Statistical characteristics of Thomson scattering data

Abstract

Thomson scattering is used to measure the electron temperature of plasmas. It is essentially a counting experiment. The distribution of detected photons (photoelectrons) in a number of channels yields an estimate of the scattered light spectrum and, thereby, of the plasma electron temperature and density. Repeated measurements at fixed plasma parameters yield a range of values of the measured electron temperature. The spread in these values is typically broader toward high temperatures and the peak (mode) of the temperature density function is lower than the nominal plasma temperature, while the mean is higher. This spread is intrinsic to these measurements, as actual data samples and simulations show. Here a two-channel analytic model of the distribution of temperature readings from non-relativistic, thermal plasmas is used to obtain an intuitive picture of the significance of data from this means of measurement. The model is applied to data from three experiments with electron densities ranging from 5⋅ 1017 m−3 to 7⋅ 1022 m−3, with similar temperatures in widely different profiles of the probability density function. The model is consistent with observed scattered and background light and can, for example, help identify artifacts of diagnostic response, such as apparent high temperatures, and, supplemented by data from additional detector channels, deviations from a thermal electron population, electron drift, and relativistic effects. This is a chance to look in a modern way at some metrological features of this well-established measurement technique.