29 research outputs found
Final Acceptance Tests of Helium Refrigerator for Wendelstein 7-X
No full textAbstractFollowing successful installation and commissioning, final acceptance tests were carried out on the helium refrigerator for Wendelstein 7-X.The tests were carried out for the normal operating modes i.e. peak power mode (3.4K), standard mode(3.9K), short standby mode (< 10K) and long standby mode (< 100K).Besides the normal modes, the transient modes including, cool-down and warm-up, auto interchanging between various modes and the purging modes were tested.In addition, the handling of quench, emergency signals such as cryostat vacuum break,utility failure, alarm and trip signals were checked
Distributions of deposits and hydrogen on the upper and lower TDUs3 target elements of Wendelstein 7-X
No full text0000-0001-5089-3642Distributions of deposits and hydrogen (H) on the graphite divertor target elements TM4h4 and TM3v5 in the test divertor units 3 (TDUs3) of Wendelstein 7-X (W7-X) are studied. The TM4h4 and TM3v5 are located at the magnetically symmetric positions in the upper and lower divertor. The microstructure of the deposition layer is characterized by a transmission electron microscope (TEM) combined with a focused ion beam (FIB). Metallic deposits such as iron (Fe), molybdenum (Mo), chromium (Cr) are detected in the deposition layer by energy-dispersive x-ray spectroscopy (EDS). The depth-resolved distribution patterns of boron (B) and metallic deposits on upper and lower horizontal (h) divertor target elements TDUs3-TM4h4 as well as upper and lower vertical (v) divertor target elements TDUs3-TM3v5 are clarified by glow discharge optical emission spectrometry (GDOES). Results for both TDUs3-TM4h4 and TDUs3-TM3v5 show that the B deposition regions exhibit higher H retention due to the co-deposition with deposits. On the other hand, up-down asymmetries in B deposition caused by particle drift exist on both TDUs3-TM4h4 and TDUs3-TM3v5. The B deposition amount on upper TDUs3-TM4h4 is 40% smaller than that on lower TDUs3-TM4h4. While for the vertical target elements, the B deposition amount on upper TDUs3-TM3v5 is 35% larger than that on lower TDUs3-TM3v5. Meanwhile, a shift of around 3 cm in B deposition peaks is observed on upper and lower TDUs3-TM4h4 and TDUs3-TM3v5. Results of numerical simulation of carbon deposition/erosion profiles on the target elements using ERO2.0 code and power flux measured by infrared cameras are shown and compared with the above mentioned B profiles.journal articl
In-vessel inspection of arc traces on the W7-X plasma facing components after the operation phase OP2.1
No full textUpdated distribution of arc traces across the interior walls after the operation phase 2.1 of Wendelstein 7-X (W7-X) is reported. A total of 105 new arc places were found, with 95% of them located around the stainless steel panels, diagnostic ports, and the glow discharge cleaning electrodes. Remaining 5% of the traces were found on carbon surfaces such as the graphite baffle and carbon fiber-reinforced composites divertor tiles, for the first time in W7-X. The arcing on the divertor tiles showed a retrograde motion, suggesting that the arcing must have occurred during the main plasma operations. A simple estimate of the mass loss due to arcing on the divertor area shows that ∼0.2-0.3 g of carbon was eroded during OP2.1. This amount turns out to be a significantly smaller amount compared to the estimated losses due to sputtering, ∼7.6 g/1800 s during OP1.2b (Brezinsek et al., 2021)
Mapping deposition pattern on Wendelstein 7-X stainless steel components by picosecond laser-based techniques
No full textMapping the deposition pattern on Wendelstein 7-X stainless steel components by picosecond laser-based techniquesD. Zhao a, b, J. Oelmann a, R. Yi a, S. Brezinsek a, M. Rasinski a, T. Dittmara, M. Mayer c, C.P. Dhard d, D. Naujoks d and the W7-X team daForschungszentrum Jülich, Institut für Energie- und Klimaforschung - Plasmaphysik, Partner of the Trilateral Euregio Cluster (TEC), 52425 Jülich, GermanybSouthwestern Institute of Physics, P.O.Box 432, Chengdu, Sichuan, 610041, ChinacMax-Planck-Institut für Plasmaphysik, 85748 Garching, GermanydMax-Planck-Institut für Plasmaphysik, 17491 Greifswald, GermanyE-mail address: [email protected] Wendelstein 7-X, the largest operating stellarators in the world, is equipped with plasma-facing components (PFCs) made of graphite and stainless steel panels in recessed areas of the vessel. The migration of once eroded C from PFCs towards recessed areas is a critical issue as thick and potentially unstable layers can be build-up in the upcoming steady-state operation of the W7-X in the near future when the actively cooled divertor is installed. Here, we analyse a stainless steel sample retrieved from the divertor closure area after the first phase of operation with uncooled graphite divertor accumulating about 3h plasma time in the first two campaigns.The sample, which was located behind the test divertor unit in half-module 21, was extracted from the vessel and investigated ex-situ by picosecond laser-based techniques including laser-induced breakdown spectroscopy (LIBS) and laser-induced ablation- quadrupole mass spectrometry (LIA-QMS). The distribution of deposited elements on the sample was analysed with high depth resolution and mapped in lateral direction by LIBS. From the spectra analysis, carbon (C) and hydrogen (H) were identified in the deposited layer. With increasing laser shot numbers at the same location, iron (Fe) lines appeared in the spectrum from the substrate material – the stainless steel. Moreover, smaller amounts of boron were identified in the near surface, resulting from three boronisations conducted during the second operation phase of W7-X. Thus, a carbon-rich layer with a non-uniformly distributed deposition pattern on the steel target build up during plasma operation. The H and C distribution is strongly correlated: the H concentration correlates with the thickness of the deposited layer and results from co-deposition processes. The deposition thicknesses were determined by using the laser ablation rates of a deposition C layer and the steel sample multiplied with the number of ablation pulses. The deposition thickness of the C-rich layer determined by LIBS varies spatially between 21 ± 3 nm and 262 ± 20 nm. Complementary, focused ion beam combined with scanning electron microscopy (FIB-SEM) as well as ellipsometry techniques were employed to verify the deposition thicknesses at the deposition zones as well as identify the H to C ratio of the optical thin C layer. These two techniques confirm in principle the LIBS results with a deposition thicknesses between 25 nm and 263 nm at the light deposition zone and the strong deposition zone, respectively. Thus, the FIB-SEM, ellipsometery and LIBS are in quantitative agreement regarding the thickness. Complementary, LIA-QMS was applied to determine independently the absolute H content at different lateral positions. The results of LIA-QMS were in good agreement with the quantitative LIBS signals for hydrogen. The analyses show that LIBS and LIA-QMS can be used to map the deposition distribution and determine the implantation of H content, which is of great interest to study the plasma-wall interaction in W7-X for further operational phases
Assessment of carbon net erosion/deposition at the divertor of W7-X
No full textThe net carbon erosion rate at the divertor strike line of W7-X was determined experimentally during the operational phases OP 1.2a and OP 1.2b [M. Mayer et al. Phys. Scr. T171 (2020) 014035; M. Mayer et al., Nuclear Fusion 62 (2022) 126049]. OP 1.2a was characterized by high concentrations of oxygen and a very high net carbon erosion rate. The oxygen concentration decreased by 1–2 orders of magnitude in OP 1.2b due to boronizations, and the experimentally observed erosion rate decreased by a factor of 5–6. The carbon erosion rate at the W7-X divertor is calculated using an analytical erosion model taking physical and chemical erosion by hydrogen, carbon and oxygen impact into account. Experimentally determined plasma parameters from several discharges and experimental surface roughnesses were used for the simulations. The calculated erosion rates for the selected discharges are in reasonable agreement with the experimental values. According to the calculations, during OP 1.2a carbon erosion at the strike line was dominated by chemical erosion by oxygen. The decrease of the oxygen impurity concentration in OP 1.2b decreased the carbon erosion rate by a factor of about five, which is in good agreement with the experimental values. During OP 1.2b carbon erosion was dominated by sputtering by incident hydrogen and carbon ions. The surface morphology has a profound influence on the net carbon erosion rate and develops during a campaign by erosion/deposition phenomena. Plasma-exposed surfaces get smoother with plasma exposure time, which can result in an increase of the erosion yield
Numerical simulation of neutral gas dynamics in the W7-X sub-divertor
No full textThe present work presents a 2D and 3D modeling of the neutral gas flow in the sub-divertor region of the W7-X. The investigations have been done using the DIVGAS code. The complex 2D and 3D geometries of the divertor components in the sub-divertor region have been considered and the Standard and High-Iota magnetic configurations have been numerically simulated. The main objective of this study is to investigate the dynamics of neutral particles in the sub-divertor region including the effects due to geometry and toroidal and poloidal leakages located at the divertor targets and baffles on the achieved pumping efficiency. A sensitivity analysis has been performed for the effect of various geometrical and flow parameters on the pumping performance, under different plasma scenarios. The considered incoming fluxes in the sub-divertor range between 10 ^20 to 10 ^22 (H _2 s ^−1 ). The main conclusions, which can be extracted from the present numerical analysis could be summarized as follows; a large fraction of incoming neutral particle flux i.e. >70% on the low iota side and >40% for the high iota side is leaked back to the main divertor region, while higher incoming neutral fluxes facilitate the increase of the pumped flux as well as the decrease of the outflux. It has been estimated that a small fraction ∼3%–4% of the incoming neutral flux is being pumped via the turbo-molecular pumps. The closure of the toroidal leakages as well as the inclination of the pumping gap panel by 9 ^o facilitate the increase of the pumped flux, but considering the all the engineering constraints, the latter option seems to be more easy to be implemented. For low incoming neutral fluxes (∼10 ^20 H _2 s ^−1 ) and for the case of AEH section, free molecular flow conditions are estimated and therefore neutral-neutral collisions could be neglected. For higher incoming neutral fluxes and for both AEH and AEP sections neutral-neutral collisions play a significant role in the flow establishment. A comparison with available experimental measurements of the neutral pressure in the sub-divertor has been performed for Standard and High-Iota plasma discharges. The 3D DIVGAS simulations predict qualitatively the experimental data with relative deviation between 25 and 63%. All the above numerical findings will actively support the optimization of the W7-X particle exhaust, in view of the experimental campaign OP2
Crystal-orientation-dependent physical sputtering from four elemental metals
No full textPhysical sputtering and its dependence on crystal orientation is well-known, but crystallinity of the target is neither commonly considered in simulations nor when publishing experimental values for its yield. Due to the recent development of detector technology and therefore of crystal orientation mapping, a measuring technique has been established to obtain the physical sputtering yield for an extensive number of orientations of polycrystalline specimens. In this work, yields for two bcc metals (W and Mo) and two fcc metals (Cu and Pt) for impacting Ga ions with 30 keV and around 5 keV energy were determined experimentally and are compared with molecular dynamics (MD) simulations in a recently established two level approach and with binary collision approximation (BCA) simulations. The agreement between experimental yields and simulations is excellent regarding distribution in the angular space and reasonable regarding absolute values. As expected, the variation in angular space among the materials with the same crystal lattice structure is minor.Peer reviewe
In-vessel colorimetry of Wendelstein 7-X first wall components after OP2.1
No full textColorimetry has been continuously utilized for the estimation of deposition layer distribution on the first wall panels and divertor target elements in each Operation Phases (OP) of Wendelstein 7-X (W7-X). In OP2.1, significant achievements were made in plasma-wall interaction studies and divertor performance, completed by the installation of actively cooled divertors together with the change of material from fine grain graphite to Carbon Fiber-reinforced Composite. This upgrade enabled the achievement of long-duration plasma discharges, reaching up to 500 s with a total injected energy of 1.3 GJ. Compared to a factor of 2.5 increase in the net average deposition thickness between OP1.2a and 1.2b, no significant change was observed between OP1.2b and OP2.1. The balance of deposition and erosion of deposition layer on the first wall panels is presumably changed in OP2.1. The considerable reasons are discussed qualitatively from the point of possibilities such as lower surface temperature of the divertor target elements, lower impurity level of bulk plasmas, and difference of the location of sputtered carbon source. Moreover, the clear pattern of the colorimetry on the divertor target elements is seen in OP1.2a and OP2.1 not in OP1.2b. However, it is not currently conclusive whether changes in reflectivity and optical properties on the graphite divertor surface are due to erosion or deposition
