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F. Delahaye, A. K. Pradhan (The Ohio State University)
Helium-like ions provide the most important spectral diagnostics in high temperature fusion and astrophysical plasmas. The primary lines lie in the X-ray region and are mainly excited by electron impact from the ground level 1s2(1S0) to the n = 2 levels 1s2s (3S1, 1S0), 1s2p (3Po0,1,2, 1Po1). However, at high temperatures n > 2 levels are also excited.
New calculations for electron impact excitation of He-like ions of C, N, O, Ne, Mg, Si, S, Ca, Fe and Ni are carried out using the Breit-Pauli R-Matrix method up to the n = 4 levels and including the effect of radiative decay of resonances in the collision strengths.
The target ion is represented by an eigenfunction expansion of 31 fine-structure levels from the spectroscopic configurations 1s2, 1sn \ell, n \leq 4 and \ell \leq n-1, and a set of correlation configurations. The forbidden M1 transition up to the metastable 23S_1 level is enhanced most by resonances effects.
The present rate coefficients, and new data for transition probabilities of He-like ions, will be used in a radiative-collisional model to calculate X-ray emission spectra. Line intensity ratios are computed for diagnostics of temperature and density of laboratory and astrophysical sources.
Partial support is provided by NASA and the CNRS (Observatoire de Meudon), and the computational work is carried out at the Ohio Supercomputer Center.
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.