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 19 97 Plasma Phys. Control. Fusion 39 1409) for the nonlocal linear radio frequency response of tokamak plasmas can be reduced to integrals over position of local expressions akin to the ones familiar in uniform pl asma theory. This is only possible in the asymptotic limit of high inh omogeneity along the guiding centre orbits, typical of cyclotron inter actions, and introducing an ad hoc simplifying assumption, namely disc arding the tangent resonance effects studied in the reference. The pre sent analysis yields results of theoretical and practical interest for the modelling of high-frequency plasma heating: (i) Connection with i mportant former theoretical work is explicitly established; in particu lar, The equivalence between the standard guiding centre and the Hamil tonian formalisms becomes evident, as it should. Within the above assu mptions, we demonstrate the equivalence of nonlocal and local formulat ions, 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 an the main qua litative 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 equilibr ium.(iii) We give a very simple (and rigorous) remedy to this problem allowing straightforward improvement of these models and of the associ ated wave codes. This remedy should significantly increase the numeric al accuracy of the rf power deposition profiles, as well as the accura cy 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 i mprovement in the available numerical descriptions of the Landau wave- particle interactions.