Nanostructured tungsten films for exposure to high-flux Deuterium plasmas

We present an experimental investigation on the relation between the nano-structural properties of W films, deposited by Pulsed laser Deposition (PLD), and deuterium (D) retention arising from exposure to high-flux deuterium plasmas. W coatings have been recently adopted to cover plasma facing surfaces of first walls in Tokamaks [1]. Neutrons, particles, e.m. radiation deeply change first wall material properties during operation, inducing modifications at the nanometric level, like vacancies, defects, and nanocrystalline growth [2].The dependence of deep diffusion in metals on particle fluxes and radiation damage is largely unknown and requires further research also because of lack of knowledge of the influence of materials properties at the nanoscale on macroscopic behavior. To investigate this issue we deposited by PLD nanostructured W coatings and characterized them by high resolution SEM, XRD, EDS [3]. Thanks to the flexibility of PLD it is possible to study wide ranges of coating structures, growths and morphologies, by changing process parameters. The coatings are dense, non-porous and exhibit different crystalline structures, from highly crystallographic-oriented with columnar structure to "amorphous-like" structure (crystalline domain size less than 2 nm) with high level of nm defects. We exposed films to high-flux (>1024 m-2s-1) D plasmas in the divertor simulator Pilot-PSI, to investigate D retention and erosion properties. Surface temperatures below 520K and between 600-950K were adopted, to investigate effects of vacancy migration during implantation. Thermal Desorption Spectroscopy measurements show that retention properties are highly influenced by nanostructure and disorder. The measured retention levels are similar or even much higher than the highest measured in pre-damaged W. Surface morphology changes (bubble formation, nano-crystalline structures) were found. W coatings have been exposed also to ion flux from capacitive coupled RF plasma, using hydrogen and noble gases. Erosion dynamics and morphology changes of the coatings as a function of nanostructure have been addressed.