Measuring the electron energy distribution in tokamak plasmas from polarized electron cyclotron radiation

In this doctoral thesis, a promising technique has been used for the measurement of non-thermal electrons in TCV. The technique employs a Vertical viewing ECE to more easily discriminate the radiation according to the energy of the electrons. Successful measurements using the Vertical ECE were achieved, for the last time on a tokamak, several decades ago. That is due, among other reasons, to a major limitation that emerged from the early attempts in the $1980$s: that of the refraction of the line of sight of the ECE antenna in the plasma. The refraction, which increases with electron density, shifts the antenna's line of sight and allows the detection of the tokamak's background radiation via multiple wall reflections.The Vertical ECE antenna, designed and installed on TCV, produces a beam of maximum waist size $\sim 3$ cm, for measurements in the frequency range from $78$ to $148$ GHz.Early simulations, to determine the extent of the refraction issue on TCV, constrained the maximum operational density to $n_{\mathrm{e}} < 1\times 10 ^{19} \mathrm{m}^{-3}$. This result drastically reduced the operational window of the diagnostic until an innocent discovery came in to change the game. In reality, refraction causes trouble only if the background radiation originates within the plasma. As far as TCV is concerned, the background radiation originates essentially from the X$2$ component of the ECE. A suitable combination of magnetic field $($which varies on TCV from $0.9$ to $1.5$ T$)$ and measured frequencies allows to keep the origin of the background radiation away from the plasma, thus reducing, almost entirely, the level of background radiation. This observation, which unleashed the potential of the Vertical ECE on TCV, also made it possible to exploit the plasma itself, in approximately $70$ ohmic discharges, for the calibration of the diagnostic. The calibration is based on the calculation of the X$3$ radiation intensity under the conditions of low optical thickness, and is validated against the plasma black-body radiation.It is therefore with a calibrated diagnostic system, and a relaxed window of operation, that we have been able to measure the radiation from non-thermal electrons in some generated non-Maxwellian distributions. These measurements of X and O polarizations were achieved in ECCD and runaway electron scenarios at very high temporal resolution, in the order of $\sim 10 \mu s$.In ECCD scenarios, the great flexibility of the ECH power on TCV has been exploited, sometimes by varying the ECH launcher angles in full firing to allow increasingly energetic electrons to drive the current. For the runaways, the measurements were carried out during simple scenarios, at high plasma current $(I_{\mathrm{p}} \sim 200$ kA$)$ and densities below $1\times 10 ^{19 } \mathrm{m}^{-3}$.Particularly interesting were the measurements of runaways with MGI, or in the presence of ECCD. In agreement with other diagnostics, the Vertical ECE has allowed the observation of a reduction of the runaway emission intensity in the presence of ECCD. In this doctoral thesis, a clear identification of the emission from non-thermal electrons could thus be achieved and attempts were made to reconstruct their energy distribution. The reconstruction shows enhancements in the parallel or perpendicular directions for the different scenarios. Concerning the runaways, the reconstruction of the distribution allowed the observation of a flat