Some Basic Physics Results for Plasma Spectroscopy and Modelling

Abstract

Spectroscopic diagnostics and modelling of technical or divertor plasmas require many results from basic physics research. Reasonable simplifications must be introduced and information is necessary about atomic and molecular processes in the plasma volume and at the walls. As a first approximation, the corona model is presented, which is applicable for excitation and ionisation at low densities. This model requires cross sections and rate coefficients for electron impact. Many of these can be found in the literature, but it is also shown, how they can be estimated from the optical f-values. Consideration of competing processes is often required, as e.g. dissociative excitation, excitation transfer, charge exchange or quenching. Some examples are reported. At higher electron densities, collisional-radiative models must be used, which are available for electron impact collisions. Redistribution by neutral particles can present a major problem in low density discharges and many questions are still open. Heavy particle reaction rates are published in the literature, but must sometimes be taken with caution. The optical thickness of resonance lines is discussed, which may considerably affect line emission also in the visible spectral range. Models and programs are described for taking into account opacity and the results are shown. In addition to reaction cross sections, the electron energy distribution function must be known, which is often non-Maxwellian. It is demonstrated, how deviations can be recognised in spectroscopic criteria with different energy thresholds and how the high-energy tail may be reconstructed for further use in diagnostics or modelling. Eventually, a self-consistent model of these plasmas is highly desirable calculating electrical and magnetic fields, all chemical reaction rates and the consequent distribution functions.