Estimates of molecular absorption cross-sections in mercury plasmas at very high pressures using self-reversed line diagnostics

Abstract

Plasmas containing mercury at pressures >100 bar are used as sources of extremely high radiance. High-resolution measurements have been made of the absolute spectral radiance of a plasma in a line of sight through its centre. Detailed comparisons of the Hg 546 nm calculated line shape with measurements provide information about the pressure, centre temperature and the temperature profile. Results are given for an arc at a pressure of 160 ± 10 bar and maximum temperature of 8350 ± 100 K. Radiation transport calculations show that absorption by mercury molecules increases progressively with decreasing wavelength towards the UV. By comparing calculations of self-reversed line maxima, a temperature-independent cross-section for Hg2 absorption has been derived. Use of this, with other published data for line and continuum radiation in radiation transport calculations, yields calculated spectra in excellent agreement with the experiment except in the extreme wings of spectral lines. The calculations can be used to predict behaviour at other pressures; at all the pressures and most wavelengths considered the plasma has appreciable optical depth. Uncertainties in pressure measurement remain a problem with this type of plasma. The 313, 365 and 435 nm line radiances are absorbed to various degrees by mercury molecules. Their radiance ratios are very sensitive to pressure and could be used, if calibrated, to provide a more satisfactory measure of pressure.