Plasma spectroscopy

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Abstract
In this review the optical and soft X-ray radiation emitted from an ionized gas is discussed. This radiation depends, not only on the atomic properties of the isolated radiating species, but also on the properties of the plasma in the immediate environment of the radiator. This dependence on the plasma properties is almost exclusively a consequence of the fact that ions and electrons interact with other species via the long-range Coulomb potential. The fact that an emitting atom or ion in a plasma is never isolated from the perturbing effects of neighbouring ions and electrons is reflected in the details of the radiation in many ways. The long-range interactions with the charged components of the plasma control the populations of bound states, shift and broaden bound energy levels, lower the ionization potentials of atomic species and are the cause of continuum radiation emission and the emission of normally forbidden lines. The distribution of particles in the possible quantum states is considered in a plasma, for which collisional excitation and de-excitation processes are controlled by the free electrons. In local thermodynamic equilibrium (L.T.E.), which corresponds to a collision-dominated plasma, the population densities can in principle be well described, but lowering of the ionization potential and partition function cut-offs become important. Conditions for the validity of L.T.E. are given. In non-L.T.E. situations the detailed processes of population and de-population have to be considered and a detailed knowledge of cross sections and rate coefficients is required. Calculations may be performed using the collisional-radiative decay model (or the simpler coronal model); but even then, for non-hydrogenic species, metastable levels and dielectronic recombination should be considered. The details of the emitted radiation (line radiation and line profiles, recombination radiation and bremsstrahlung) and the possible effects of radiative transfer are considered. The theoretical assumptions are examined and stressed, and experiments are discussed only when they have a direct bearing on the theoretical predictions. In particular, a large section is devoted to line broadening theory. This is because it is in this field, especially through the generalized impact approximation for electrons, that great improvements in theory have been made in recent years. Since it is necessary to know the properties of plasmas, both in fusion research and in experiments to measure atomic parameters of highly excited and highly ionized species, methods for measuring number densities and temperatures are described. In particular, methods are discussed which can be used in high temperature plasmas which are not in L.T.E. Radiation losses due to impurities of high electronic charge are shown to be an extremely important energy loss mechanism for the hydrogen (deuterium) plasmas studied in controlled fusion research. Finally, some of the outstanding problems in plasma spectroscopy are mentioned.

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