[en] The analysis of magnetic fluctuations in the JET and TCA tokamaks has been focused for this thesis on three subjects: i) magnetic turbulence and its correlation with the degradation of energy confinement ii) magnetic perturbation associated with the sawtooth iii) analysis of the magnetic coherent activity associated with stable locked-mode. The spatial structure of the turbulent magnetic activity has been investigated in JET and it is shown that the turbulent fluctuations are strongly correlated along the equilibrium magnetic field with k.B_o∼0. In TCA and JET, the level of magnetic fluctuations is observed to increase when the energy confinement time decreases. The analysis of a magnetic perturbation - the gong - appearing at the time of the sawtooth instability growth showed that the gong is in fact the magnetic signature of the instability itself. Different types of analysis are proposed which could improve our understanding of the sawtooth instability. The analysis of the coherent magnetic fluctuations allowed to deduce a toroidal plasma rotation and to estimate the radial ambipolar electric field. The results for TCA and JET are comparable with measurements performed in other tokamaks. It has been possible to show the existence of a new type of magnetic oscillations which is present during the stable phase of a locked-mode. This mode is resonant at the edge with n=1, m∼int(q(a)) and has been interpreted as an interchange mode destabilized by the new helical structure due to the locked-mode. (author) 93 figs., 4 tabs., 141 refs