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The performance of tokamaks is usually described in terms of plasma temperature, density and confinement time. The term temperature implies that the plasma is in thermal equilibrium and its particles have maxwellian (normal) velocity distribution. However, in several conditions, the plasma contains a significant number of suprathermal or 'fast' particles, whose energy is several times higher than thermal energy. The number of such particles can be significantly higher than that corresponding to the maxwellian distribution. Such electrons produced by different acceleration mechanisms in the TCV tokamak have been analyzed in the course of this thesis. This thesis consists of three parts. The first part describes the design, construction and implementation of a new 24-channel Electron Cyclotron Emission (ECE) radiometer, viewing the plasma from the Low Field Side (LFS) in the frequency range 65-100 GHz. The new LFS ECE radiometer, together with the existing 24-channel ECE radiometer viewing the plasma from the High Field Side (HFS) in the frequency range 78-114 GHz are the main diagnostics used in the analysis. The second part includes the development of a numerical code to simulate the ECE signals. The code is based on the WKB approach and solves the equation of radiation transport to calculate ECE signals using a numerically defined Electron Distribution Function (EDF). A method to reconstruct EDF from HFS and LFS ECE, based on the ECE code, is presented. The third part discusses the experimental data obtained on the TCV tokamak. In regimes with Electron Cyclotron Current Drive (ECCD), a quantitative explanation of the anomalously high absorption of the RF power at 118 GHz and a validation of the power balance analysis based on Thomson scattering measurements are provided. An analysis of the EDF during sawtooth instability shows that suprathermal electrons are generated in ohmic, Electron Cyclotron Heated (ECH) and ECCD plasmas. In ECH plasmas, typical energies and densities are found to be E = 15–30 keV and nst = 2·1017 m–3. This corresponds to 30-40 % of the energy lost during the crash, which proves that acceleration of electrons is an important mechanism for the dissipation of energy released during magnetic reconnection in a tokamak plasma. The observed fast electron dynamics suggests a high radial diffusion in the plasma during and just after the sawtooth crash (Dst = 25 m2/s), which dominates over the classical slowing down process. |
