  The understanding of the behaviour of impurities is a critical issue in
tokamak physics. The ion effective charge Zeff provides a measure for impurity
concentration. On the TEXTOR tokamak (Julich, Germany), we run a diagnostic to
determine Zeff from the bremsstrahlung emissivity E. From radial profiles of E,
electron density ne and temperature Te, profiles for Zeff can be reconstructed.
However, their interpretation is difficult outside the plasma centre, because
of various uncertainties in E, ne and Te at the edge, which render the radial
matching of the different profiles problematic. Conversely, if it were possible
to obtain a set of line-integrated values for Zeff directly from the
line-integrated measurements of E, ne and Te, then these problems would be
avoided. Now, recent advances in the field of statistical signal processing
allow the extraction of an unknown signal from a signal mixture. In particular,
we describe a procedure for the single-channel Bayesian source separation of a
line-integrated Zeff signal from a line-integrated emissivity source, using as
a forward model a linearized version of the known functional dependence of Zeff
on E, ne and Te. Here, a line-integral over a traditionally obtained Zeff
profile may serve as a prior for the line-integrated Zeff signal. In this way,
precise information on the electron density and temperature may even become
superfluous for the determination of Zeff.
