In addition to the propagation and absorption of the waves into the plasma, a crucial point in ion cyclotron radiofrequency heating (ICRH) is to know how the energy is coupled to the plasma. The coupling efficiency is experimentally estimated through measurements of the antenna coupling resistance and it is of primary importance to precisely establish its dependence on the plasma parameters. The coupling resistance is highly connected to the density profile, which determines the cut-off positions of the magnetosonic waves for a given radiated wave number spectrum. For a large size tokamak like Tore Supra these cut-off densities range between 1018 and 1019 m−3, corresponding to the plasma edge. In this study, the RF coupling is investigated with respect to the plasma edge density profile measured by X-mode reflectometry and a characteristic cut-off position of the magnetosonic wave. We show that the edge density is not only a function of the average density but also depends in a complex way, on various edge plasma regimes, the power deposition, or the kind of limiter materials or divertor perturbation used to control the edge. Over a large variety of plasma edge regimes, it is shown how the distance between a characteristic ICRH cut-off density layer and the antenna is the relevant parameter to address the problem of the RF coupling efficiency. Simulation of the coupling process with the three-dimensional antenna code Ion Cyclotron ANTenna compares well with RF measurements, provided that the experimental density profiles are used in the computations.
Edge density profile effects for lower hybrid waveguide coupling