ICRF HEATING IN TOKAMAKS

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Abstract
The current status of ICRF heating of Tokamak plasmas is described. Two aspects of the subject are discussed : the physics of wave damping and the evaluation of ion heating. Recent experiments in several Tokamak devices at the second harmonic of the deuteron cyclotron frequency have exhibited large discrepancies between the observed and predicted wave damping. Current thinking attributes these discrepancies to the influence of a two-ion hybrid resonance between the majority deuterium ions and impurity protons. A number of experimental observations appear to be consistent with this view. However, the details of the physical processes which lead to wave damping are still unclear. One theory attributes the damping to a linear mode conversion at the two-ion hybrid layer. This process is important, if the proton-to-deuteron density ratio is large (> 10 %). Another possible explanation, based upon enhanced cyclotron harmonic damping due to the proximity of the two-ion hybrid and cyclotron resonance layers, is discussed in this paper. The boundary value problem for fast wave propagation in an inhomogeneous two-ion plasma has been solved explicitly to obtain the wave field structure. For modest proton concentrations (< 5 %) a steep gradient in the left-hand polarized electric field is found in the region between the hybrid and cyclotron resonant layers. This gradient appears to provide sufficient enhancement of the second harmonic damping strength to account for the observed discrepancies. However, a reliable measurement of the proton concentration is needed to choose between the two possibilities. Ion heating was examined in detail in recent experiments in the ATC device. It is shown that ICRF heating can heat the bulk of the ions in the core of a Tokamak plasma with efficiencies ranging from 10 to 40 %, without causing any significant change in the ion energy confinement

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