Heat loads on JET plasma facing components from ICRF and LH wave absorption in the SOL

Abstract

In JET, lower hybrid (LH) and ion cyclotron resonance frequency (ICRF) wave absorption in the scrape-off layer can lead to enhanced heat fluxes on some plasma facing components (PFCs). Experiments have been carried out to characterize these heat loads in order to: (i) prepare JET operation with the Be wall which has a reduced power handling capability as compared with the carbon wall and (ii) better understand the physics driving these wave absorption phenomena and propose solutions for next generation systems to reduce them. When using ICRF, hot spots are observed on the antenna structures and on limiters close to the powered antennas and are explained by acceleration of ions in RF-rectified sheath potentials. High temperatures up to 800 °C can be reached on locations where a deposit has built up on tile surfaces. Modelling which takes into account the fast thermal response of surface layers can reproduce well the surface temperature measurements via infrared (IR) imaging, and allow evaluation of the heat fluxes local to active ICRF antennas. The flux scales linearly with the density at the antenna radius and with the antenna voltage. Strap phasing corresponding to wave spectra with lower k∥ values can lead to a significant increase in hot spot intensity in agreement with antenna modelling that predicts, in that case, an increase in 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 the LH hot spot surface temperature from IR measurements allows a quantification of the power flux along the field lines: in the worst case scenario it is in the range 15–30 MW m−2. 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 the LH power density. The local electron density in front of the grill increases with the LH launched power; this also enhances the intensity of the LH hot spots.