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Abstract:
The quasilinear model is commonly used to determine the heating rate and the current drive efficiency of the fast wave in the ion cyclotron range of frequency. The validity of this model at these frequencies is a controversial issue [Becoulet et al.. Phys. Plasmas 1, 2908 (1994)]. Here this problem is addressed from the single-particle point of view by deriving an approximated Hamiltonian which accounts also for the inhomogeneity of the confining magnetic field. Realistic values of the wave field are calculated by means of the toroidal full- wave code TORIC for typical scenarios of an ASDEX Upgrade [Gruber et al., Nucl. Fusion 41, 1369 (2001)] experiment, and apply the Chirikov criterion to determine the threshold for the development of the intrinsic stochasticity. These results are verified by numerically integrating the Hamiltonian equations. It is found that, due to the discreteness of the parallel phase velocity spectrum of a fast wave at these frequencies, for the parameters of ASDEX Upgrade the intrinsic stochasticity is marginal, especially close to integer and low-rational values of the safety factor. The failure of intrinsic stochasticity to guarantee the validity of the conventional quasilinear model, however, does not imply the onset of nonlinear saturation in the power absorption and in the driven current, since in all realistic situations Coulomb collisions suffice, by a large margin, to ensure the applicability of the linearized Vlasov equation. Under this condition, power and momentum transfer rates to resonant electrons are identical to those predicted by the quasilinear theory. (C) 2003 American Institute of Physics.
