Deposition of 13C tracer and impurity elements on the divertor of Wendelstein 7-X

Carbon impurity transport and deposition were investigated in the Wendelstein 7-X stellarator by injecting isotopically labelled methane ((CH4)-C-13) into the edge plasma during the last plasma operations of its Operational Phase (OP) 1.2B experimental campaign. C-13 deposition was measured by secondary ion mass spectrometry (SIMS) on three upper divertor tiles located on the opposite side of the vessel to the(13)CH(4) inlet. The highest C-13 inventories were found as stripe-like patterns on both sides of the different strike lines. These high deposition areas were also analysed for their impurity contents and the depth profiles of the main elements in the layers. Layered deposition of different impurity elements such as Cr, Ni, Mo and B was found to reflect various events such as high metallic impurities during the OP1.2A and three boronizations carried out during OP1.2B.Peer reviewe

Exposure of tungsten heavy alloys at high thermal loads in LHD

Tungsten has been considered a plasma-facing material in a future fusion reactor because of its low sputtering yield and low fuel retention. It has been examined in several tokamaks. In stellarators, it has recently been used for some plasma-facing components. However, in addition to its high cost, W is difficult to machine due to its hardness and brittleness and therefore alternative materials in the form of tungsten heavy alloys are being investigated and some tests have already been performed in the ASDEX upgrade [1]. WNiFe materials are magnetic, but since magnetization saturates at ∼ 2 Tesla for W97NiFe [1], these could also be investigated for use in stellarators. Samples were prepared from pure W, W95NiCu, W97NiFe and W95NiFe alloys. The samples were exposed in the Large Helical Device (LHD) stellarator during three recent operation campaigns. The samples were inserted by means of the divertor manipulator at the positions of the strike line under H-, D- and He plasma conditions. These experiments were designed to test the samples at high thermal loads by adjusting the exposure conditions to achieve sample temperatures above, around and below the melting temperatures of Ni, Fe and Cu. During some of these exposures, although the temperatures reached above the melting limit, resulting in segregation of Ni, Fe and Cu and partial release of alloying materials, normal plasma operation continued without any radiative collapse. Scanning electron microscopy with focused ion beam (SEM/FIB), energy dispersive X-ray spectroscopy (EDX) and glow discharge optical emission spectroscopy (GDOES) measurements confirmed the observed change in surface morphology

Depth resolved analysis of hydrogen in W7-X graphite components using laser-induced ablation-quadrupole mass spectrometry (LIA-QMS)

A deep understanding of the plasma-wall interaction processes in fusion devices like Wendelstein 7-X is necessary for an efficient plasma operation and a long lifetime of the plasma-facing components.In this work we present an approach employing residual gas analysis after picosecond laser-induced ablation (ps LIA-QMS) of graphite limiter tiles, exposed in the first plasma operational phase of Wendelstein 7-X, for depth-resolved and quantitative hydrogen content analysis. A series of poloidal and toroidal locations are analyzed at three of the five limiters, showing up to 2.3 × 1022 hydrogen atoms/m2 in net-deposition areas after a total plasma exposure of about 311 s in mixed hydrogen and helium operation. Shallow implantation of hydrogen is observed in erosion zones, where a low fuel content is present due to the high surface temperature during plasma operation. The hydrogen content spans between (1.1 and 3.7) × 1021 hydrogen atoms/m2 in the net-erosion areas. Moreover, oxygen has been analyzed and its appearance in both the implantation and deposition zone was verified. Results are compared to thermal desorption spectrometry and to simultaneously performed laser-induced breakdown spectroscopy (LIBS) measurements. Keywords: Laser-induced ablation, Quadrupole mass spectrometry, Laser-induced breakdown spectroscopy, Graphite plasma-facing components, Wendelstein 7-X, Limite

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

3-Dimensional (3D) analysis is crucial for many materials and can be used to study their structure and properties. Laser-Induced Breakdown Spectroscopy (LIBS) is a versatile tool to get depth information quickly, but the poor depth resolution and in general a difficult quantification are the two main drawbacks. To solve these problems, a LIBS method based on picosecond-laser pulses is introduced. The ablation depth is measured and associated with the laser pulse number. A series of layer-structured graphite samples was tested by this method and the 2D and 3D layer structures of these samples were identified with a resolution of up to 24 and 102 nm for Mo and C elements under a residual pressure of 1 × 10−5 Pa, respectively. This shows the great potential of picosecond Laser-Induced Breakdown Spectroscopy (ps-LIBS) in the field of depth analysis

Hydrogen content in divertor baffle tiles in Wendelstein 7-X

Plasma–wall interactions play a crucial role for the performance of fusion devices and the lifetime of plasma-facing components. In this work the results of some plasma–wall interaction processes, namely the erosion/deposition pattern and the fuel content of divertor baffle tiles exposed in Wendelstein 7-X in the initial phase of island divertor, are presented. Net-deposition of carbon with co-deposited oxygen and hydrogen is determined on the graphite tiles after about 1 hour of plasma operation in hydrogen and helium in operation phase OP 1.2a. C is predominately a result of net-erosion of the graphite target plates and oxygen is the strongest intrinsic impurity in Wendelstein 7-X in OP 1.2a. The hydrogen content distribution on a set of tiles exposed in equivalent positions in three of the five modules of the stellarator was quantified for the first time. Ex-situ performed laser-induced breakdown spectroscopy measurements show the depth-resolved fuel content in deposited layers as well as implantation and diffusion in the base material. Complementary, gas analysis after laser-induced ablation offers quantitative hydrogen content determination in the deposited layers up to 1018 hydrogen atoms/cm that is non-uniformly distributed over the 95 mm 125 mm sized tiles. The results show a toroidal asymmetry with 60% more hydrogen in the stellarator module 2 with respect to module 1 as well as a slight top-down asymmetry for the baffle tiles in upper and lower half modules. A clear dependence of the hydrogen content on the surface temperature during plasma exposition in stellarator discharges in standard magnetic divertor configuration with edge transformation 5/5 was not observed