PLASMA POLARIZATION SPECTROSCOPY

In the density matrix formalism, the ensemble of atoms or ions is expr essed in terms of population and alignment of excited levels and the g round state. The population of an excited level determines the intensi ty of an emission line originating from this level and the alignment g ives rise to the linear polarization of this line. Spatially anisotrop ic collisions induce transitions between the populations and the align ments; for example, an alignment along with a population is produced b y a beam of electrons in an excited level from the ground-state popula tion. Thus, an anisotropic plasma produces polarized radiations, and b y plasma polarization spectroscopy (PPS) we can investigate the anisot ropic, and thus non-thermal, features of the plasma. The angular distr ibution of the perturber velocities is expanded in terms of Legendre p olynomials. The expansion coefficients and the relevant cross sections for the transitions among the populations and the alignments yield th e rate coefficients for these transitions. For the ensemble of atoms o r ions immersed in the plasma, coupled rate equations are constructed for the populations and the alignments of excited levels. We review th e present status of our knowledge of the collision cross sections for various collision processes. Several experimental observations of PPS performed so far are reviewed, which include those on a tokamak plasma , laser produced plasmas, various discharge plasmas and solar Bares. W e discuss what kind of information could be obtained from PPS.