Investigation and plasma cleaning of first mirrors coated with relevant ITER contaminants: beryllium and tungsten

Abstract

In order to extend the investigation of the plasma cleaning of ITER first mirrors, a set of molybdenum mirrors was coated in a laboratory with ITER-relevant contaminants, namely beryllium and tungsten. Different coating techniques as well as several contaminant compositions were used to ensure a large variety of films to clean, completing a previous study conducted on mirrors exposed in the JET ITER-like wall (tokamak deposits) [1]. Due to the toxicity of beryllium, the samples were treated in a vacuum chamber specially built for this purpose. The cleaning was performed using capacitively coupled RF plasma and evaluated by performing reflectivity measurements, scanning electron microscopy, x-ray photoelectron spectroscopy and ion beam analysis. The removal of all types of contaminants was achieved by using different plasma compositions (argon, helium and mixtures of the two) with various ion energies (from 200-600 eV) and in some cases the mirror's reflectivity was restored towards initial values. Pure helium discharges were capable of removing mixed beryllium/ tungsten layers and oxidized molybdenum. In addition, no significant increase in the diffuse reflectivity of the mirrors was observed for the helium cleaning, though this was the case for some samples cleaned with argon. Helium is therefore appropriate for cleaning all mirrors in ITER leading to a possible cleaning regime where the entire vessel is filled with He and all mirrors are cleaned simultaneously without damaging their surfaces.

Deuterium as a cleaning gas for ITER first mirrors: experimental study on beryllium deposits from laboratory and JET-ILW

“…During the first 72 h, it was expected to have a reduction of the equivalent Be thickness from 591 to 231 nm but the sample only had 33 nm left, implying that JET-ILW deposits are removed faster than pure Be laboratory deposits (factor 1.5). Such an effect was already observed in a previous study where the cleaning was conducted with Ar and He [13]. The following cleaning steps (measurements after 80 and 86 h) displayed a reduction in the erosion rates calculated after each cleaning and plotted in figure 5.…”

“…This enables one to calculate absolute reflectivity of samples from in situ reflectivity measurements for given wavelengths (for example at 400, 550 and 700 nm wavelength as displayed in figure 2). A typical evolution of the in situ relative reflectivity is shown in figure 1 of [13].…”

“…The core level spectrum of Rh3d was fitted with one doublet at a binding energy (BE) of 307.2 eV, corresponding to metallic Rh [33,34]. SEM images taken after the D 2 cleaning displayed a flat surface without any peculiar structures (that would be indicative of remaining contamination but are not shown here, see for example figure 7 of [13]). The surface was similar to the image shown in figure 3 after D 2 plasma cleaning, where the initial nano-crystalline structure of the deposited Rh film can still be observed after the plasma cleaning.…”

“…A first experimental campaign devoted to plasma cleaning of Be contaminated deposits has been carried out in the JET Beryllium Handling Facility. Laboratory made deposits (Be and W) [12] and tokamak deposits grown in JET-ILW on typical FM material [13] (molybdenum (Mo) and rhodium (Rh)), were cleaned with helium (He), argon (Ar) or a mix of both with ion energies ranging from 200 to 600 eV. The deposits thickness was significantly reduced, often fully removed and accompanied by an increase of the reflectivity.…”

Plasma cleaning of ITER edge Thomson scattering mock-up mirror in the EAST tokamak

“…However, as the film thickness of alumina was monitored with quartz microbalance and cross-checked with SEM cross-sections, the only possibility for the strong decrease in mirror reflectivity is the oxidation of the mirror surface in addition to the 10 nm of alumina. For T1 and T2, the higher reflectivity after cleaning is typically observed when removing the surface oxidized the Mo layer [25]. Due to physical sputtering during the cleaning process, the diffuse reflectivity of the mirrors was increased, especially in the visible range.…”