# Probing Collisional Plasmas with MCRS: Opportunities and Challenges
## 4 Recommendations  
With the lessons learned from the previous sections, the authors would like to hint towards the following potential avenues for future research. * Investigating the laser-droplet interaction in extreme ultraviolet light sources [56] during the laser pre-pulse impact, main pulse impact, and temporal plasma afterglow phase using MCRS could contribute to improved insight in the dynamics under these complex and highly interesting conditions;
* Experimenting on tunable monochromatic ionizing radiation--e.g., from a free-electron laser [57; 58; 59; 60]--makes the interpretation of measurement data easier and could, for example, be used to gain a fundamental understanding of photoionization processes and improved design of an ionizing beam monitor based on MCRS [14; 49];
* Developing a multiple-frequency microwave interferometer could enable the investigation of plasmas, which do not allow for the presence of a cavity as is the case in the experiment described in, e.g., reference [61]. Advantageous of such a system is that the permittivity is different for each frequency and a fit function can be used to improve the accuracy of the results. This approach would be additionally accurate when frequencies well below and above the electron collision frequency are used;
* The background ionization plays an important role in the ignition of a plasma [62]. This property is very difficult to measure directly and the required resolution of MCRS is now within reach thanks to recent developments. Two MCRS-based approaches to investigate the background ionization level are proposed below:
* The first approach requires a change in the collision frequency without a change in the electron density and the collision frequency of one of the states, which can be obtained by e.g., calculations based on cross-section data. As the expected changes in resonant behavior are only \(\Delta f/f_{1}\sim-10^{-9}\) and \(\Delta(1/Q)\sim 10^{-10}\) (for \(\
u_{1}=50\) GHz, \(\
u_{2}=1\) THz, \(n_{\rm e,1}=n_{\rm e,2}=10^{9}\) m\({}^{-3}\) and \(f_{\rm res}=3.5\) GHz), it will require a high sensitivity in the measurement approach;
* In the second approach, no restrictions to changes in the electron dynamics are required. The collision frequencies and electron densities need to be determined which requires the usage of at least two resonant modes or another diagnostic method to determine the collision frequencies. The polarizability of the medium depends on its temperature [17] and this complicates the execution of the proposed experiments. Other factors which need to be taken into account are the expansion of the cavity volume and the change in conductance of the walls due to temperature variations. Some general advice and ideas for (potential) users of the MCRS method are listed below. * As MCRS is sensitive to almost everything, it is crucial to separate the different contributions to the detected change in resonant behavior. A step forward would be to perform 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. However, these could be used to study plasma-produced acoustic waves. This is another approach to separate contributions to changes in resonant behavior;
* The measurement time or the number of cavities can be reduced by applying multiple frequencies simultaneously in one cavity and separating the responses by an electronic filter;
* Resonant modes with the same cut-off frequency are referred to as degenerate modes.