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Plasma polarimetry is a diagnostic technique used in nuclear fusion reactors to measure the line integral of some plasma parameters, such as the electron density and the magnetic field, and constrain, analyse and validate the equilibrium models. Despite the strong link between the plasma properties and light polarisation propagation, the interpretation of plasma polarimetry remains complex and sometimes uncertain. The type 1 approximation is the most common hypothesis used to link the polarisation effects, such as the Faraday rotation and the Cotton Mouton phase shift, with the plasma properties (electron density and magnetic fields). However, this approximation is valid only in specific conditions, which depend on both the plasma configuration and initial polarisation of the electromagnetic wave. Moreover, the uncertainty propagations of these measurements are affected by regions where their values are too high to guarantee accurate values, making the measurement unsuitable. The need to measure an unsteady physical environment, which goes from the condition of no plasma to the flat top of the discharge, makes the setting of the measurement more constrained. The aim of this work is to analyse, both analytically and numerically, the behaviours of the interpretability and uncertainty propagation of polarisation measurements, in order to give a detailed and the most general as possible description of these issues, ensuring an easier, more performant and reliable understanding and design of plasma polarimetry. The results will show that an input linear polarisation around 45° degree is the most suitable for plasma polarimetry when type 1 approximation is adopted and which the choice of the laser wavelength governs the performances of the polarimeter.

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Tomography is applied daily to bolometric data in magnetically controlled nuclear fusion devices to infer important quantities of the studied plasmas, such as the emissivity profiles or the radiated power in different location of the main chamber. Tomographic reconstructions are also crucial to perform power balance analysis and to estimate heat fluxes for turbulence studies. One of the issues, associated to any tomographic technique, resides in the ill-posed nature of the mathematical problem, meaning that more than one possible emissivity profile is compatible with the measurements within the estimated uncertainties. Therefore, a careful evaluation of the quality of the obtained reconstructions is an important step of the analysis. On JET, since the two cameras are located in different toroidal locations, axial-symmetry of the plasma radiation must be assumed to perform traditional tomographic reconstructions. Such specific hypothesis, however, cannot be guaranteed in case of symmetry-breaking mechanisms, such as strong local gas puffing for fuelling. The present contribution addresses this problem of local plasma fuelling, focusing on the observed significant mismatch between measured and back-calculated projections in specific pulses, including the last JET DT campaign (DTE2) carried out in 2021. The analysis indicates that the observed discrepancy, due to the gas injected primarily from valves located in the same octant where one of the bolometers array is located, can be rectified acceptably well by a physically based correction. The sensible improvements of the bolometric Maximum Likelihood tomography’s outputs are also documented and the possible physical explanation for such phenomenon is discussed. The study performed in this article is expected to have a significant impact on the analysis of JET data and on the physical interpretation of the results obtained during the recent DTE2 experiments, for which a proper evaluation of the radiated power estimates is of not negligible relevance.