Effective K. cap alpha. x-ray excitation rates for plasma impurity measurements

Metal impurity concentrations are measured by the Pulse-Height-Analyzer (PHA) diagnostic from K..cap alpha.. x-ray peak intensities by use of an averaged excitation rate <sigmav>. 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, <sigmav> is a weighted sum of rates; coronal equilibrium is usually assumed. The <sigmav> used earlier omitted the intercombination and forbidden lines from the dominant helium-like state. The result was an overestimate of metals and an underestimate of low-Z impurities in cases where metals were significant. Improved values of <sigmav> 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 VUV spectroscopy