Collisional rate coefficients for the iron ions Fe(VIII), Fe(IX), and Fe(X)

Relative values of rate coefficients for excitation by electron collisions were determined experimentally for various transitions in Fe viii, Fe ix, and Fe x at two plasma conditions in a small theta-pinch. Collisional ionization rates were also obtained for these ions. The maximum errors of the relative values of the excitation rate coefficients are estimated to be plus-or-minus30% to plus-or-minus40% and those of the ionization rates to be a factor of 1.5. Iron pentacarbonyl Fe (CO)5 was used as an impurity in hydrogen or helium gases. The electron temperature and density were obtained from the analysis of scattered laser light. They were in the range 60--150 eV and 1015 to 2 times 1016 cm-3, although this range was not available for each ion. The excitation rate coefficients were calculated by the use of the Coulomb-Born approximation. Comparison with experiment is made within each ion and also between ionization stages. A discrepancy in previous Coulomb-Born calculations is noted for the case of Fe viii