An Overview of the Comprehensive First Mirrors Test in JET with ITER-Like Wall

First Mirror Test in JET with the ITER-Like Wall was performed with polycrystalline molybdenum mirrors. Two major types of experiments were done. Using a reciprocating probe system in the main chamber a short-term exposure was done during 0.3h of plasma operation in 71 discharges. The impact on reflectivity was negligible. In a long-term experiment, 19hs with 13h of X-point plasma during the entire campaign, twenty Mo mirrors were exposed, including four coated with a 1µm thick Rh layer. Optical performance of all mirrors exposed in the divertor has been degraded by up to 80% because of beryllium, carbon and tungsten co-deposits on surfaces. Total reflectivity of most Mo mirrors facing plasma in the main chamber was only slightly affected in the spectral range 400-1600nm, while the Rh-coated mirror lost its high original reflectivity by 30% thus decreasing to the level typical for molybdenum surfaces. Specular reflectivity was decreased most strongly in the UV range 250-400nm. Surface measurements with XPS and depth profiling with SIMS and Heavy Ion ERDA revealed that the very surface region on both types of mirrors had been modified by neutrals resulting eventually in the composition change: Be, C, D at the level below 1¥1016 cm-2 mixed with traces of Ni, Fe in the layer 10-30 nm thick. In several cases the original matrix material (Mo) has remained as the major constituent of the modified layer. The data obtained in two major phases of JET operation with carbon and full metal walls are compared. The implications of these results for first mirrors and their maintenance in a reactor-class device are discussed.
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