# Probing Collisional Plasmas with MCRS: Opportunities and Challenges
## 4 Recommendations
The usage of these type of modes can be cumbersome because the microwave field in a cavity tends to alternate over time between these modes. Furthermore, the fitting becomes more complex when resonance peaks (almost) overlap. For these reasons, degenerate modes should be avoided or all but one excluded in the design of a cavity;
* Integrating the antenna with (a part of) the cavity walls would result in a smaller perturbation from the ideal resonant field. Hence, this would result in more accurate usage of calculated electromagnetic field profiles;
* Manufacturing a stable cavity proved to be difficult. Vacuum soldering of the parts to create a metal structure--which forms the cavity--from one solid metal block showed the best approach. This approach with a solid cavity and correcting the response for temperature fluctuations is completely contradictory to the inventions in which the response of the cavity is temperature-stabilized by moving parts [63; 64; 65];
* The accuracy in determining the quality factor is lower than that of the resonance frequency. For this reason, it might be beneficial to use the change in the resonance frequency of multiple modes to separate the contributions of the electron density and the collision frequency to changes in resonant behavior. This approach requires the cavity to be filled homogenously with plasma and large differences between the resonance frequencies of the modes. Other methods to measure \(Q\)[2] potentially provide a higher resolution than the approach used in our prior works. Some practical advice with a strong focus on the experimental setup and method used in references [18; 19; 20]. * The current approach--with frequency-resolved measurements--requires many reproducible discharges. A more direct scheme with good temporal resolution would be a huge step forward. Examples of approaches that require less complicated analysis, although with a poor temporal resolution, are described in the following articles [7; 41; 67]. * In all MCRS experiments performed by the authors, an equidistant frequency domain is used. The spectral positions of the probing frequencies can be optimized by allowing non-equidistant positions between two subsequent positions in the frequency domain. This will reduce the measurement time or increase the resolution in permittivity;
* Commercial network analyzers provide a calibration procedure to compensate for non-ideal properties of the load. The setup used here does not have such a function and therefore undesired oscillations are present in the spectral response. An example of a spectrum with these oscillations can be seen in Figure 2 of reference [18]. These undesired features increase the error in the results and can be troublesome for the user. Users would benefit from similar functionality as available in network analyzers;* In the experiments performed by the authors, only cylindrical cavities were used. Cuboid cavities do not suffer from rotation of the resonant field of higher-order azimuthal modes due to e.g., perturbations by the plasma and could help to increase the accuracy of the results;
* Tuning of the antenna in a cavity is a very important step to obtain good results. Positioning the antenna with a micrometre screw gauge would be a good improvement to the current design of the cavities.