Analysis of impurity effects on X-ray measurements in the PFRC-2*

In the Princeton Field-Reversed Configuration-2 (PFRC-2) plasma experiment, X-ray line radiation vs. Bremsstrahlung measurements are of interest in order to measure electron energy distribution functions (EEDFs) from which electron number density n_e and temperature T_e can be extracted. Three Silicon Drift Detector (SDD) X-ray pulse-height detectors in the center cell energy-analyze the X-ray radiation emissions above 50 eV from three viewing chords, where two are near the central (z=0) plane and the third is near the nozzle. There is a coupling between the origins of the Bremsstrahlung and line radiation spectra: the energy dependence of the EEDF impacts the line radiation, while the effective ionic charge of the plasma (Z_eff) impacts the EEDF that is extracted from the measured Bremsstrahlung. In our experiments with liquid-nitrogen-cooled flux conservers, impurities have been shown to increase the measured Bremsstrahlung by an order of magnitude. We have incorporated into our EEDF inversion algorithm the relevant cross-sections to model X-ray line spectra production from plasma impurities, which can allow us to extract n_e without reference to Z_eff. Additionally, by including models of Bremsstrahlung from high-Z atoms and calibration data from an X-ray gas target tube, we can improve our EEDF fits to the measured spectra and hence obtain more accurate measurements of T_e. We describe X-ray measurements of PFRC-2 plasma along with tests of our new model towards improving our measurements of n_e and T_e.