## 4 Conclusions  
The simulation of diagnostics is a fundamental step in the design process of tokamaks, in order to ensure that the best performances are obtained, and that the degradation due to electromechanical, thermal and nuclear effects is the most limited as possible. These simulations require the use of multiphysics models which can take into account all the effects of degradation and misalignment.  
In this work, the modelling of interferometry, polarimetry and Thomson scattering effects have been introduced and implemented on the ray optics module of COMSOL multiphysics. COMSOL was chosen since it allows a very easy interface of the various modules (thermal, mechanics, electromagnetic, etc.), reducing the efforts required to run a multiphysics simulation. However, all the models can be implemented on other platforms.  
The validation of the models has been performed by comparing the expected value with the simulated one. All the effects implemented in the model have been tested in an ITER-like geometry, using a simple circular plasma model (which does not affect the functioning and the validation of the model) with properties comparable with the ones expected in ITER. In the case of the interferometry, three lines of sight have been used. The phase differences between the probing beams and the reference beams, and thus the line integrated electron densities, were calculated. They were compared with the line integral of the electron density of each line of sight, and it was shown that the data are overlapped. In the case of plasma polarimetry, the same probing beams of the interferometer were used, and the simulated Faraday rotation and Cotton-Mouton phase shift were calculated. Then, the line integral density was been calculated using the type 1 approximation of the Cotton-Mouton phase shift. This value was been compared with both the expected and the interferometer quantities, showing perfect accordance and also validating the polarimetric model. The spectral Thomson scattering, since it provides local information, was validated by just comparing simulated spectrum with the expected one, calculated from the local values, while the polarimetric TS was validated calculating the electron temperature from the simulated depolarisation of the scattered radiation and by comparing it with the temperature of the scattering volumes.  
This work aimed to analyse the possibility to accurately simulate the optical laser-based diagnostics of thermonuclear plasma and to describe how the model can be implemented in a ray optics code. The validation section clearly shows that the phenomena can be simulated with no errors (between the expected or imposed and the simulated values). In the future, these models may be used to design, test and evaluate the performances of nuclear reactor diagnostics in several conditions, helping in understanding the best setup of the instruments to ensure the best performances and their longest lifetime.  
Figure 9: The first component of the reduced scattered Stokes vector as a function of the electron temperature on the four scattering volumes (**left**) and expected vs. simulated electron temperature for each channel at each numerical simulation (**right**).  
**Author Contributions:** Conceptualization, R.R. and P.G.; methodology, L.S., I.W. and R.R.; software, L.S. and I.W.; validation, L.S. and I.W.; investigation, L.S., I.W. and R.R.; writing--original draft preparation, R.R.; writing--review and editing, L.S., I.W. and P.G.; supervision, R.R. and P.G.; project administration, P.G.; funding acquisition, P.G. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Data sharing not applicable.  
**Conflicts of Interest:** The authors declare no conflict of interest.