2-dimensional mapping of ICRF-induced scrape-off layer modifications with a retarding field analyser on ASDEX-Upgrade

Using a reciprocating Retarding Field Analyser (RFA), Scrape-Off Layer (SOL) modifications were investigated on ASDEX-Upgrade during heating with waves in the Ion Cyclotron Range of Frequencies (ICRF), suspected for enhanced impurity production in this all-metal machine. Two quantities involved in the sputtering were measured: the current I_slit on a saturated slit plate, proportional to the parallel ion flux and the mean parallel energy <W_‖>_t of collected ions, averaged over many RF cycles. Combining multiple RFA reciprocations over a scan of q₉₅ provided 2D poloidal/radial resolution. In the outer SOL a localized RF-perturbed zone was evidenced on the RFA side magnetically connected to an active ICRF antenna. A flat 2D I_slit pattern surrounded by steep gradients was observed, correlatively with <W_‖>_t exceeding 150eV. The centre of the zone is connected radially slightly behind the leading edge of antenna side limiters, with a radial extension up to ±2cm. The zone is broadest and <W_‖>_t is largest near the bottom of the active antenna. This is interpreted as a zone of local plasma biasing via sheath rectification, creating density convection around it. The I_slit pattern is qualitatively consistent with simple considerations about E×B particle convection.