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

Abstract

The first mirror test for ITER in JET with carbon walls has been completed. Thirty polycrystalline Mo mirrors including four coated with a 1 µm rhodium (Rh) film were exposed to plasma in the divertor region and in the main chamber. The mirrors were installed in eight cassettes of pan-pipe shape. The reflectivity of all mirrors exposed in the divertor has been degraded by 80-90% because of the formation of thick (>20 µm) flaking co-deposits on surfaces. Only small reflectivity losses (5-10%) occurred on mirrors located at the channel mouth of the cassettes from the main chamber wall. This is due to the in situ removal of deposited species by charge exchange neutrals. Deuterium, 12 C and 9 Be are the main isotopes detected on surfaces, but other isotopes ( 13 C) are also found in some locations, thus indicating differences in the material migration. Rhodium coatings with an initial reflectivity that is 30% better than that of pure Mo survived the test without detachment, but their resultant reflectivity was the same as that of the exposed Mo surfaces.

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

“…All specimens exposed in the main chamber, both the long-term samples from the midplane holders on the outer wall and the short-term probes exposed from the top of the machine, look clean and shiny after exposure. This observation is notably different from the results after JET operation with the carbon wall where small deposition was seen only on the mirrors located near the channel mouth (0cm and 1.5cm), while significant amount of deposition was observed on the wall mirrors placed deeper (3cm and 4.5cm) in the cassette channels [3][4][5].…”

“…During the previous exposures of mirrors in JET with carbon wall a clear dependence between the reflectivity and the depth in the cassette channel was observed. The reflectivity loss for mirrors close to plasma was larger than deep in the channel [3][4][5]. The recent results after exposure in JET-ILW do not fully follow this pattern.…”

“…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.…”

Optical Coatings as Mirrors for Optical Diagnostics

“…Tests were carried out in TEXTOR [3,33,34], DIII-D [35], JET-C [36,37] and JET-ILW [38] showing no critical failure of the coated mirrors like delamination or strong degradation of the optical properties. For example in TEXTOR for Rh coated mirror exposure, the film survived under erosion conditions although the reflectivity was decreased by 8% at 250 nm after exposure [3].…”