Special section on the physics and technology of plasma heating by ICRF power

This special section brings together much of what is currently at the forefront of ion cyclotron resonance frequency (ICRF) research. Which theories are people working on? Where is progress being made? What results are being obtained?The present Nuclear Fusion section on ICRF is not—and was explicitly meant not to be—an overview or review of ICRF systems, research achievements or theories. It is more a snapshot of the leading edge of the investigations. It is based, in part, on presentations to the 16th Topical Conference on RF Power in Plasmas, Park City, Utah, USA, April 2005.The forefront of ICRF research currently being actively pursued covers a wide range of topics: theoretical, experimental and technological.As can be expected, most of the papers in this section have direct relevance to ITER. Elements that will be important in ITER, and that are being addressed and developed in the papers, are the presence of fast particles with their influence on wave propagation and damping, the non-linear mechanisms in the edge—in particular close to the wave launcher—and steady-state aspects. Specific ITER components as well as RF scenarios are studied.Continued efforts to improve the analytical description of wave damping and absorption and the availability of gradually more powerful computers led to significant progress in incorporating the effect of particles with non-thermal velocity distributions—the presence of which has already become significant in present-day machines due to massive RF and/or NBI heating which forces the particles away from thermo-dynamical equilibrium (Brambilla et al, Jaeger et al).The exact role that RF-created and fusion-born fast particles will play is still a matter of lively debate. As shown in the papers by Choi et al and Pinsker et al, the presence of energetic particles is a significant factor in the wave absorption, even at high harmonics.Accounting for the actual magnetic topology allows the capture of RF induced effects which were treated in less detail in earlier modelling. Murakami et al present a wave–particle description that accounts for the complex geometry of the Large Helical Device (LHD). Finite Larmor radius effects and changes to the guiding centre orbits are addressed theoretically in the papers by Johnson et al and Hellsten et al.In the edge, the plasma is not fully ionized, the machine walls and protruding objects are close-by, and the magnetic topology can be complicated. Despite these difficulties an understanding and appropriate modelling of the plasma edge is imperative as the plasma edge is an essential feature in ICRF coupling and RF-plasma interaction. The full richness of the antenna near-field plasma dynamics and the non-linear plasma–wave interaction close to the launchers is addressed by Myra et al, while Bobkov et al present experimental evidence of some key players in the area of wave coupling.Advances are being made in treating increasingly realistic and detailed antenna geometries with plasma in coupling computations (Lancellotti et al) and on describing the presence and effect of the plasma in front of the antenna box.The role of edge interactions becomes of greater consequence as the pulse length grows towards steady state conditions. Colas et al provide evidence gathered during the long pulses in Tore Supra.Proper design and tuning of the wave launcher is essential to ensure coupling to a variety of experimental conditions. Messiaen et al and Bosia et al focus on specific technological elements of the ICRF system foreseen for ITER: the antenna and the matching system.In the initial phase of ITER, where H will be used as the majority ion species, special non-fusion ICRF scenarios will need to be used. Such scenarios are explored by Mayoral et al in JET.We hope that this special section will give you a flavour of the ongoing research in the ICRF area and that it will contribute to advancing ICRF in particular for ITER.