## 1 Introduction  
Plasma diagnostics is one of the most important research fields in nuclear fusion, since they are the inputs of the control system, which has the scope to control the confinement stability of the plasma, and they are fundamental to understand and analyse the plasma physics and equilibrium [1; 2; 3; 4; 5]. Contact measurements are prohibited for core plasma investigations, because they are invasive, their life would be too short, and their interactions with plasma would produce impurities, causing off-normal events, such as disruptions. Magnetic diagnostics provide only boundary or integrated information, causing a lack of knowledge of the interior of the plasma, making optical diagnostics one of the best methods to probe the plasma interior [6; 7]. For example, bolometers are widely used to measure the total emission of the plasma [8]; fundamental for the power balance, and, by tomographic inversion methods, they are able to evaluate the radiation pattern on the poloidal plane [9; 10; 11]. Between the optical diagnostics, the laser-based ones found many applications, where the most valuable are interferometry, polarimetry and Thomson scattering (TS) diagnostics [12]. The interferometer, measuring the phase difference between the probing beam and the reference beam, is used to measure the line-integrated electron density along the line of sight. It is considered to be one of the most important plasma diagnostics [13; 14]. For example, the Joint European Torus (JET) mounts four vertical and four horizontal interferometric lines of sight, which are used to probe the electron density of the plasma and control the plasma discharge. Plasma polarimetry is based on the birefringent characteristic of magnetised plasmas, which involves two components of the electric field of the probing beam propagate differently, involving a changing of the polarisation of the beam. Measuring the polarisation variation effects (Faraday rotation and Cotton-Mouton phase shift) of the beam, it is possible to obtain information about the line integrated electron density and magnetic field [15; 16; 17]. Moreover, the polarimeter and the interferometer can be performed on the same probing beam, as done at JET, making the interferometer-polarimeter a unique, reliable, and informative diagnostic. At ITER, it is expected to have different polarimeter-interferometer diagnostics, both toroidal and poloidal, and both for the study and the control of the plasma. The Thomson scattering analyses the scattering radiation of a probing beam. The scattered radiation intensity is directly proportional to the local electron density, while the scattered spectrum is dependent on the local electron temperature, because of the Doppler effect which occurs due to the interaction of the electromagnetic radiation with the moving electrons [18; 19]. The design of these diagnostics is a crucial point of the building process of a tokamak since several factors must be considered. At first, since the interaction between the probing radiation and the plasma are strongly influenced by the plasma properties, it is important to simulate the effects at different plasma configuration and find the best setup which is optimised for all scenarios. For example, the laser wavelength of the polarimeter at JET is 195 \(\upmu\)m, which is unusable for ITER or DEMO plasma scenarios [20; 21; 22]. Moreover, in the present tokamaks, the environment is not critical in terms of neutron and heat fluxes, as well as electromagnetic forces, while in the future tokamaks, these loads will be much higher and they will play an important role on the design of the diagnostics, since they may involve the degradation of the performances. The high neutron flux may change the optical properties of the mirrors, windows, and other optics, while the heat fluxes and the electromagnetic forces may vary the alignment, involving a decrease of the performances of the diagnostics [23; 24; 25].