Applicability of ICRH as a heating method for fusion plasmas crucially depends on the possibility of restricting unwanted impurity production connected with this method. One of the sources of impurities is the Faraday screen which protects the current-carrying antenna strap from the main plasma. A possible mechanism of impurity production is the sputtering of metal atoms from the screen by energetic plasma ions. These plasma ions may gain their energy as follows:. The magnetic flux of the antenna creates a certain RF voltage across the gaps between adjacent strips of the Faraday screen. If no plasma were present at the screen, the electric field generating this voltage would extend across the whole gap. In the presence of a plasma, on the contrary, the electric field is essentially localized within a thin sheath in front of the strip with the lower voltage. The electric field in this case is therefore much larger and the time necessary to accelerate the ions across the sheath to the strip is much shorter than in the case where no plasma is present. Thus, ions within a plasma of sufficient density at the screen can gain energy of the order of the voltage across the gap between adjacent strips within a half period of the IC wave. This non-resonant mechanism of ion energy gain is studied with a numerical 1-D electrostatic particle code. Ion and electron orbits are followed in the self-consistent electric and prescribed magnetic fields. The voltage induced is taken into account as a time-varying boundary potential. The code is used to calculate particle and energy fluxes to the boundary as function of time, energy and incidence angle. From these fluxes the sputtering yield of target material can be determined. © 1990.
