In non-LTE arc plasmas, O II excited state number densities were measured relative to the O II ground and metastable states. The results were compared with collisional-radiative code calculations on the basis of the JET ADAS programs. Stationary He plasmas with small oxygen admixtures, generated in a 5 mm diameter cascade arc chamber (pressures 13-70 hPa, arc current 150 A), were investigated spectroscopically in the visible and the VUV spectral range. The continuum of a 2 mm diameter pure He arc (atmospheric pressure, current 100 A) served for calibration of the VUV system response. Plasma diagnostics on the basis of H beta Stark broadening yielded electron densities between 2.4*1014 and 2.0*1015 cm-3 for the low-pressure O II mixture plasmas. The intensities of a He II line were analysed with respect to electron temperatures, resulting in values from 3.5 to 8.2 eV. The O II ground and metastable number densities were derived from resonance lines in the VUV on the basis of the corona excitation balance and the relevant rate coefficients. Subsequently, the population of five 3p levels and one 3d level was obtained from lines in the visible spectrum. The agreement of measured and calculated excited state populations is generally very satisfactory, thus confirming the rate coefficients in the code. This is of particular interest in this intermediate region between corona balance and LTE, where many atomic data are required in the simulation. Clear indications were found for the diffusion of metastables lowering their number densities significantly below their statistical values.
High-power 915 MHz atmospheric pressure microwave air plasma characterized by optical emission spectroscopy and imaging