Scanning electron microscopy analyses of an ITER plasma-facing unit mockup exposed to extreme ion fluences in Magnum-PSI

Abstract

A small-scale tungsten monoblock mockup was exposed to continuous Magnum-PSI plasma beams for 6 different condition: including pure H, D, and He plasmas as well as mixed D/He plasmas with low electron temperature (1.5-5 eV), with maximum surface temperatures up to 1600 °C, and ion fluences up to 10 30 m -2 . The recrystallization in the centre of the hottest exposure spots, the absence of cracking on all monoblocks, the formation of a several microns thick nanostructured fuzz layer by pure He exposure, and the presence of various impurities from the exposure in Magnum-PSI on the surface were observed and analysed with a dedicated scanning electron microscope able to handle the entire mockup without any prior cutting.

Accelerated/reduced growth of tungsten fuzz by deposition of metals

“…<1000 K, and fuzz was not formed at the peripheral region. Concerning the local isolated fuzzy islands, similar isolated fuzz balls 310 have been identified previously after a long time irradiation [18,31]. In this work, the fraction of Mo was found to be higher on the location where fuzzy islands were formed from the EDS analysis.…”

“…On the other hand, h FN of D1 and D2 was greater than the cases without deposition at the similar fluences. In the PISCES-B and Magnum-PSI devices, respectively, 6.5 µm thick and 7.5 µm thick fuzzy layers have been identified previously [18,6]. However, the He fluence was about two orders of magnitudes greater than that of D1 and D2 (1×10 28 m −2 in the PISCES-B and 3×10 28 m −2 in the Magnum-PSI).…”

“…Also, it has been revealed that mm-thick large-scale fiberform nanostructures (LFNs) can be formed at an accelerated growth rate with an additional deposition [16,17]. In the Magnum-PSI device, plasma irradiations to a W monoblock mockup have revealed that metallic impurities from the source led to some unexpected phenomena such as an enhanced diffusion of molybdenum (Mo) along the grain boundaries and sponge-like tin structures [18]. In short, the effects of deposition of metals on He plasma induced morphology changes have not been fully understood, and they will be important to assess the impact of He effects in fusion devices.…”

“…It is understandable that no changes occurred on W1, because the incident ion energy, E i , should be lower than the threshold energy for fuzz growth of ∼20 eV. The threshold energy could be lower than 20 eV especially when the He fluence is high [3,18], but E i on W1 was likely too low to lead Table 1: Experimental conditions for the preparation of the samples used in this study. The shown parameters are as follows: the sample potential (V W ), the surface temperature (T s ), the potential of the sputtering plate (V spu ), duration, and He fluence.…”

Latest results of Eurofusion plasma-facing components research in the areas of power loading, material erosion and fuel retention

“…Tests of ITER-like MBs at elevated temperature were performed in high fluence experiments in MAGNUM-PSI [52,54] mission from [52]-figure 1). The target used in this study is a mock-up consisting of 7 MBs attached to a cooling tube.…”