Tungsten coating on jet mk IIGB-SRP divertor tiles

Erosion and deposition at the JET vessel walls and divertor have been investigated over the last years by means of various coating structures. This work is still ongoing and a new set of CFC tiles with tungsten stripes was installed in the divertor during the 2001 shutdown at JET. Six divertor tiles were coated with tungsten marker stripes using the DIARCr plasma coating method. The thickness of the coating was measured with profilometry and Rutherford Back-scattering Spectroscopy (RBS). The thickness profile of the deposited layer was determined by measuring the thickness of a tungsten layer grown simultaneously on a silicon wafer by profilometry. On the 2 cm wide marker stripe, the thickness varied ± 0.1 µm and the average thickness was 3.4 µm. The thicknesses measured directly on the coated tiles by RBS were in excellent agreement with the profilometry results. The contents of major impurities were determined using Time-of-Flight Elastic Recoil Detection Analysis (TOF-ERDA), Particle Induced X-ray Emission (PIXE), RBS and Secondary Ion Mass Spectrometry (SIMS). The impurities found were O, Fe, Ni and Cr in concentrations typically less than 1 at.%. Thickness and impurity measurements were performed prior to mounting in the vessel. The endurance of the tungsten coatings under severe conditions was tested in the Neutral Beam Test-bed at JET. For testing purposes, a block of CFC material 200 mm 150 mm 50 mm (which is approximately half the area of a divertor tile) was coated with a tungsten layer of approximately 3 µm thickness. The tungsten coated block was exposed to a total of 113 pulses with a maximum peak power density of 15.7 MWm-2. At 15.7 MWm-2 maximum surface temperature went up to 1280°C. Visual inspection of the coating revealed no obvious changes as a result of thermal cycling. No sputtering of the film by deuterium ions could be detected either. RBS spectra along the sample, taken before and after the NB tests were compared and found to be identical. This work has been conducted under the European Fusion Development Agreement and is partly funded by EURATOM.