Localized Scrape-Off Layer Density Modifications by Ion Cyclotron Near Fields in JET and ASDEX-Upgrade L-mode Plasmas
Combining Lithium beam emission spectroscopy and edge reflectometry, localized Scrape-Off Layer (SOL) density modifications by Ion Cyclotron Range of Frequencies (ICRF) near fields were characterized in JET L-mode plasmas. When using the ICRF wave launchers connected magnetically to the Li-beam chord, the density decrease was steeper 2-3 centimetres outside the last closed flux surface (when mapped onto the outer mid-plane) and its value was halved at the outer limiters. The depletion depends on the ICRF power and on the phasing between adjacent radiating straps. Convection due to ponderomotive effects and/or E¥B0 drifts is suspected. During ICRF-heated H-mode discharges in 2013, DC potentials up to 70V were measured locally over a few centimetres in the outer SOL by a floating reciprocating probe, located toroidally several metres from the active antennas. These observations are compared with probe measurements on ASDEX-Upgrade. Their implications for wave coupling, heat loads and impurity production are discussed.