Reliability of temperature determination from curve-fitting in multi-wavelength pyrometery

Abstract

This paper examines the reliability of a widely used method for temperature determination by multi-wavelength pyrometry. In recent WDM experiments with ionbeam heated metal foils, we found that the statistical quality of the fit to the measured data is not necessarily a measure of the accuracy of the inferred temperature. We found a specific example where a second-best fit leads to a more realistic temperature value. The physics issue is the wavelength-dependent emissivity of the hot surface. We discuss improvements of the multi-frequency pyrometry technique, which will give a more reliable determination of the temperature from emission data.Keywords: warm dense matter, temperature measurement, pyrometry, heavy ion beam heating. IntroductionWarm dense matter can be produced by several heating technologies including lasers, pulsed electric current, electron, ion and x-ray beam heating, shock-wave heating, etc (DOE, 2009;Hoffmann et al. 2005;Sasaki et al. 2006;Ng et al. 2005; Lee et al. 1998). Temperature measurements are essential to WDM science. In most cases the deposited energy is determined accurately but at present there is no universal method to directly measure the temperature. As a consequence, the specific heat C = dE/dT is not accurately determined and this is a key unknown aspect of the equation of state (EOS). There is also a class of experiments, which aims to study phase transitions: these may be high-temperature evaporation of refractory metals, a liquid-vapor critical point, or an abrupt change of electronic structure (metal-insulator transition). Such transitions typically occur at definite temperatures and an accurate measurement of the transition temperature is obviously a key step for building up the quantitative science of warm dense matter. Today the critical points of refractory metals are uncertain to something like 50 % and this uncertainty is a leading source of EOS uncertainty for WDM expanded below solid density. At higher energy-density (HEDP), shock Hugoniot measurements can accurately determine for the material density, energy-density and pressure, but different EOS models predict different temperatures for the same (energy/pressure/density) shocked conditions, so again temperature is a key uncertainty for HEDP EOS data. Finally, almost all the computer codes used to predict the properties of warm dense matter (or hotter plasmas) use density and temperature as inputs and for this reason it will be easier to directly compare theory and experiment if one can accurately measure the temperatures attained in any given experiment.