Radiofrequency sheaths and impurity generation by ICRF antennas

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 > 2∊0V/eg2 ∼ 109cm−3 (V being the voltage across the gap and g the gap spacing) is present in the gap between the elements, the 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 an intense electric field. This intense electric field accelerates ions up to an appreciable fraction of the gap voltage (∼1 kV), sufficient to cause physical sputtering of the screen material. Impurities so generated constitute the principal limitationon power density for ICRF antennas. Principles of ICRF antenna and Faraday screen design which minimize sputtering are discussed.