Material erosion and deposition on the divertor of W7-X

Abstract

The net erosion and deposition pattern of carbon from the Test Divertor Unit (TDU) of the stellarator W7-X was determined. Special target elements with marker layers consisting of about 300 nm molybdenum and 5-10 µm carbon on top were used during the operation phase OP 1.2a. The thicknesses of the marker layers were determined by elastic backscattering spectrometry (EBS) using 2.5 MeV protons before and after plasma exposure and laser-induced breakdown spectroscopy (LIBS) on selected target elements after exposure. Scanning electron microscopy (SEM) was used for investigating the surface morphology before and after exposure. Massive erosion of up to 20 µm carbon was observed at the strike line, in total 48±14 g carbon were eroded from the 10 TDUs. The erosion was laterally nonuniform on the micro-scale. Strongly eroded surfaces were considerably smoother as compared to the original material. Only very little deposition of carbon is observed on the TDU: This means that the TDU is a large net erosion source.

3-Dimensional analysis of layer structured samples with high depth resolution using picosecond laser-induced breakdown spectroscopy

“…These results consequently contained information in poloidal and depth direction, which makes this method a 2D analysis. Similar erosion results of graphite and Mo were obtained in the EBS results, both the results showed that the erosion dominated area locates in the B3 sample [27]. Figure 8 shows depth information in poloidal and toroidal direction, which makes this method a 3D analysis.…”

Overview of the results from divertor experiments with attached and detached plasmas at Wendelstein 7-X and their implications for steady-state operation

“…W7-X plasma-facing components in the previous campaigns were made predominantly from graphite, making them prone to erosion due to plasma-wall interactions. Almost 50 g of carbon has been accounted for after the first half of the last campaign with a total plasma duration of nearly 3800 s [36]. During the second half of the campaign boronization of the first wall widened the operational window [37,38].…”

Inspection of W 7-X plasma-facing components after the operation phase OP1.2b: observations and first assessments

“…A detailed inspection of the PFCs was carried out after the completion of OP1.2b and a number of observations about melting, depositions, arcing and loosely bound particles were made. A small part of the eroded 48 g C from the divertor strike line positions [5] was found to be deposited on the vessel wall, inner divertor closures and the panels. From the estimated small amount of depositions it is still difficult to establish the C balance, for which the investigation of a large number of tiles and the estimation of the pumped out fractions through the vacuum pumps would be necessary, which is presently ongoing.…”

“…The beam spot diameter was about 1.8 mm. A Passivated Implanted Planar Silicon (PIPS) detector with 300 µm thickness and about 12 keV nominal energy resolution was used [5]. Nuclear Reaction Analysis (NRA) was carried out with incident 2.5 MeV 3He ions using the 12 C( 3 He,p0) 14 N and 10 B( 3 He,p1) 12 C reactions.…”