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Nuclear fusion is a promising candidate to supply energy for future generations. At the high temperatures needed for the nuclei to fuse, ions and electrons are no longer bound into atoms. Magnetic fields confine the resulting plasma. One of the heating methods is the ion cyclotron resonant absorption of waves emitted by an external Ion Cyclotron Radio Frequency (ICRF) antenna. The efficiency of ICRF heating is strongly affected by rectified RF electric fields at antenna and other in-vessel components (so-called ‘sheath effects’). The chapter presents an overview of ICRF principles. Attention is given to characterising the detrimental sheath effects through experiments on a dedicated test facility (IShTAR: Ion cyclotron Sheath Test ARrangement). IShTAR has a linear magnetic configuration and is equipped with an independent helicon plasma source. The configuration and capabilities of the test-bed and its diagnostics are described, as well as an analysis of the plasmas.

Experiments on several tokamaks convincingly demonstrated that a deliberate seeding of selected impurities can have a positive effect on the plasma performance. On the one hand, a significant reduction of the head load on divertor plates, one of the main concerns by constructing a thermonuclear reactor, has been achieved due to the increase of edge radiation. On the other hand, in some devices impurity seeding has led to an improvement of the energy confinement and the so called radiation improved (RI) mode has been established with the same or even better confinement than in the H-mode. However, in order to make use of these positive impacts, the behaviour of seeded impurity has to be strictly controlled and such negative developments as the accumulation of impurity ions in the plasma core accompanied by a strong increase of the central radiation losses should be avoided.Plasma heating by radio-frequency (RF) waves has been proven to be a useful tool to control the behaviour of puffed impurities. In order to asses the prospective of impurity control by RF waves in larger devices and under reactor conditions, proper modelling approaches have been developed. One of the important parameters, which should be evaluated, is the averaged energy or temperature of heated impurity ions. The latter determines, in particular, the power transported to the main species, and, thus, the heating efficiency. Besides, the temperature of impurity ions characterizes the intensity of particle losses for heated impurities. An approach to compute the impurity temperature under such conditions is elaborated. It is based on the construction of a hierarchy of approximate solutions to the impurity heat balance equation and takes into account that the density and, thus, the heat conductivity of heated ion species can change by many orders of magnitude with the position in the plasma. The developed method has been incorporated into 1D transport code RITM. Coupled with the full wave code TORIC, the particle and heat balances for impurity and main plasma species provide a self-consistent approach to model the ion cyclotron resonance heating (ICRH) scenario. The modelling of various heating scenarios for several tokamaks displays the impacts of impurity heating on the heat and particles transport and heating efficiency. To investigate the possibility of impurity control at the large tokamak the experiment on selective impurity heating in the mode conversion H/D plasma was prepared and carried out in the tokamak JET., Doctorat en sciences, Spécialisation physique, info:eu-repo/semantics/nonPublished