A qualitative comparison of theoretical models of radiofrequency wave propagation and absorption in tokamak plasmas

We show how the theoretical expressions obtained earlier (Lamalle P 1997 Plasma Phys. Control. Fusion 39 1409) for the nonlocal linear radiofrequency response of tokamak plasmas can be reduced to integrals over position of local expressions akin to the ones familiar in uniform plasma theory. This is only possible in the asymptotic limit of high inhomogeneity along the guiding centre orbits, typical of cyclotron interactions, and introducing an ad hoc simplifying assumption, namely discarding the tangent resonance effects studied in the reference. The present analysis yields results of theoretical and practical interest for the modelling of high-frequency plasma heating: (i) Connection with important former theoretical work is explicitly established; in particular, - The equivalence between the standard guiding centre and the Hamiltonian formalisms becomes evident, as it should. - Within the above assumptions, we demonstrate the equivalence of nonlocal and local formulations, and stress the ability of the latter to also validly incorporate toroidal effects such as particle trapping and radial guiding centre drifts in the plasma rf response. (ii) Emphasis is put on the main qualitative differences between theoretical approaches. Strikingly, in a tokamak with rotational transform, we show that some simplified models of radiofrequency wave propagation allow a nonphysical 'nonresonant' contribution to the power absorption, i.e. depending on the real part of the plasma dispersion function in the case of a Maxwellian equilibrium. (iii) We give a very simple (and rigorous) remedy to this problem, allowing straightforward improvement of these models and of the associated wave codes. This remedy should significantly increase the numerical accuracy of the rf power deposition profiles, as well as the accuracy of the global balance between the rf power launched by the antennae and absorbed by the plasma. (iv) Finally, we stress the need for an improvement in the available numerical descriptions of the Landau wave-particle interactions.