Nanostructured Rhodium Films for Advanced Mirrors Produced by Pulsed Laser

This work presents a laboratory-scale experimental investigation of the erosion and the behavior under redeposition of rhodium (Rh), which is of interest for diagnostic first mirrors (FMs) in ITER due to its high reflectivity in a wide wavelength range and acceptable sputtering yield. Erosion by the plasma particles and redeposition of the sputtered material can lead to a dramatic decrease of the specular reflectivity of FMs, posing a serious threat on the performance of diagnostic systems [E.E. Mukhin et al., Nucl. Fusion 52 (2012) 013017]. Among all possible techniques, pulsed laser deposition (PLD) permits the production of Rh films as metallic coating in FMs; in particular, the PLD capability of tailoring the morphology and the structure of the coatings at the nanoscale [A. Uccello et al., J. Nucl. Mater. 432, (2013), 261] could be exploited to minimize problems related to erosion. With the aim of understanding the erosion characteristic of the Rh films, three different PLD morphologies were exposed to pure deuterium plasma at ITER-relevant conditions. The results show how the nanostructure of the films influences their erosion and optical properties. In order to experimentally simulate the redeposition phenomenon onto mirrors, PLD was employed also to produce carbon deposits with controlled morphology and nanostructure, resembling the characteristics of those found in the currently operating tokamaks. The same laser system has been exploited trying to develop a suitable laser cleaning of Rh films from these contaminants, preserving at the same time the integrity of Rh mirrors.