Application of Excitation Cross-Section Measurements to Optical Plasma Diagnostics

Abstract

Abstract Natural and human-produced plasmas cover a vast parameter space with a rich array of properties that bear on science and engineering problems spanning many disciplines. A property common to all plasmas is their glow, in which information about plasma conditions is encoded. Photon emission, for many plasma conditions, results primarily from electron-impact excitation, and the emission spectrum thus contains embedded information about the distribution (often nonequilibrium) of electron energies in the plasma. With knowledge of the magnitudes and energy dependence of the optical emission cross sections for various emission lines, it is possible to use measurements of the plasma glow to extract the energy distribution of the plasma electrons. In this chapter we discuss the fundamental principles of electron-impact excitation of atoms (mainly illustrated with argon) and experimental results that are relevant to plasma applications. Plasma emission models which link the cross sections to the observable plasma glow measurements are described. With the aid of a radiation model that includes the excitation cross sections of ground-state and metastable argon atoms, the observed plasma spectra enable the characterization of nonequilibrium electron energy distributions. Individual features of the energy dependence of the cross sections for exciting the various radiating levels are exploited to develop diagnostics to address anomalous distributions of high-energy electrons (i.e., distributions with either an excess or reduced number of high-energy electrons) that can occur in different types of plasmas. Because electron collisions also govern other important plasma mechanisms, such as ionization and electron-driven chemistry, noninvasive optical diagnostic tools to measure energy distributions are of great interest.