Sputtering of Mo and Al in D2/N2 plasma cleaning discharge

Sputtering of Mo and Al (as Be proxy) in mixed D2/N2 DC glow discharge was studied in view of the first mirror performance. The composition of the working gas was varied from 100% D2 to 100% N2, while keeping a total pressure of 15Pa. The ion energies striking the sample surface were defined by its 100V biasing negative to a floating potential. It has been obtained that the sputtering yield of Mo and Al increases gradually with N2 concentration up to 4‒16mol% and decreases with further N2 addition. In contrast, the sputtering yield of Be remains unchanged up to 10mol% of N2. Adding 16mol% leads to three-fold decrease in the sputtering rate. The sputtering behavior is discussed in context of surface data analysis and mass spectroscopy of the discharge gas exhaust. Variation in reflectivity of a single crystalline Mo due to plasma exposure under similar conditions is also presented and discussed

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

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