Stochastic aspects in the theory of spectral-line broadening. II. Noncommutative cluster expansions

Generalizing ideas of von Waldenfels we develop a systematic procedure to define truncatedn-point operators which are reminiscent of Ursell functions of statistical mechanics. The truncation procedure is adapted to factorization relations obeyed by the operators in question. The results are applied to spectral-line broadening in plasmas. We derive cluster expansions for the line-shape function in terms of these truncated operators, where the ions are treated quasistatistically. The first order approximation for the line-shape function is discussed. The results are carried over to several moving perturber species, in particular to nonquasistatic ions.