Studies of impurity deposition/implantation in JET divertor tiles using SIMS and ion beam techniques

Deposition in JET divertor tiles has been observed to be asymmetric; i.e. heavy deposition occurs in the SOL at the inner divertor whereas there is little deposition at the outer divertor. Heavy deposition at the inner divertor has lead to flaking on the water-cooled louvres and after the DTE1 tritium experiment at JET it was observed that majority of the retained tritium is in the flakes that have spallen from the louvres. Erosion/deposition issues have thus a great impact on the tritium retention and lifetime of first wall components in present and future fusion devices. The asymmetry in the deposition implies a drift in the SOL. To investigate material transport and SOL flows observed at JET, 13CH4 and SiH4 were injected into the plasma boundary in the last day of discharges prior to the JET 2001 shutdown. Methane was puffed at the top of the vessel, and the silane at the outer divertor wall. Silicon was observed spectroscopically as an impurity in the plasma. A set of special tiles was installed in the Mk IIGB divertor during the 1999 shutdown. The tiles were coated with poloidal stripes of a boron doped carbon layer on a rhenium interlayer. These tiles were removed in the 2001 shutdown, and have been analysed in 2002 to determine the erosion/deposition profile, and 13C and 28Si distribution over the entire divertor cross-section. The analysis techniques used have been Secondary Ion Mass Spectrometry (SIMS) and the Ion Beam Analysis (IBA) methods Time of Flight Elastic Recoil Detection Analysis (TOF-ERDA) and Rutherford backscattering (RBS). 13C could be found in large amounts on the surfaces of the inner divertor tiles whereas in the outer divertor it was hardly detected. Under the 13C layer a deuterium rich carbon layer was found on the inner divertor tiles. Thick deposits of beryllium were buried under the carbon layer. Also the silicon distribution was recorded. The observed 13C and Si distributions are discussed in relation to the position of strike point and the ion fluxes measured with divertor Langmuir probes. This work has been conducted under the European Fusion Development Agreement and is partly funded by EURATOM.