# Probing Collisional Plasmas with MCRS: Opportunities and Challenges
## 5 Conclusions  
In this review article, a manifesto is presented for probing collisional plasmas with Microwave Cavity Resonance Spectroscopy. The discussed topics might be able to help to further develop the diagnostic method and explain unexpected results. The main messages of this article can be summarized as follows:  
1. The resolution in the resonant behavior--and therefore in the permittivity--is improved by several advancements in the measurement approach. Based on an experimental study, the dependency of this resolution on the number of averages and the size of the frequency sweep step is explained and translated to the optimal parameters with respect to the needed measurement time and resolution. Additionally, the influence of the quality factor on the resolution is explained;
2. As smaller changes in the resonant behavior can be resolved due to recent improvements of the method, the electron dynamics are no longer the only contributor to changes in the permittivity during plasma experiments. The improvements to the diagnostic method enabled measuring--non-selectively--a new range of physical properties, e.g., gas composition, gas pressure, and the number density of metastables, etc. For this reason, we recommend thinking of permittivity as the main parameter when analyzing MCRS measurements rather than \(n_{\text{e}}\) and \(\
u_{\text{m}}\). Furthermore, a more detailed description is given of the temperature-corrected apparent frequencies that have been introduced in [19];
3. For plasmas where the collision frequency of the electrons could not be neglected with respect to the applied angular microwave frequencies and also have a spatial dependency, the interpretation of the measurement data and the obtained \(n_{\text{e}}\) requires special care. The determination of the collision frequency of the electrons in a plasma with a spatial profile is very susceptible to errors of multiple orders of magnitude;
4. In the two MCRS studies of atmospheric-pressure discharges [18; 19], a camera was used to determine the volume which is occupied by the plasma. This volume together with the calculated resonant field profile is used to estimate the effective volume ratio \(\mathcal{V}\). The uncertainty in the volume occupied by the plasma, and subsequently in \(\mathcal{V}\), limits the accuracy of the determination of the electron density to its order of magnitude [18]. A set of improvements concerning this topic is proposed;
5. The linearity of the shift in resonance frequency with the change of permittivity--or more traditionally the electron density--is investigated for a wide range of plasma parameters such as geometric dimensions, electron-neutral collision frequencies, and electron densities. Moreover, the correlation between the radius and the position of the plasma column with the mode structure is explored in relation to the lower limit of the permittivity or in archic terms the upper limit of the electron density. In all investigated conditions, the putative limit \(\omega_{\text{pe}}\ll\omega\) was too stringent;
6. Penetration of the microwaves into the plasma is crucial for successful MCRS measurements. For this reason, the skin depth is investigated for a variety of plasma parameters. Moreover, the calculated skin depths are compared with the results obtained in the previous section. This comparison showed that the limit \(r_{\text{p}}\ll\delta\) is in some cases to pessimistic in limiting the operational regime of MCRS. Although there are still significant challenges for users of the diagnostic method, the advancements of the last few years have brought many new opportunities. **Author Contributions:** Conceptualization, B.P. ; Methodology, B.P. ; Software, B.P. and P.K. ; Validation, B.P. ; Formal analysis, B.P. ; Investigation, B.P. and T.S. ; Data curation, M.v.d.S. ; Writing--original draft preparation, B.P. ; Writing--review and editing, T.S., P.K., M.v.d.S., J.B., and W.I. ; Supervision, J.B. and W.I. All authors have read and agreed to the published version of the manuscript.