Ray-tracing modelling of the ICRF heating of large tokamaks

The wave-front concept in conjunction with the ray-tracing technique is applied to solving the problem of fast-wave ion cyclotron resonance heating of tokamaks, leading to realistic power deposition profiles. The different features of the complete model are reviewed, including the 2-D computation of the antenna near-field, the concept of a constant-phase surface and the derivation of the initial conditions for ray tracing, the geometric optics expansion scheme, the ray and power transport equations and their implementation in general toroidal geometry, as well as the procedures leading to the production of final power deposition profiles for the various plasma species. The validity and consistency of the approach are investigated both by means of numerical checks and by using analytic asymptotic expansions in simplified cases. The range of validity and the limitations of the present approach are fully discussed. Using asymptotic theory, it is possible to bridge the gap between the antenna coupling theory (a full-wave solution) and ray tracing (a geometric optics solution), showing how the power spectrum radiated by the antenna constrains the field distribution over a constant-phase surface. Examples of power deposition profiles for the plasma species are generated for several minority heating scenarios of JET in which the antenna is located on the low-field side. In most cases, a very peaked power deposition profile is obtained, which is centred around the minor radius at which the cyclotron layer cuts the meridian plane of the tokamak.