Abstract
In general, Faraday screen elements in an ICRF antenna are not aligned precisely along the combined toroidal and poloidal magnetic fields. When plasma of density n > 2epsilon/sub 0/V/eg/sup 2/ /approximately/ 10/sup 9/cm/sup -3/ is present in the gap between elements, electron response to the parallel electric field shorts out the electric field over most of the gap, leaving a narrow sheath of positive space charge and intense electric field. Here V denotes the voltage across the gap and g the gap spacing. This intense electric field accelerates ions up to an appreciable fraction of the gap voltage (/approximately/ 1 kV), sufficient to cause physical sputtering of the screen material. Impurities so generated constitute the principal limitation on power density (kW/cm/sup 2/) for ICRF antennas. ICRF antenna and Faraday screen design principles which minimize sputtering are discussed. 24 refs., 9 figs., 1 tab.
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