Effective Kα x-ray excitation rates for plasma impurity measurements

Abstract

Metal impurity concentrations are measured by the pulse-height-analyzer (PHA) diagnostic from Kα x-ray peak intensities by use of an averaged excitation rate 〈σv〉. Low-Z impurity concentrations are inferred from the continuum enhancement (relative to a pure plasma) minus the enhancement due to metals. Since the PHA does not resolve lines from different charge states, 〈σv〉 is a weighted sum of rates; coronal equilibrium is usually assumed. The 〈σv〉 used earlier omitted the intercombination and forbidden lines from the dominant heliumlike state. The result was an overestimate of metals and an underestimate of low-Z impurities in cases where metals were significant. Improved values of 〈σv〉 using recent calculations for H-, He-, and Li-like Fe range from 10% to 50% larger than the earlier rates and yield metal concentrations in better agreement with those from vacuum ultraviolet (VUV) spectroscopy.