XSNQ-U: a non-LTE emission and absorption coefficient subroutine

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Abstract
This report describes the non-LTE atomic-state computer program, XSNQ-U. The original classified report was issued in 1971. Since then, important changes and improvements have been included. XSNQ-U provides frequency-dependent emission and absorption coefficients for a material not in LTE (local thermodynamic equilibrium). As in XSNB, an LTE opacity subroutine, a compromise was sought between accuracy and computer speed. The result, XSNQ-U, is intended for use as a subroutine in any transport code. This report surveys the basic non-LTE equations for the average ion model, as pioneered by Grasberger (UCRL-12408 and -5135), and gives details for the approximations and technique used in XSNQ-U. Also included are some illustrative numerical examples. Since the writing of the classified report, the code has been modified to handle the case of multiple elements. The extension is straight forward; the notation of this report would only be more cumbersome if the single-element notation were replaced by the multiple-element notation. Also, the method of solving the rate equations by using analytical differentials as discussed in the classified report was replaced by a more direct incremental differences scheme, which is discussed in the present version. 12 figures, 2 tables.

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