Description
A probabilistic formulation and a generalised version of the two-foil method for estimating electron temperature from soft X-ray radiation are presented, motivated by applications to the COMPASS-U tokamak. Using synthetic data representative of the planned diagnostic configuration, the performance of the original method is benchmarked and the impact of tungsten line radiation on accuracy is investigated. The results show that neglecting tungsten line radiation, especially for high performance scenarios, can lead to a substantial underestimation of the electron temperature, highlighting a key limitation of the standard approach.
To address this issue, a generalised two-foil method is proposed that explicitly incorporates line radiation from plasma impurities into the probabilistic model. This formulation relates the ratio of emissivities, measured by two detectors with different filters, to the electron temperature, electron density, effective charge, and tungsten density. The method is demonstrated using simulations of COMPASS-U plasma scenarios with a synthetic model of a planned soft X-ray diagnostic system capable of tomographic inversions. Benchmarking is performed under different a priori assumptions on plasma parameters, considering both the standalone method and cases where additional constraints from auxiliary diagnostics are included. Three distinct regimes are identified in the plasma parameter space in which the diagnostic sensitivity is dominated by the electron temperature, by the impurity content, or by both simultaneously.