Speaker
Description
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. They are thus the most vulnerable optical component in the optics chain inside port plugs, being subject to erosion, especially by fast charge-exchange neutrals, or to deposition of impurities at flux rates which can reach 0.1nm/s. The material selected for the reflecting surface must combine, among others, a high optical reflectivity in a wide spectral range and a sufficient resistance to physical sputtering − both during normal operation and during cleaning discharges, if any is installed. Rhodium (103Rh, atomic no. 45) was identified early as a possible or even promising candidate.
Rhodium combines several attractive properties, for instance a mass which leads in most cases to low sputtering yields (cascades of binary collisions) together with an optical reflectance (~75%) which is much higher than for some heavier elements considered (molybdenum or tungsten) and, unexpectedly, fairly insensitive to large temperature changes. It is rather inert and its low oxidation is an appreciable advantage in case of steam ingress events (e.g. water ingress from leaks).
Whereas the original diagnostic baseline for the core CXRS diagnostic in ITER (upper port plug No.3) was relying on Mo, we have now turned to Rh as a preferable option which allows for an integrated design in the sense that a cooled holder and an embedded cleaning system, should it be decided for, seem to be feasible. The authors aim to procure monocrystalline rhodium which would prevent to a large extent the diffuse reflection to increase with the damage due to physical sputtering.
Pros and cons of the use of rhodium for a first mirror will be presented with respect to optical, mechanical and robustness aspects and achievements.
