Effect of the coupling of modes on the accessibility of ''low hybrid'' waves in a plasma

It is now agreed that ohmic heating is insufficient for reaching ignition in fusion machines (for example, the TOKAMAKS). Very fast extra heating is required, i.e. quickly giving energy to the plasma particles. Two methods have been suggested for this: the injection of very energetic neutral particles or use of radio-frequency fields. Where the latter method is concerned, various frequency fields have been investigated: the cyclotron frequency of electrons (ECH), low hybrid frequency (LHH), cyclotron frequency of ions (ICH) and very low frequencies (TTMP, ALFVEN waves). This thesis aims to study certain problems raised by low hybrid heating. An accurate study is made of the propagation of the two hybrid modes in the plasma, the slow mode and the fast mode. The coupling phenomenon and its effects will be seen to appear. The results obtained will be used to describe the particular case where the radio-frequency source is a set of two guides mounted in phase opposition. (The experiments have shown that the optimum coupling conditions between the ''grid'' and the plasma are given by two guides in phase opposition or four monted with a π phase shift between two adjacent guides). This work affords a better understanding of propagation phenomena in the low hybrid field. It confirms the hypotheses made on surface waves and it shows the existence of an energy flux corresponding to a fast wave. In certain cases, this could give an interpretation for a part of the unrecovered energy, since the fast wave possesses too great a phase velocity to be able to release its enegy in the plasma in a short enough time to be efficient