First Mirrors Test in JET for ITER: An overview of optical performance and surface morphology

Abstract

a b s t r a c tMetallic mirrors will be essential components of all optical systems for plasma diagnosis in ITER. A comprehensive First Mirror Test (FMT) has been carried out in the JET tokamak. The exposure was followed by a characterisation of mirrors and their carriers. The main results obtained by optical and surface analysis methods are: (i) reflectivity of all tested mirrors is degraded either by erosion with charge exchange neutrals or by the formation of thick deposits; (ii) deuterium and carbon are the main elements detected on all mirror surfaces and the presence of beryllium is also found in several cases; (iii) thick deposits show columnar structure; (iv) bubble-like structures are detected in deposits; their formation is a probable reason for deposits' disintegration and peeling-off; (v) the deposition in channels in the divertor cassettes is pronounced at the very entrance and then it sharply decreases with the distance from the plasma, l$5-7 mm.

Overview of the second stage in the comprehensive mirrors test in JET

“…3.0 and 4.5 cm, and (ii) all divertor mirrors are covered by deposits that are rather thick and flaking, and deposition decreases with depth in the channel. These results are in full agreement with findings after the first step of the mirror test in 2005-2007, as reported in [2,3]. The results imply that the surface state of mirror in the divertor is governed by net deposition, whereas net erosion decides the state of mirrors located close to the channel mouth in cassettes on the main chamber wall.…”

“…The loss of reflectivity is more pronounced in the visible range than in the infrared region. The results are very consistent with those obtained after the exposure in 2005-2007 [2,3]. The strongest loss is measured on mirrors located close to the channel mouth.…”

“…Therefore, on the request of the ITER design team, the first mirror test (FMT) has been carried out at JET [1,2]. The entire research program comprises: (a) selection of material for the test mirror, (b) manufacture of mirrors and their carriers (cassettes of pan-pipe shape) for in-vessel installation, (c) optical pre-characterization of mirrors, (d) exposure in the plasma boundary of JET during a complete operational campaign, (e) a broad range of post-exposure analyses by means of optical and surface analysis methods, (f) analyses of deposition in the channels of the mirror carriers [3] and (g) photonic cleaning of the exposed mirrors [4], followed by analyses of cleaned surfaces and examination of products generated by the laser light impact. The FMT is embedded in tritium retention studies [5] aiming at a detailed description of erosion-deposition patterns in JET measured by several types of diagnostic tools.…”

An overview of the comprehensive First Mirror Test in JET with ITER-like wall

“…Plasma-wall interaction (PWI) phenomena leading to material erosion and migration [1] may be decisive for the state of mirror surfaces and the degradation of their properties. To recognize the extent of changes a thorough First Mirror Test (FMT) has been carried out at the JET tokamak (Joint European Torus) on the request of the ITER Design Team [2][3][4][5]. The major goals of the test are to assess the optical performance, i.e.…”

“…The first phase, in JET with carbon walls, was completed. Mirrors exposed for up to 80h with 59.6h of X-point operation in 2005-2009 campaigns were examined and described in detail [3][4][5]. The morphology of all exposed specimens was modified by neutral species although their impact on reflectivity was dependent on the mirror location.…”