Heat-Loads on JET Plasma Facing Components from ICRF and LH Wave Absorption in the Scrape-Off-Layer
In JET, Lower Hybrid (LH) and Ion Cyclotron Resonance Frequency (ICRF) wave absorption in the Scrape-Off Layer (SOL) can lead to enhanced heat fluxes on some Plasma Facing Components (PFCs). When using ICRF, hot spots are observed on the antenna structures and on limiters close to the powered antennae and are explained by acceleration of particles in RF­rectified sheath potentials. High temperatures up to 800oC can be reached on locations where a deposit has built up on tile surfaces. Modelling taking into account the fast thermal response of deposit layers can well reproduce surface temperature measurements via IR imaging, and allow evaluation of the heat fluxes local to active ICRF antennae. The flux scales linearly with the SOL density and with the antenna voltage. Strap phasing corresponding to wave spectra with lower k|| values can lead to a significant increase of hot spot intensity in agreement with antenna modelling that predicts, in that case, an increase of RF sheath rectification. LH absorption in front of the antenna through Electron Landau Damping of the wave with high N|| components generates hot spots precisely located on PFCs magnetically connected to the launcher. Analysis of LH hot spot surface temperature from IR measurements allows quantification of the power flux along the field lines: in worst case scenarios it is in the range of 15-30 MW/m2. The main driving parameter is the LH power density along the horizontal rows of the launcher, the heat fluxes scaling roughly with the square of LH power density. The ionisation of neutrals in the SOL by LH power plays an important role in the absorption mechanism and far SOL density enhancement.