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On the use of rhodium mirrors for optical diagnostics in ITER

Profile image of Philippe MERTENSPhilippe MERTENS

2019, Fusion Engineering and Design

https://doi.org/10.1016/J.FUSENGDES.2019.04.031
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Abstract

The first mirrors of optical diagnostics in ITER are exposed to high radiation and fluxes of particles which escape the plasma, in the order of 10 20 m −2 s −1. At the position of the mirror, the flux may still reach about 10 18 m −2 s −1. First mirrors are thus the most vulnerable in-vessel optical components, being subject to erosion, esp. by fast charge-exchange neutrals, or to deposition of impurities at flux rates which can reach 0.05 nm/s. The material selected for the reflecting surface must combine a high optical reflectivity in a wide spectral range and a sufficient resistance to physical sputtering during normal operation and during mirror cleaning discharges, if any is installed. Rhodium (103 Rh) was identified early as a possible or even promising candidate. It combines several attractive properties, for instance a mass which leads in most cases to low sputtering yields together with an optical reflectance (75% Rh) which is much higher than of some other options. Rh is insensitive to large temperature changes. Rhodium is fairly inert and its low oxidation is an appreciable advantage in case of steam ingress events. The core-plasma CXRS diagnostic in ITER (UPP 3) have now turned to Rh as a baseline. The aim is to procure monocrystalline rhodium (SC-Rh) to mitigate the increase of the diffuse reflection with the damage due to physical sputtering.

Figures (5)
Fig. 2. Top: cross-section of a conceptual setup for a first mirror made of solid rhodium [19,20]; bottom: distortion at operating temperature of the mirror shown above, given as a contour plot of the displacement perpendicular to the reflecting surface. Fig. 1. Front part of the core CXRS optics (mirrors M1-M3). The DSM is the Diagnostic Shield Module, the FGU is the Front Guard Unit. The latter is placed in a cavity of the DFW — Diagnostic First Wall — which is hidden on the picture.
Fig. 2. Top: cross-section of a conceptual setup for a first mirror made of solid rhodium [19,20]; bottom: distortion at operating temperature of the mirror shown above, given as a contour plot of the displacement perpendicular to the reflecting surface. Fig. 1. Front part of the core CXRS optics (mirrors M1-M3). The DSM is the Diagnostic Shield Module, the FGU is the Front Guard Unit. The latter is placed in a cavity of the DFW — Diagnostic First Wall — which is hidden on the picture.
Fig. 3. A good joining attempt (vertical interface in the middle of the picture) between W/Cu “alloy” and Rh, magnification: 1 um scale. Left side: EDX min- iatures in false colours show the distribution of W, Cu and Rh. Scale bar: 5 um. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. A good joining attempt (vertical interface in the middle of the picture) between W/Cu “alloy” and Rh, magnification: 1 um scale. Left side: EDX min- iatures in false colours show the distribution of W, Cu and Rh. Scale bar: 5 um. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Pilot test of Rh crystal growth, by courtesy of MaTecK [23]. A cleaning system can be installed for the first mirror, it is in Because M1 is subjected to frequent bombardment by impurities from the plasma, mainly charge exchange neutrals in the energy range 0.1-1 keV, or by the ions of the gas used for repetitive cleaning (see Section 5 below), it should withstand it without significant degradation of the reflective properties: the specular reflectivity must not decrease and the diffuse reflectivity must be kept below a few percent. This is best attained by avoiding a polycrystalline material with different grain orientations which give rise to these spurious reflections after differ- ential sputtering of the grains [21,22]. A single crystal provides the
Fig. 4. Pilot test of Rh crystal growth, by courtesy of MaTecK [23]. A cleaning system can be installed for the first mirror, it is in Because M1 is subjected to frequent bombardment by impurities from the plasma, mainly charge exchange neutrals in the energy range 0.1-1 keV, or by the ions of the gas used for repetitive cleaning (see Section 5 below), it should withstand it without significant degradation of the reflective properties: the specular reflectivity must not decrease and the diffuse reflectivity must be kept below a few percent. This is best attained by avoiding a polycrystalline material with different grain orientations which give rise to these spurious reflections after differ- ential sputtering of the grains [21,22]. A single crystal provides the
principle a mandatory sub-system to reach the required lifetime in view of the expected deposition rates. The approach of the European Domestic Agency is to use a radio frequency (RF) discharge to remove the impurity deposits by physical sputtering [10]. A DC glow discharge is the fallback solution. With a depth of material to be removed which can reach 50 nm per cleaning cycle (compare Fig. 5 in [34]) and = 100 cycles specified, the availability of an “End-of-Cleaning Indicator” (ECI) is of utmost importance.
principle a mandatory sub-system to reach the required lifetime in view of the expected deposition rates. The approach of the European Domestic Agency is to use a radio frequency (RF) discharge to remove the impurity deposits by physical sputtering [10]. A DC glow discharge is the fallback solution. With a depth of material to be removed which can reach 50 nm per cleaning cycle (compare Fig. 5 in [34]) and = 100 cycles specified, the availability of an “End-of-Cleaning Indicator” (ECI) is of utmost importance.

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