Plasma-wall interaction studies in W7-X: main results from the recent divertor operations

Abstract

Wendelstein 7-X (W7-X) is an optimized stellarator with a 3-dimensional five-fold modular geometry. The plasma-wall-interaction (PWI) investigations in the complex 3D geometry of W7-X were carried out by in situ spectroscopic observations, exhaust gas analysis and post-mortem measurements on a large number of plasma-facing components extracted after campaigns. The investigations showed that the divertor strike line areas on the divertor targets appeared to be the major source of carbon impurities. After multistep erosion and deposition events, carbon was found to be deposited largely at the first wall components, with thick deposits of >1 μm on some baffle tiles, moderate deposits on toroidal closure tiles and thin deposits at the heat shield tiles and the outer wall panels. Some amount of the eroded carbon was pumped out via the vacuum pumps as volatile hydrocarbons and carbon oxides (CO, CO2) formed due to the chemical processes. Boron was introduced by three boronizations and one boron powder injection experiment. Thin boron-dominated layers were found on the inner heat shield and the outer wall panels, some boron was also found at the test divertor unit and in redeposited layers together with carbon. Local erosion/deposition and global migration processes were studied using field-line transport simulations, analytical estimations, 3D-WallDYN and ERO2.0 modeling in standard magnetic field configuration.

Distributions of deposits and hydrogen on the upper and lower TDUs3 target elements of Wendelstein 7-X

“…If GDC was the reason for the discretized Fe and Mo deposition peaks, larger depositions should be seen at larger depth. During OP 1.2b, two steel components melting events were observed [1,16]. These results show that the discretized deposition peaks of Fe and Mo are quite likely occurred during OP 1.2b.…”

“…Based on the EDS results shown in figure 3 The small peaks of B concentration in figure 4(b) are related to the boronization history of W7-X. Three boronizations by means of glow discharge with 10% B 2 H 6 + 90% He mixture gas were applied in OP 1.2b which totally introduced 17.86 g B [16]. Furthermore, an in situ B 4 C powder injection was performed before the third boronization by the probe mounted particle injector (PMPI) [17].…”

“…Furthermore, an in situ B 4 C powder injection was performed before the third boronization by the probe mounted particle injector (PMPI) [17]. The amount of injected B by PMPI was 1.57 g [16]. According to figure 4(d) which is an enlargement of figure 4(c), B also exists on the surface of low PSI region.…”

Raman microscopy to characterize plasma-wall interaction materials: from carbon era to metallic walls

“…These observations contributed to conclusion that most of the deuterium trapped in the walls was not trapped on the neutralizers, that were heated close to 1000 °C (as deduced from the shape of the Raman spectra confirming temperature was high during operation) but on some regions of the limiter. Recent results on the stellarator W7-X revealed the presence of deuterated amorphous carbons with similar dispersion in the σ Vs Γ plot, as the one found on the Tore Supra limiter [31]. For beryllium, the trends are different: all data, regardless if they come from the JET tokamak or are reference materials synthetized in laboratory [49,68,73,74] [6], are localized in roughly the same region of the band position with respect to the band width.…”

“…Linear plasma devices are also of interest as they deliver intermediate flux between laboratory plasma and tokamaks [24][25][26]. Post-mortem analyses are also used to obtain a before/after comparison and understand how elements migrate inside the nowadays machines [14,[27][28][29][30][31][32][33][34][35]. Ion beam analyses [36] and thermal desorption spectroscopy are key techniques that give absolute and relative content.…”