Interaction of ICRF Fields with the Plasma Boundary in AUG and JET and Guidelines for Antenna Optimization

W sputtering during ICRF on ASDEX Upgrade (AUG) and temperature rise on JET A2 antenna septa are considered in connection with plasma conditions at the antenna plasma facing components and E-parallel to near-fields. Large antenna-plasma clearance, high gas puff and low light impurity content are favorable to reduce W sputtering in AUG. The spatial distribution of spectroscopically measured effective W sputtering yields clearly points to the existence of strong E-parallel to fields at the antenna box ("feeder fields") which dominate over the fields in front of the antenna straps. The picture of E-parallel to fields, obtained by HFSS code, corroborates the dominant role of E-parallel to at the antenna box on the formation of sheath-driving RF voltages for AUG. Large antenna-plasma clearance and low gas puff are favorable to reduce septum temperature of JET A2 antennas. Assuming a linear relation between the septum temperature and the sheath driving RF voltage calculated by HFSS, the changes of the temperature with dipole phasing (00 pi pi, 0 pi pi 0 or 0 pi 0 pi) are well described by the related changes of the RF voltages. Similarly to the AUG antenna, the strongest E-parallel to are found at the limiters of the JET A2 antenna for all used dipole phasings and at the septum for the phasings different from 0 pi 0 pi. A simple general rule can be used to minimize E-parallel to at the antenna: image currents can be allowed only at the surfaces which do not intersect magnetic field lines at large angles of incidence. Possible antenna modifications generally rely either on a reduction of the image currents, on their short-circuiting by introducing additional conducting surfaces or on imposing the E-parallel to=0 boundary condition. On the example of AUG antenna, possible options to minimize the sheath driving voltages are presented.