• Energy Research

Nuclear materials and energy 34, 101366 - (2023). doi:10.1016/j.nme.2023.101366 Published by Elsevier, Amsterdam [u.a.]

This paper summarizes the emissivity measurements performed on the shaped ITER-like plasma facing units (PFU) of the WEST lower divertor installed for the second phase of WEST. Laboratory measurements before their installation in WEST, as well as measurement of one ITER-like PFU after exposure to an experimental campaign are shown. The measurements of 24 standard ITER-like PFU show the emissivity variability from 0.07 to 0.2 even for a serial production. Measurements performed after high heat flux tests (10 MW/m2 cycles) on two different facilities show the emissivity modification induced by this heat load capability test with mainly a decrease of the emissivity in the heat load area. Finally, the exposed ITER-like PFU exhibit a complex pattern with strong emissivity variation from 0.05 in the net erosion area at the strike point locations up to 0.85 in the deposition area close to the strike point location.

Post mortem analyses of dust collected in Alcator C-Mod have highlighted a production of large size dust particles. The quantities of such large particles are higher than in any other tokamak. They are divided in two classes as a function of their shape and consequently, their origin. Rounded dust particles such as spheres and splashes constitute the first class. These particles are the result of high heat loads on various leading edges of plasma facing components and possibly, their melting during plasma operation. The heated or already molten material can be destabilized during disruptions and droplets are emitted across the vacuum chamber. After solidification, the resulting rounded particles are either in pure elements or in alloys. Flake-like dust particles, which are mainly due to light material coating delamination, constitute the second class of dust particles.

Two populations of dust particles were found during the first phase of operation of WEST. The one that dominates by size and weight comes from the delamination of tungsten coatings covering graphite tiles and the emission of droplets of molten materials during off-normal events. Sizes vary from several microns to tens of microns. More generally, micron-sized dust particles due to the erosion of all materials present in the vacuum vessel were collected. In addition, nanocavities were found at the surface of tungsten dust sampled after He plasmas and were attributed to He trapping in the form of nanobubbles. Tungsten nanoparticles constitute the second unexpected dust population. They are dominant by their number and were essentially found at the surface of micron-sized particles. They may result either from the condensation of an oversaturated vapor above molten tungsten or come from ion-neutral clusters growing in plasma regions of low temperature until the appearance of solid particles.

This paper summarizes the observations on the Unidentified Flying Object (UFO) observed during the first high fluence campaign performed in WEST. The campaign was characterized by high erosion and deposition rates, generating dusts and deposits prone to flaking during plasma operation. A total of 686 UFOs have been detected for 384 discharges executed, in which 133 UFOs lead to a disruption. These UFOs appear mostly during the low hybrid heating ramp-up early in the discharges. The mechanisms leading to a disruption is described as well as the delay available to react after the UFO entering in the plasma. The UFOs originate mostly from the high field side corresponding to the thick deposit area. These UFOs can be debris from a preceding disruption (≈27 %) or come from progressive loss of adherence of the thick deposits (≈55 %) under plasma cycling or more likely by disruption on this area. Finally, the thermal resistance of the poorly attached deposits has been calculated in the range from 2.10−4 °C m2/W to 1.10−3 °C m2/W coherent with previous observations in the high field side of the JET divertor.

The evolution of the tungsten microstructure and properties under helium irradiation could cause problems for both operational and safety reasons in fusion reactors. In particular, the presence of helium bubbles, formed mainly in the subsurface area, modifies the mechanical and retention properties of radioactive elements. In that context we have investigated the shape of helium bubbles in tungsten induced by helium irradiation under well-defined experimental conditions. We have used single crystals with controlled bombardment energy, flux, fluence and post-irradiation annealing at high temperature under ultra-high vacuum in order to get closer to the thermodynamic equilibrium of the helium bubbles. The bubble shape has been characterized by a combination of TEM and 3D reciprocal space map by in-situ GISAXS. We have shown the presence of facetted bubbles exposing {110} and {100} facets of similar size. Using Wulff theorem and experimental results from both techniques, we derive a surface energy ratio of γ100γ110= 1.03 ± 0.03.