is underlined. CN and NO density increases in the boundary layer are explained through a dissociative adsorption of N2(v) or N2(A 3Σ+). Such a process implies a significant action of nitrogen in surface erosion. Excitation of CN and NO by electrons is rejected and an excitation transfer with vibrationally excited and metastable states of N2 and CO is proposed.