Argon-hydrogen plasma jet investigated by active and passive spectroscopic means

A supersonically expanding argon cascaded arc plasma, with different amounts of hydrogen added (0, 0.7, and 1.4 vol % ${\mathrm{H}}_2$), was studied using Thomson-Rayleigh scattering and optical emission spectroscopy. With hydrogen added, the electron density profile as a function of the distance from the onset of the expansion shows a large extra ionization loss (compared to the pure argon case), especially after the stationary shock front. This anomalous loss of ionization is attributed to molecular processes, such as associative charge transfer between ${\mathrm{Ar}}^{+}$ and ${\mathrm{H}}_2$, and dissociative recombination of the resulting ${\mathrm{ArH}}^{+}$ molecular ion. Spatially resolved emission spectroscopy shows that in the expansion the radial hydrogen emission profiles are broader than the argon profiles. The addition of hydrogen appears to change the characteristic shock behavior of pure argon. For both the argon and the hydrogen system it is shown that the uppermost levels are in Saha equilibrium with their adjacent continuum. The measurements are in agreement with the view that association of hydrogen atoms at the vessel walls results in a reentry flow of hydrogen molecules in the plasma beam.