Plasma Diagnostics, Optical Emission and Absorption Spectroscopy

Abstract

This chapter covers optical emission spectroscopy (OES) and absorption spectroscopy (ABS) which are non-intrusive, commonly used, plasma diagnostic techniques that rely on the observation of the plasma characteristics through its own emission or light absorption properties. The diagnostic tool in this case can be in a remote location relative to the plasma and does not have any influence on the plasma properties. These techniques, however, rely on the assumption that the plasma is in local thermodynamic equilibrium (LTE), with the electron temperature Te equal to that of heavy species Th. While this is generally valid in the core of atmospheric pressure discharges with electron densities ne > 1022 or 1023 (m−3), non-equilibrium conditions in which Te ≫ Th generally exist in plasmas generated at low pressures or in the boundary layers of thermal plasmas close to solid walls or electrodes. In spite of their limitation, these techniques have been among most valuable tool that played a key role for the study of the characteristics of thermal plasma sources and the development of their applications in a wide range of technologies. In this chapter, the basic concepts of OES are presented followed by a review of the principal methods used such as absolute line intensity measurement, intensity ratio techniques, the Boltzmann plot, intensity distribution of spectral lines, and electron density measurements using Stark broadening. One of the challenges meet in OES results from the fact that most spectrometric methods are based on side-on observation of the radiation emitted from a thin slice of the radiating plasma volume. Measurements are therefore inherently made of integrated radiation intensities for a specified wavelength or frequency interval along a particular line of sight. Consequently, the determination of the local distribution of the emission coefficients in the thermal plasma source requires the de-convolution of the measured integrated radiation intensities using standard Abel’s inversion for axially symmetric sources. In the absence of axial symmetry of the plasma source, computed tomography offers a viable alternative involving the use of multiple individual measurements made at different angles around a half-circle of the plasma source. In the last part of this chapter, a brief review is presented of absorption spectroscopic diagnostic techniques covering basic concepts and examples of measurements in different plasma source configurations.