Experimental evidence suggests that unabsorbed wave energy in ion cyclotron range of frequency fast wave (FW) experiments can result in deleterious edge interactions. A model describing the formation of far field sheaths due to FW interaction with material surfaces is presented. Near conductors that do not conform to flux surfaces, an incoming FW causes the generation of a slow wave (SW) component. The E∥ of the SW drives an RF sheath, in a manner similar to what has been previously discussed for antenna (near field) sheaths. To assess the importance of the proposed mechanism, a heuristic scaling model of the resultant sheath voltage V is developed and compared with a numerical code. The model illustrates the important dependencies of V on the single pass absorption, edge density, FW frequency, FW cutoff location, and limiter/wall geometries and yields qualitative agreement with the experimental observations.

It is shown that radio-frequency (rf) antenna sheaths can bias the edge plasma potential and drive steady-state convective cells in the scrape-off layer (SOL). The resulting E×B convective flow opposes the direction of the sheared flow in the SOL induced by the radially decaying Bohm sheath potential. A two-dimensional fluid simulation shows that the interaction of the opposing poloidal flows produces secondary vortices, which connect the edge of the confined plasma to the antenna limiters, when the antenna–plasma separation is typically of order a few times the local electron skin depth at the antenna. Estimates for typical tokamak edge parameters suggest that the transit time of particles and energy across these vortices is rapid enough to cause the broadening of SOL density and temperature profiles observed during high-power heating with ion cyclotron range of frequency (ICRF) antennas in monopole phasing. Radio-frequency-sheath-driven convection is also a good candidate to explain the phasing dependence of the global confinement properties of ICRF H modes on the Joint European Torus (JET) [Fusion Technol. 11, 13 (1987)]. A comparison of the JET H-mode data with the theoretical modeling supports this idea and suggests that ICRF convection may be a useful tool to spread the heat deposition in the divertor and to extend the lifetime of the H mode.

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A model is developed to calculate the impurity influx causcd by induced radio-frcquency(r.f.j sheaths near ion-cyclotron rnnge of frequencies (ICRF) antennas, which accelerate ions to energies sufficient lo Sputter the Faraday screen and other adjacent matcrial surfaces. The eiistmce o f several types of r.f. sheaths is discussed, and detailed models of Faraday-screen-gap and front-face sheaths are doscribed which arc relevant lo thhe antenna geometry in the Joint European Tarus (JET) experiment. Calculations o f the local influx of mctal impuritics from the Faraday screen show good quditatiw agreement with the obscrved dependence of the impurity influx in JET on phasing, magnetic field angle, and screen coatings, and the model providcs a useful frsmcwork for understanding other aspects of the data. It is also shown that a great increase in the metal influx due 10 self-sputtering of thc screen C'iwalanche") can occur under certain conditions. Howevcr, the model calculations also demonstrate that proper antenna design can virtually climinatc the impurity influx from thc ~creens. N O T A T I O N d total self-sputtcring amplificationFactor .db A,, sputtering area A,, B equilibrium magnetic field D, radial diffusion cocficient f subscript denoting sputtering region contribution of charge state k to self-sputtering tunplificatian factor area of region p ( = s or r) with unit normal in radial direction i, Faraday jcrecn gep dinlension F;K, 2) L , Faraday scrccn periodicity dimension (see Fig. 2) L,, parallel connection length in region p ( = s or I) I,, radial half-length of region p [=s or f) n,, elcctron dcnsitg in region p (=s or f) ne, electron density at ATTS Nps number of FS elemenls for the antenna N phasing factor describing area of FS covercd by gap sheaths , F$ volumc-averaged density of FS ions in ionization state in region p (=s or F) too volume-averaged density of FS neutrals in region p (=s or f j PLk' probability fiaaclorr [sec equations (14) and (19)-[23)1 R: limiter impurity density ratio at A I T S L nJn, K, sputtering yield angle enhancement fdctor (assumcd the Same for a11 species) s subscript denoting privatc SOL region T. electron tcmpeiature [equal in regions s and f ) V , antenna voltagz ( p p j V, gap voltage V, transmission line voltage (0-p) U" neutral thermal (free-streaming) velocily Vsh rectitied shcath potential

During r.f. experiments on JET a strong modification of the plasma edge is generally observed. Density and temperature profiles in the scrape-off layer usually flatten and enhanced impurity and neutral influx is observed. Metallic impurity influx originating from the antennae screens is also observed. This is especially true when the antennae are phased so that the excited spectrum contains a large fraction of power in the waves with k = 0-3 m-', A radially localized enhancement of the particle diffusivity D L ( u ) across the plasma edge results. When the antennae are phased in the quadrupole configuration with the maximum power emitted at k , , = 7 m-' a strong modification of the edge is not observed. The scrape-off layer profiles scale as in the ohmic discharges. Then both the light and metal impurity influxes become substantially reduced.