Full wave modeling of ICRF edge fields and far-field sheaths in tokamaks

Summary form only given. ICRF experiments on JET and DIII-D clearly show that the loading resistance and heating efficiency of the 4-strap antennas are strongly dependent on phasing. Namely, coupling resistance increases and heating efficiency decreases at low k/sub /spl par//, which suggests significant RF power losses at phasings corresponding to low k/sub /spl par//. In this paper, several possible mechanisms relevant to these observations are modeled using the 3D distribution of RF fields from the ALFA code. It is found that the RF electric field distribution is significantly different for different phasings, which suggests strong sheath interactions with walls and limiters [far-field sheaths (FFS)] for strap phasings corresponding to low toroidal mode numbers. To obtain qualitative results, the previously developed RF sheath theory is used for estimation of the power absorbed near the plasma edge.