1,056 research outputs found
J. Likonen
No full textthis paper is to explain high concentration Si diffusion in GaAs. The implantation dose used in this study is considerably higher than in earlier studies and the annealings also extend to lower temperatures. The solid solubility of substitutional Si in GaAs as a function of temperature is given, and the observed exponential behavior is explained qualitatively
Corrigendum to “Thermal desorption spectrometry of beryllium plasma facing tiles exposed in the JET tokamak” (Fusion Engineering and Design (2018) 133 (135–141),
No full textA mistake regretfully appears in Section 3.2 of the paper in the Fig. 4a and b. The authors make a sincere apology for the mistake and would like to apologise for any inconvenience caused. During preparation of the plot for the publication a mistake had occurred in the y-axis unit. This has been attributed to a typo error in the y-axis value column used in the preparation of the plot. Therefore, the y-axis value is in wrong decade, it should appear in 1014instead of 1015. The values from the plot were not used in any calculations presented in the paper therefore the change does not affect any conclusions of the paper. Fig. 4(a) and (b) should be as follows: [Figure presented]</p
Deuterium loading of redeposited-like W coatings present in tokamaks by ion implantation
No full textPulsed Laser deposition (PLD) is a suitable technique to reproduce W, W–O and W–N–O coatings resembling those present in tokamaks walls, as protective W coatings covering plasma facing components (PFCs) or W-based layers redeposited in PFCs during reactor operation. Nevertheless, difficulties still exist to codeposit deuterium in all such layers, and parallel methods need to be implemented to load it. W-based coatings with porous and columnar microstructures grown by PLD where loaded with flat depth contents of deuterium down to 0.4–0.6 μm using multiple ion implantation steps at distinct incident energies and fluences. Deuterium amounts close to 7 at.% were easily achieved. Similar deuterium contents are commonly observed in deposits in now-a-days tokamaks. Microstructural characterization of the coatings was carried out by scanning electron microscopy (SEM). Quantitative elemental analysis of as-deposited and as-implanted materials was accessed by elastic backscattering spectroscopy (EBS), nuclear reaction analysis (NRA) and by time-of-flight elastic recoil detection (ToF-ERDA). Secondary ion mass spectrometry (SIMS) revealed the depth profiles of the existing isotopes and, particularly, of retained deuterium. Beyond their use in ex-situ experiments, the production is useful to calibrate laser induced breakdown spectroscopy (LIBS) and SIMS setups, aiming a standard free depth-quantification of deuterium in W-based coatings
Material migration and fuel retention studies during the JET carbon divertor campaigns
No full textThe first divertor was installed in the JET machine between 1992 and 1994 and was operated with carbon tiles and then beryllium tiles in 1994–5. Post-mortem studies after these first experiments demonstrated that most of the impurities deposited in the divertor originate in the main chamber, and that asymmetric deposition patterns generally favouring the inner divertor region result from drift in the scrape-off layer. A new monolithic divertor structure was installed in 1996 which produced heavy deposition at shadowed areas in the inner divertor corner, which is where the majority of the tritium was trapped by co-deposition during the deuterium-tritium experiment in 1997. Different divertor geometries have been tested since such as the Gas-Box and High-Delta divertors; a principle objective has been to predict plasma behaviour, transport and tritium retention in ITER. Transport modelling experiments were carried out at the end of four campaigns by puffing 13C-labelled methane, and a range of diagnostics such as quartz-microbalance and rotating collectors have been installed to add time resolution to the post-mortem analyses. The study of material migration after D-D and D-T campaigns clearly revealed important consequences of fuel retention in the presence of carbon walls. They gave a strong impulse to make a fundamental change of wall materials. In 2010 the carbon divertor and wall tiles were removed and replaced with tiles with Be or W surfaces for the ITER-Like Wall Project
Deuterium retention in the divertor tiles of JET ITER-Like wall
No full textDivertor tiles removed after the second JET ITER-Like Wall campaign 2013–2014 (ILW-2) were studied using Secondary Ion Mass Spectrometry (SIMS). Measurements show that the thickest beryllium (Be) dominated deposition layers are located at the upper part of the inner divertor and are up to ∼40µm thick at the lower part of Tile 0 exposed in 2011–2014. The highest deuterium (D) amounts (>8 · 1018at./cm2), in contrast, were found on the upper part of Tile 1 (2013–2014), where the Be deposits are ∼10µm thick. D was mainly retained in the near-surface layer of the Be deposits but also deeper in tungsten (W) and molybdenum (Mo) layers of the marker coated tiles, especially at W–Mo layer interfaces. D retention for the ILW-2 divertor tiles is higher than for the first campaign 2011–2012 (ILW-1) and probable reasons for the difference are that SIMS measurements for the ILW-2 samples were done deeper than for the ILW-1 samples, some of the tiles were exposed during both ILW-1 and ILW-2 and therefore had a longer exposure time, and the differences between ILW-1 and ILW-2 campaigns e.g. in strike point distributions and injected powers. Keywords: JET, Fuel retention, Erosion, Depositio
Erosion and deposition in the JET divertor during the second ILW campaign
No full textErosion of plasma-facing materials and successive transport and redeposition of eroded material are crucial processes determining the lifetime of plasma-facing components and the trapped tritium inventory in redeposited material layers. Erosion and deposition in the JET divertor were studied during the second JET ITER-like wall campaign ILW-2 in 2013-2014 by using a poloidal row of specially prepared divertor marker tiles including the tungsten bulk tile 5. The marker tiles were analyzed using elastic backscattering with 3-4.5 MeV incident protons and nuclear reaction analysis using 0.8-4.5 MeV He-3 ions before and after the campaign. The erosion/deposition pattern observed during ILW-2 is qualitatively comparable to the first campaign ILW-1 in 2011-2012: deposits consist mainly of beryllium with 5-20 at.% of carbon and oxygen and small amounts of Ni and W. The highest deposition with deposited layer thicknesses up to 30 mu m per campaign is still observed on the upper and horizontal parts of the inner divertor. Outer divertor tiles 5, 6, 7 and 8 are net W erosion areas. The observed D inventory is roughly comparable to the inventory observed during ILW-1. The results obtained during ILW-2 therefore confirm the positive results observed in ILW-1 with respect to reduced material deposition and hydrogen isotopes retention in the divertor
LIBS diagnostics of Be-based samples with different gas impurities
No full textControlling plasma fuel retained in the plasma facing components of the first wall of a fusion reactor is one of the most important challenges influencing safe operation of the International Thermonuclear Experimental Reactor in the first place. This issue is proposed to be addressed by the laser-induced breakdown spectroscopy (LIBS) diagnostics, which is particularly powerful in studying the near-surface deposits and analyzing their composition. The main goal of the present study is determining the depth profiles of different elements in beryllium-based materials and the possible co-deposited layers that are formed on the walls of the Joint European Torus (JET) fusion device. Depth profiles estimated by LIBS are compared with those measured by secondary ion mass spectrometry, furthermore, the differences are discussed. In particular, the evolution of spectral lines of Be, as well as the main gaseous elements, such as Ne, N, O, and D, incorporated into the samples were extracted at different depths in the layers. LIBS diagnostics allowed making a fairly accurate analysis of the detected spectral lines of the elements on the samples. The effect of variations of the ablation rate and uncertainty that it introduces in LIBS measurements was also discussed. This investigation will have a significant impact on the development of pre-processing algorithms for machine learning models in terms of adaptation models operating on synthetic data for processing experimental spectra and is important from a point of view of LIBS tests being under preparation at JET
Laser-induced breakdown spectroscopy for helium detection in beryllium coatings
No full textLaser Induced Breakdown Spectroscopy (LIBS) method is considered to be a promising tool for analyzing the retention of hydrogen isotopes (D and T) and helium (He) on the first walls and divertor regions of future fusion reactors. Helium will be produced in DT reactions but could also be used in the initial non-nuclear phases of DEMO concepts. The present study investigates the He detection by LIBS method in the Be coatings simulating the deposits on the divertor plasma-facing components of JET while the results are also relevant for He detection in the deposits of other wall materials. The study was carried out in a vacuum vessel filled with 2–40 mbar argon background gas. It was shown that 2.8 at. % of He was confidently detectable by LIBS at optimized measurement conditions and the estimated limit of detection at used experimental conditions is approximately 0.7 at. %. The intensity of the He emission line at 587.56 nm was the strongest at the center of the laser-induced plasma plume. The He line intensity increased with the pressure of Ar gas but the broadening of the He line and the increase of the background emission and noise set an upper limit to the Ar background pressure usable for He detection. The application of the calibration-free LIBS procedure resulted in the overestimation of the He/Be ratio by several orders of magnitude. The overestimation can be explained by the deviation of LIBS plasma from the local thermodynamic equilibrium, which is caused by the very high excitation energy of He atoms
Thermal desorption spectrometry of beryllium plasma facing tiles exposed in the JET tokamak
No full textThe phenomena of retention and de-trapping of deuterium (D) and tritium (T) in plasma facing components (PFC) and supporting structures must be understood in order to limit or control total T inventory in larger future fusion devices such as ITER, DEMO and commercial machines. The goal of this paper is to present details of the thermal desorption spectrometry (TDS) system applied in total fuel retention assessment of PFC at the Joint European Torus (JET). Examples of TDS results from beryllium (Be) wall tile samples exposed to JET plasma in PFC configuration mirroring the planned ITER PFC is shown for the first time. The method for quantifying D by comparison of results from a sample of known D content was confirmed acceptable. The D inventory calculations obtained from Ion Beam Analysis (IBA) and TDS agree well within an error associated with the extrapolation from very few data points to a large surface area
Impurity re-distribution in the corner regions of the JET divertor
No full textThe International Thermonuclear Experimental Reactor (ITER) will use a mixture of deuterium (D) and tritium (T) as the fuel to generate power. Since T is both radioactive and expensive the Joint European Torus (JET) has been at the forefront of research to discover how much T is used and where it may be retained within the main reaction chamber. Until the year 2010 the JET plasma facing components were constructed of carbon fibre composites. During the JET carbon (C) phases impurities accumulated at the corners of the divertor located towards the bottom of the chamber in regions shadowed from the plasma where they are very difficult to reach and remove. This build-up of C and the associated H-isotope (including T) retention were of particular concern for future fusion reactors therefore, in 2010 JET changed the wall protection to (mainly) Be and the divertor to tungsten (W)-the JET ITER-like wall (ILW)-the choice of materials for ITER. This paper reveals that with the JET ILW impurities are still accumulating in the shadowed regions, with Be being the majority element, though the overall quantities are very much reduced from those in the C phases. Material will be transported into the shadowed regions principally when the plasma strike points are on the corner tiles, but particles typically have about a 75% probability of reflection from line-of sight surfaces, and multiple reflection/scattering results in deposition over all surfaces
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