ABSTRACT

Measurements of plasma parameters in high-pressure plasma environment offer challenges and opportunities which usually have to satisfy requirements that are different compared to both partially ionized and highly ionized lowpressure plasmas. The highly collisional nature of atmospheric pressure plasma, compared to lower «lOtorr) pressure plasmas, can significantly modify the data analysis procedure and, more importantly, sometimes even the applicability of methods used to measure plasma characteristics in diffusion-dominated lower pressure plasmas. Also, the scaling laws of collision ally dominated self-sustained plasmas are usually bounded by ionization and thermal instabilities, which impose different operating requirements for maintaining self-sustained non-equilibrium plasmas at atmospheric pressure compared to low-pressure plasmas. Well developed low-pressure plasma diagnostics methods for both partially ionized (Auciello and Flamm 1989, Herman 1996) and highly ionized plasmas (Fonck and den Hartog 2002, Hutchinson 2002) can be adopted for collisionally dominated plasmas. The examples of applicability of electron density measurement by millimeter wave and mid infrared interferometric methods, with appropriate modifications for collisionally dominated plasmas, are discussed in this chapter in sections 8.3 and 8.4, respectively. Also, elastic and inelastic laser light diagnostic methods which are better suited for characterizing plasmas at elevated gas density are described in section 8.2. In section 8.2, both theoretical and experimental descriptions of Rayleigh scattering, pure rotational and ro-vibrational Raman scattering and Thomson scattering measurements in air plasma are described.