Abstract
Since the 1940s, Microwave Cavity Resonance Spectroscopy (MCRS) has been used to investigate a variety of solids, gases, and low-pressure plasmas. Recently, the working terrain of the diagnostic method has been expanded with atmospheric-pressure plasmas. This review discusses the advancements that were required for this transition and implications of studying highly collisional, with respect to the probing frequencies, plasmas. These developments and implications call for a redefinition of the limitations of MCRS, which also impact studies of low-pressure plasmas using the diagnostic method. Moreover, a large collection of recommendations concerning the approach and its potential for future studies is presented.
Highlights
The high collisionality of the electrons in atmospheric-pressure plasmas suppresses the change in the real part of the permittivity, which is responsible for the shift in f res
A step forward would be to perform Microwave Cavity Resonance Spectroscopy (MCRS) measurements using multiple cavities in parallel in one solid block of metal. An example of this approach would be one cavity—preferably in vacuum conditions—could be used to monitor the conductivity and the dimensions of the metal structure surrounding the cavity, another without a discharge to compensate for the properties of the gas and a third to monitor the discharge; Section 3.5.2 demonstrated that not all higher-order modes are sensitive enough—when the plasma is placed at a node—to probe the electron dynamics of an atmospheric-pressure plasma jet
A more detailed description is given of the temperature-corrected apparent frequencies that have been introduced in [19]; For plasmas where the collision frequency of the electrons could not be neglected with respect to the applied angular microwave frequencies and have a spatial dependency, the interpretation of the measurement data and the obtained ne requires special care
Summary
Since the introduction of Microwave Cavity Resonance Spectroscopy (MCRS) as a plasma diagnostic in the 1940s [1], the technique has been solidified by a series of publications [2,3,4,5]. The changes in the position as well as the width of a resonance peak were used to separate and determine the individual contributions This ‘atmospheric-pressure’ avenue was continued by exploring the full temporal evolution of electron dynamics in the spatial afterglow of an RF-driven atmospheric-pressure plasma jet [19]. Ωpe denotes the electron plasma frequency—which depends on the electron density—and ω the angular frequency of the applied microwave field This limit, the shift of the resonance frequency no longer scales linearly with the increase in the electron density and translating measurements into tangible physical parameters becomes difficult. The main messages of this article are related to the required improvement of the resolution, contributors to changes in resonant behavior, inhomogenous plasmas, determination of the effective plasma volume, lower limit for the permittivity, and the skin depth of the plasma.
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