Theory of excitation of He-like and Li-like atomic sublevels by directive electrons: application to X-ray line polarisation

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Abstract
The presence of an axially symmetric angular distribution of energetic electrons in hot plasmas can be detected by measuring the linear polarisation of the X-ray lines emitted from collisionally excited levels of multiply charged ions. The authors have evaluated the degree of linear polarisation of Fe24+ resonance lines and Fe23+ inner-shell excitation satellite lines observed in soft X-ray emission of solar flares and tokamaks. Collision strengths for exciting upper line magnetic sublevels were calculated using a distorted-wave method for the electron impact energy range up to 20 times the excitation threshold. It is shown how a plasma diagnostics of these non-Maxwellian electrons can be derived in a practical case.