Recent advances in self-consistent RF sheath modeling and relatedphysical properties: Application to Tore Supra IC antennae

Non-linear wave-plasma interactions in the plasma edge often set the operational limits for radio-frequency (RF) heating systems. Some peripheral Ion Cyclotron (IC) wave energy loss is attributed to a net direct current (DC) biasing of the edge plasma by RF-sheath rectification. In order to understand the underlying mechanisms driving these interactions, SSWICH (Selfconsistent Sheath and Wave for IC Heating) models self-consistently the interplay between RF wave propagation and edge plasma biasing, using a two-field fluid approach. RF and DC parts are coupled through non-linear RF and DC sheath boundary conditions (SBC). The iterative resolution of the coupled RF+DC model experiences convergence issues in the wide sheaths regime, i.e. large DC biasing relevant in the vicinity of high-power RF launchers. In order to overcome this difficulty and provide a first guess of the solution, the self-consistent RF+DC system was solved explicitly for the first time in this asymptotic wide sheaths limit, while keeping the possibility to excite the system by any realistic RF field map. Several original physical properties were observed. The radial penetration of the RF sheaths along lateral walls at both ends of the open magnetic field lines can be far deeper than the skin depth characteristic of the slow wave (SW) evanescence. This is interpreted invoking the Sheath-Plasma Wave, only appearing in the presence of sheaths and carrying them along material boundaries up to their leading edge. The RF voltages driving sheaths were shown to scale as the square root of the input RF power, consistent with some experimental observations and valuable for antenna design. This is illustrated in the case of Tore Supra antennae by the comparison of two Faraday screens with different electrical design that were experimented upon last year simultaneously. Since only the edge plasma is actually meshed, another issue is the emulation of radiating conditions in order to avoid reflections at the inner boundary of the domain. A versatile perfectly matched layer (PML) adapted to any medium was thus developed and extensively tested as an artificial dielectric material. As tokamak magnetized plasmas present two wave eigenmodes with opposite group velocities, simultaneous adaptation is impossible. The PML is accordingly adapted only for the fast wave if the plasma domain is far thicker than the SW skin depth. The PML should also be placed beyond the coupling zone of the fast wave.