Superadiabatic behavior of fast particles accelerated by ion cyclotron resonance heating

This paper addresses the problem of superadiabatic motion of ions interacting resonantly with waves in the ion cyclotron range of frequency (ICRF). In the classical quasilinear treatment of ion cyclotron resonance heating, the phase difference between the wave and the cyclotron motion of the particles must be assumed to be completely uncorrelated between two resonance crossings. However, beyond a critical particle energy, the so-called adiabatic barrier, this assumption is no longer valid, and particles exhibit superadiabatic behavior. In order to study in detail the influence of superadiabatic motion on the efficiency of ICRF heating the MOKA code was developed. In the MOKA code, complete trajectories of fast particles in a toroidal plasma magnetic equilibrium modified by a realistic radio-frequency perturbation are calculated. This paper shows that the location of the adiabatic barrier depends strongly on the particle orbits. Also, the injected toroidal spectrum is of high importance for determining the correlation effects, while at high energies, collisions are found to play a negligible role. A realistic description of this adiabatic barrier is given for an ICRF minority heating pulse of Tore Supra [J. Pamela, Fusion Engineering and Design 46, 313 (1999)].