A supersonically expanding argon cascaded arc plasma, with difFerent amounts of hydrogen added (0, 0.7, and 1. 4 vol % H2), was studied using Thomson-Rayleigh scattering and optical emission spectroscopy. %'ith 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 Ar+ and H2, and dissociative recombination of the resulting 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.

A potential buildup in front of a magnetized cascaded arc hydrogen plasma source is explored via E x B rotation and plate potential measurements. Plasma rotation approaches thermal speeds with maximum velocities of 10 km/s. The diagnostic for plasma rotation is optical emission spectroscopy on the Balmer-beta line. Asymmetric spectra are observed. A detailed consideration is given on the interpretation of such spectra with a two distribution model. This consideration includes radial dependence of emission determined by Abel inversion of the lateral intensity profile. Spectrum analysis is performed considering Doppler shift, Doppler broadening, Stark broadening, and Stark splitting.

Cascaded arc plasma sources with channel diameters between 4 and 8 mm were experimentally investigated at discharge currents up to 900 A and hydrogen (H 2 ) flow rates up to 10 slm. Pressure measurements at the arc exit showed that the heavy particle temperature in the discharge channel was about 0.8 eV. The electron temperature was calculated from the electron mass balance, taking into account electron losses due to ambipolar diffusion and convection out of the source channel. This calculation showed that the electron temperature was 1.5-4 eV, increasing with decreasing density in the channel (i.e. with decreasing H 2 flow rate and increasing diameter). The results of Thomson scattering measurements at 1 and 5 cm distance from the source exit showed the same trends. Using measurements of the average axial electric field, the effective size of the current-carrying 'active' plasma was calculated, expressed in terms of the filling fraction ρ 2 = (r eff /R) 2 . The data showed that the filling fraction increased linearly with the input power and was independent of the diameter and flow rate. The ionization degree in the active center was estimated to be 20-30% from an evaluation of the electron energy balance, Thomson scattering measurements and H β emission measurements. The highest gas efficiency was obtained when the channel was completely filled at a maximum current of 900 A (65 kW input power, 8 mm channel, 4 slm flow rate) and was 19%. The highest energy efficiency was 7%.