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.

Measuring the pressure in ultrahigh-pressure mercury arcs

“…This pressure range has been confirmed by measurements of n and T on the axis 10 ͑with n from the width of the 1014 nm Hg line 19 ͒ and recently also at the wall ͑with n from a Brewsterangle measurement 6 ͒. A lower pressure ͑16 MPa͒ has been estimated by Wharmby,20 but for a nonstandard experimental lamp and with a less direct method. In spite of the successful pressure measurements in single lamps, 6,19 the existing techniques are quite complex as to measurement and analysis and thus of limited use as standard methods in industry.…”

“…This is especially interesting for the 1014 nm Hg line 10,19 at higher pressures ͑25-30 MPa͒ and for the near-UV and blue Hg lines. 20 The present experiment could still be improved in several respects. First, ⌬ can be measured more accurately than here ͑2%͒.…”

On the atomic line profiles in high pressure plasmas

“…These figures show that the oscillations which start to occur around b ¼ 50 arise when the line profile is dominated by the Levy contribution. Wharmby 23 already mentioned that impact and quasistatic theory have their limitations and that for high densities the Levy profile can cause unrealistically large red wings. He was still able to obtain accurate results by convolving again with a Gaussian profile to artificially limit these wings.…”

Study of photometric properties of high-pressure mercury discharge with thallium iodide additives (HgTℓI) using the ray-tracing method

“…In a previous paper [8] we treated a pure mercury Hg lamp, and it was demonstrated that, assuming a temperature profile derived from experimental measurements (Bartels' method [9], from spectral analysis of the Hg 577 nm line) the numerical model successfully reproduced the observed Hg spectra. Wharmby [10] has noted that Bartels' method was an appropriate tool to derive the gas temperature in the 1950s, before the ready availability of computers. However, he recommended that the numerical technique described here should replace Bartels' method to compute the temperature profile.…”

“…Bartels' method requires a number of quite restrictive assumptions [11] with limited ranges of validity, whereas numerical integration of line radiation is only dependent on the knowledge of the line broadening coefficients. For a more detailed discussion of these issues, the reader is referred to [10,11].…”