Determination of Fe charge-state distributions in the Princeton large torus by Bragg crystal x-ray spectroscopy

A curved-crystal Bragg x-ray spectrometer has been used to measure Kα or 1s-2p radiation from highly stripped Fexviii-Fexxv impurity ions in the Princeton large torus tokamak. The spectrometer has sufficient energy resolution ({}4 eV at 6400 eV) to distinguish between the different ionization states of iron by measuring the energy shift of the Kα x rays. The measured wavelengths agree well with theory and with spectra from solar flares and from laser-produced plasmas. The distribution of Fe charge states in the center of the discharge has been inferred from a comparison of the measured x-ray spectrum with theory. The shape of the spectrum depends strongly on electron temperature (Tₑ) in the range Tₑ=800-1500 eV. Within the factor of 2 uncertainty in L-shell ionization cross sections, measured intensities agree with theory, which is based on coronal equilibrium, indicating that the ion lifetime in the center of the plasma is approximately equal to or greater than the equilibration time.