Scattering, absorption and short wavelength excitation at Faraday screens during ion cyclotron heating

摘要

Skin currents and plasma sheath currents induced at the Faraday screen bars cause RF scattering and mode conversion during ion cyclotron heating of tokamaks. The scattering of the incoming magnetosonic mode in the poloidal direction is found to be small and practically independent of the presence of plasma. The excitation of short wavelength modes due to the oscillating sheath dipoles is substantial when the wavenumber matches the Bragg diffraction condition from the screen periodicity. It is found that power losses from scattering in the Bernstein mode are comparable to dissipation losses from ion acceleration inside the sheaths at low edge densities. Subsequent Landau damping of the short wavelengths contributes to the edge plasma heating.