Preparation of tungsten coatings on graphite by electro-deposition via Na2WO4–WO3 molten salt system

Tungsten coating on graphite substrate is one of the most promising candidate materials as the ITER plasma facing components. In this paper, tungsten coatings on graphite substrates were fabricated by electro-deposition from Na₂WO₄–WO₃ molten salt system at 1173 K in atmosphere. Tungsten coatings with no impurities were successfully deposited on graphite substrates under various pulsed current densities in an hour. By increasing the current density from 60 mA cm⁻² to 120 mA cm⁻² an increase of the average size of tungsten grains, the thickness and the hardness of tungsten coatings occurs. The average size of tungsten grains can reach 7.13 μm, the thickness of tungsten coating was in the range of 28.8–51 μm, and the hardness of coating was higher than 400 HV. No cracks or voids were observed between tungsten coating and graphite substrate. The oxygen content of tungsten coating is about 0.022 wt%.     

An advance process of aluminum rich coating as tritium permeation barrier on 321 steel workpiece

We have proposed an advance three-step process, Al-electroplating in ionic liquid followed by heat treating and selectively oxidation, preparing aluminum rich coating as tritium permeation barrier (TPB). In present work, the advance process was applied to 321 steel workpieces. In the Al-electroplating, pieces were coated by galvanostatic electrodeposition at 20 mA/cm² in aluminum chloride (AlCl₃)–1-ethyl-3-methylimidazolium chloride (EMIC) ionic liquid. The Al coating on those pieces all displayed attractive brightness and well adhered to surface of pieces. Within the aluminizing time from 1 to 30 h, a series of experiments were carried out to aluminize 321 steel pieces with Al 20 μm coating at 700 °C. After heat treated for 8 h, a 30 μm thick aluminized coating on piece appeared homogeneous, free of porosity, and mainly consisted of (Fe, Cr, Ni)Al₂, and then was selectively oxidized in argon gas at 700 °C for 50 h to form Al₂O₃ scale. The finally fabricated aluminum rich coating, without any visible defects, had a double-layered structure consisting of an outer γ-Al₂O₃ layer with thickness of 0.2 μm and inner (Fe, Cr, Ni)Al/(Fe, Cr, Ni)₃Al layer of 50 μm thickness. The deuterium permeation reduction factor, PRF, of piece (Φ 80 × 2, L 150 mm) with such coating increased by 2 orders of magnitude at 600–727 °C. The reproducibility of the process was also showed.     

Simulation of residual thermostress in tungsten after repetitive ELM-like heat loads

Brittle destruction of tungsten armour under action of edge localised modes of plasma instabilities (ELMs) in ITER is an important issue determining the lifetime of the divertor. Besides, cracking of the armour produces tungsten dust with characteristic size of 1–10 μm flying from the armour surface with velocities up to 10 m/s. Influx of the tungsten dust into the ITER confinement decreases the temperature of the plasma, reduces the thermonuclear gain and even may run the confinement into disruption.This paper describes experiments in QSPA-Kh50 plasma gun and modeling, which has been performed for providing more insight into the physics of tungsten cracking under action of ELMs and for confirmation of the important result on stabilization of the crack development at the tungsten armour surface, predicted in our previous paper – the same authors, 2010.The threshold value of energy density deposition for start of tungsten cracking has been measured as 0.3 MJ/m² after 5–10 shots. From analytical considerations three times smaller threshold value has been predicted with increasing number of shots.     

Tungsten coating prepared on V-4Cr-4Ti alloy substrate by electrodeposition from molten salt in air atmosphere

Tungsten coatings were prepared by electrodeposition on vanadium alloy (V-4Cr-4Ti) substrate from Na₂WO₄-WO₃ molten salt at 1173 K in atmosphere. The crystal structure, microstructure, microhardness, oxygen content of the coating as well as bond strength between coatings and substrates were investigated. Pure and compact tungsten coatings were successfully obtained with columnar structure. The tungsten coatings thickness is 505 μm and the oxygen weight ratio is 0.025 wt.%, when the electrodeposited time is 80 h. The tungsten coatings adhesive strength determined by tensile test exceeds 39.28 MPa.     

Powder Injection Molding – An innovative manufacturing method for He-cooled DEMO divertor components

At Karlsruhe Institute of Technology (KIT), a He-cooled divertor design for future fusion power plants has been developed. This concept is based on the use of modular cooling fingers made from tungsten and tungsten alloy, which are presently considered the most promising divertor materials to withstand the specific heat load of 10 MW/m². Since a large number of the finger modules (n > 250,000) are needed for the whole reactor, developing a mass-oriented manufacturing method is indispensable. In this regard, an innovative manufacturing technology, Powder Injection Molding (PIM), has been adapted to W processing at KIT since a couple of years. This production method is deemed promising in view of large-scale production of tungsten parts with high near-net-shape precision, hence, offering an advantage of cost-saving process compared to conventional machining.The complete technological PIM process for tungsten materials and its application on manufacturing of real divertor components, including the design of a new PIM tool is outlined and, results of the examination of the finished product after heat-treatment are discussed. A binary tungsten powder feedstock with a solid load of 50 vol.% was developed and successfully tested in molding experiments. After design, simulation and manufacturing of a new PIM tool, real divertor parts are produced. After heat-treatment (pre-sintering and HIP) the successful finished samples showed a sintered density of approximately 99%, a hardness of 457 HV0.1, a grain size of approximately 5 μm and a microstructure without cracks and porosity.     

Development of W coatings for fusion applications

The paper gives a short overview on tungsten (W) coatings deposited by various methods on carbon materials (carbon fibre composite – CFC and fine grain graphite – FGG). Vacuum Plasma Spray (VPS), Chemical Vapor Deposition (CVD) and Physical Vapor Deposition (PVD) techniques are analyzed in respect with the characteristics and performances of the W coatings.A particular attention is paid to the Combined Magnetron Sputtering and Ion Implantation (CMSII) technique, which was developed during the last 4 years from laboratory to industrial scale and it is successfully applied for W coating (10–15 μm and 20–25 μm) of more than 2500 tiles for the ITER-like Wall project at JET and ASDEX Upgrade. This technique involves simultaneously magnetron sputtering and high energy (tens of keV) ion implantation. Due to the ion bombardment a stress relief occurs within the coating enabling its growth without delamination to a relatively large thickness. In addition, in order to adjust the thermal expansion mismatch between CFC and W, a Mo interlayer of 2–3 μm is currently used. Experimentally, W/Mo coatings with a thickness up to 50 μm were produced and successfully tested in the GLADIS ion beam facility up to 23 MW/m².     

Tungsten and CFC degradation under combined high cycle transient and steady state heat loads

Within the ITER divertor lifetime millions of transient events are expected during H-mode operation due to edge localized modes (type I ELMs). These will deposit their energy on plasma facing materials that are pre-heated to various surface temperatures, depending on the steady state heat load (SSHL) at the respective location, leading to synergistic effects. An electron beam facility was used to simulate ELM-like heat loads with ITER relevant power densities (≈0.5 GW/m²) and pulse duration (0.5 ms). At the same time additional SSHL was applied to obtain different base temperatures. Experiments were performed on actively cooled pure tungsten and the carbon fiber composite (CFC) NB41, applying 10³–10⁶ pulses of 0.5 ms duration with a power density of 0.14–0.55 GW/m² and 0.55–0.68 GW/m² on tungsten and CFC, respectively. Surface temperatures were about 200 °C, 400 °C and 700 °C for tungsten and about 450 °C for CFC. Crack formation in tungsten was preceded by roughening due to plastic deformation. In case of T_surf ≈ 200 °C cracks propagated comparably fast (brittle material), while slow propagation and recrystallization around the crack edges indicated fatigue damage at higher temperatures. Compared to tungsten, CFC showed a higher damage threshold.     

Electro-deposition metallic tungsten coatings in a Na2WO4–WO3 melt on copper based alloy substrate

The tungsten coating was prepared by electro-deposition technique on copper alloy substrate in a Na₂WO₄–WO₃ melt. The coating's surface and cross-section morphologies as well as its impurities were investigated by XPS, SEM and line analysis. Various plating durations were investigated in order to obtain an optimal coating's thickness. The results demonstrated that the electro-deposited coating was compact, voidless, crackless and free from impurities. The tungsten coating's maximum Vickers hardness was measured to be 520 HV. The tungsten coating's minimum oxygen content was determined to be 0.018 wt%. Its maximum thickness was measured to be 1043.67 μm when the duration of electrolysis was set to 100 h. The result of this study has demonstrated the feasibility of having thicker tungsten coatings on copper alloy substrates. These electrodeposited tungsten coatings can be potentially implemented as reliable armour for the medium heat flux plasma facing component (PFC).     

The effect of displacement damage on deuterium retention in ITER-grade tungsten exposed to low-energy,high-flux pure and helium-seeded deuterium plasmas

Samples prepared from polycrystalline ITER-grade tungsten were damaged by irradiation with 20 MeV W ions at room temperature to a fluence of 1.4 × 10¹⁸ W/m². Due to the irradiation, displacement damage peaked near the end-of-range, 1.35 μm beneath the surface, at 0.89 displacements per atom. The damaged as well as undamaged W samples were then exposed to low-energy, high-flux (10²² D/m² s) pure D and helium-seeded D plasmas to an ion fluence of 3 × 10²⁶ D/m² at various temperatures. Trapping of deuterium was examined by the D(³He,p)⁴He nuclear reaction at ³He energies varied from 0.69 to 4.0 MeV allowing determination of the D concentration at depths up to 6 μm. It has been found that (i) addition of 10% helium ions into the D plasma at exposure temperatures of 440–650 K significantly reduces the D concentration at depths of 0.5–6 μm compared to that for the pure plasma exposure; (ii) generation of the W-ion-induced displacement damage significantly increases the D concentration at depths up to 2 μm (i.e., in the damage zone) under subsequent exposures to both pure D and D–He plasmas.     

AC operation of large titanium sublimation pumps in a magnetic field: Results of the test stand for the W7-X neutral beam injectors

A neutral beam injection (NBI) system is being built for the Stellarator experiment Wendelstein 7-X (W7-X) currently under construction at IPP Greifswald. The NBI system consists of two injectors which are essentially a replica of the system present in the Tokamak experiment ASDEX-Upgrade at IPP Garching. A vacuum system with high pumping speed and large capacity is required to ensure proper vacuum conditions in the neutral beam line. For this purpose, large titanium sublimation pumps (TSP) are installed inside the NBI boxes, consisting of 4 m long hanging wires containing Ti and the surrounding condensation walls. The wires are DC ohmically heated up with 142 A to Ti sublimation temperature. A TSP system has been operated since many years in the AUG-NBI system, sublimating Ti in the pauses between the plasma discharges, when no magnetic field is present. However, at W7-X the superconducting coils generate a magnetic field permanently during experimental campaigns, whose stray B field with a maximum of 30 mT, affects the TSPs. Operated with DC, the wires would be deflected against the surrounding panels due to the Lorentz force. A simple possible solution is heating with AC, which reduces the wire deflection amplitude, inducing a risky wire oscillation. The feasibility of the AC operation in an equivalently strong B field such as the stray B field around W7-X has been demonstrated in a test stand for different AC waveforms and frequencies. Several test campaigns have shown no qualitative difference in the pumping properties between AC and DC operation of the TSP and no critical dynamic behaviour of the wires.     

Experimental set-up for time resolved small angle X-ray scattering studies of nanoparticles formation using a free-jet micromixer

Recently, we have designed, fabricated and tested a free-jet micromixer for time resolved small angle X-ray scattering (SAXS) studies of nanoparticles formation in the <100 μs time range. The microjet has a diameter of 25 μm and a time of first accessible measurement of 75 μs has been obtained. This result can still be improved. In this communication, we present a method to estimate whether a given chemical or biological reaction can be investigated with the micromixer, and to optimize the beam size for the measurement at the chosen SAXS beamline. Moreover, we describe a system based on stereoscopic imaging which allows the alignment of the jet with the X-ray beam with a precision of 20 μm. The proposed experimental procedures have been successfully employed to observe the formation of calcium carbonate (CaCO₃) nanoparticles from the reaction of sodium carbonate (Na₂CO₃) and calcium chloride (CaCl₂). The induction time has been estimated in the order of 200 μs and the determined radius of the particles is about 14 nm.     

Hardening of self ion implanted tungsten and tungsten 5-wt% rhenium

Tungsten is one of the most promising materials for high temperature components in any future nuclear fusion tokamak. In this study tungsten-ion implantation has been used to simulate the damage caused by neutrons in pure tungsten and tungsten 5 wt% rhenium. This damaged layer is only 300 nm deep so conventional mechanical tests cannot be used to investigate it. Nanoindentation has been used to measure the change in hardness as a function of six damage levels (0 dpa, 0.07 dpa, 0.4 dpa, 1.2 dpa, 13 dpa and 33 dpa). In pure tungsten the hardness increase is seen to saturate by 0.4 dpa at ≈0.8 GPa. Transmission electron microscopy of the damage structure sees a similar saturation of the loop volume number density at the same damage level. In the tungsten 5 wt% rhenium the increase in hardness is constant between 0.07 and 1.2 dpa, ≈0.85 GPa. The loop volume number density as measured using TEM is also shows little change in this region. At a damage level of 33 dpa the hardness increase is 2.88 GPa; this corresponds with the formation of small 3–5 nm rhenium clusters as observed using atom probe tomography.     

Investigating permeation and transport of H isotopes in tungsten by first-principles

We have investigated permeation and transport of hydrogen (H) isotopes in tungsten (W) single crystal employing first-principles calculations in junction with Fick’ law. Permeability was approximately evaluated according to the solubility and diffusion coefficient of H. The solubility for H in bulk W from present calculation is consistent with the experimental results measured by Frauenfelder. The permeation fluxes of H isotopes are examined at the different thickness of W wall. The permeation fluxes of deuterium with the W thickness of 21 μm at the temperature of 770 K and with the W thickness of 50 μm at the temperature of 893 K were 0.68 × 10¹³ atom/m²s and 0.34 × 10¹⁴ atom/m²s, respectively. The dissociation coefficients of H isotopes are also evaluated. We believe that the present first-principles combined with Fick’ law method can be also generalized to investigate permeation and transport of H isotopes in most metals since such H isotopes behaviors in most metals are similar to those of H isotopes in W.     

Novel granulation process of beryllide as advanced neutron multipliers

Advanced neutron multipliers with low swelling and high stability at high temperatures are desired for pebble bed blankets of demonstration fusion power (DEMO) reactors. Beryllium intermetallic compounds (beryllides) are the most promising advanced neutron multipliers. In order to fabricate the beryllide pebbles, beryllide with shapes of block and/or rod is necessary when a melting granulation process is applied such as a rotating electrode method. A plasma sintering method has been proposed as new technique which uses a non conventional consolidation process. It was clarified that the beryllide could be simultaneously synthesized and jointed by the plasma sintering method in the insert material region between two beryllide blocks. Beryllide rod of Be₁₂Ti with 10 mm in diameter and 60 mm in length has been successfully fabricated by the plasma sintering method. Using this plasma-sintered beryllide rod, fabrication of prototype beryllide pebble was performed by a rotating electrode method as one of the melting methods. The prototype pebbles of Be₁₂Ti with 1 mm in average diameter were successfully fabricated.     

The impact of thermal fatigue and carbidization on the W coatings deposited on CFC tiles for the ITER-like Wall project at JET

Since August 2011 JET operates with the ITER-like wall comprising bulk Be tiles, bulk W tiles and W coated CFC tiles with a thickness of 10–15 μm and 20–25 μm. In order to evaluate behavior of the W coatings to a cyclic thermal loading relevant to JET operation, high heat flux (HHF) tests have been carried out up to 5100 pulses with an electron beam facility at peak temperatures of 1000 °C, 1250 °C and 1450 °C. The pulse duration was 24 s. Optical inspections of the W layer performed periodically by interrupting the test revealed small delaminations with the size of 50–500 μm. The dependence of the delamination percentage on the number of pulses can be seen as a degradation curve for each particular W coating. In this way the thermo-mechanical properties of the W coatings can be characterized quantitatively. Thermal fatigue and carbidization of the tungsten due to the diffusion of the carbon from the substrate have been recognized as mechanisms for degradation of the coatings. Tungsten carbides have been identified by using TEM (transmission electron microscopy) diffraction analysis on FIB (focused ion beam) prepared cross-section samples subjected to HHF tests. Nano-pores developed at the CFC–Mo and Mo–W interfaces during the tests might be also responsible for the degradation of the coating.     

Application of laser-induced breakdown spectroscopy for characterization of material deposits and tritium retention in fusion devices

Laser-induced breakdown spectroscopy (LIBS) is discussed as a possible method to characterize the composition, tritium retention and amount of material deposits on the first wall of fusion devices. The principle of the technique is the ablation of the co-deposited layer by a laser pulse with P (power density) ≥ 0.5 GW/cm² and the spectroscopic analysis of the light emitted by the laser induced plasma. The typical spatial extension of the laser plasma plume is in the order of 1 cm with typical plasma parameters of n_e ≈ 3 × 10²² m^?3 and T_e ≈ 1–2 eV averaged over the plasma lifetime which is below 1 μs. In this study “ITER-Like” mixed deposits with a thickness of about 2 μm and consisting of a mixture of W/Al/C and D on bulk tungsten substrates have been analyzed by LIBS to measure the composition and hydrogen isotopes content at different laser energies, ranging from about 2 J/cm² (0.3 GW/cm²) to about 17 J/cm² (2.4 GW/cm²) for 7 ns laser pulses. It is found that the laser energies above about 7 J/cm² (1 GW/cm²) are needed to achieve the full removal of the deposit layer and identify a clear interface between the deposit and the bulk tungsten substrate by applying 15–20 laser pulses while hydrogen isotopes decrease strongly after the first laser pulse. Under these conditions, the evolution of the spectral line intensities of W/Al/C/hydrogen can be used to evaluate the layer composition.     

Progress of He-cooled divertor development for DEMO

A He-cooled divertor concept for DEMO [1] has been developed at Karlsruhe Institute of Technology (KIT) since a couple of years with the goal of reaching a heat flux of 10 MW/m² anticipated for DEMO. The reference concept HEMJ (He-cooled modular divertor with multiple-jet cooling) is based on the use of small cooling fingers – each composed of a tungsten tile brazed to a tungsten alloy thimble – as well as on impingement jet cooling with helium at 10 MPa, 600 °C. The cooling fingers are connected to the main structure of ODS Eurofer steel by brazing in combination with a mechanical interlock. This paper reports progress to date of the design accompanying R&Ds, i.e. primarily the fabrication technology and HHF experiments. For the latter a combined helium loop and electron beam facility (200 kW, 40 keV) at Efremov Institute, St. Petersburg, Russia, has been used. This facility enables mock-up testing at a nominal helium inlet temperature of 600 °C, a pressure of 10 MPa, and a maximal pressure head of 0.5 MPa. HHF test results till now confirm well the divertor design performance. In the recent test series in early 2010 the first breakthrough was achieved when a mock-up has survived over 1000 cycles at 10 MW/m² unscathed.     

Inactive and mutagenic effects induced by carbon beams of different LET values in a red yeast strain

To evaluate biological action of microorganism exposed to charged particles during the long distance space exploration, induction of inactivation and mutation in a red yeast strain Rhodotorula glutinis AY 91015 by carbon beams of different LET values (14.9–120.0 keV μm^?1) was investigated. It was found that survival curves were exponential, and mutation curves were linear for all LET values. The dependence of inactivation cross section on LET approached saturation near 120.0 keV μm^?1. The mutation cross section saturated when LET was higher than 58.2 keV μm^?1. Meanwhile, the highest RBE_i for inactivation located at 120.0 keV μm^?1 and the highest RBE_m for mutation was at 58.2 keV μm^?1. The experiments imply that the most efficient mutagenic part of the depth dose profile of carbon ion is at the plateau region with intermediate LET value in which energy deposited is high enough to induce mutagenic lesions but too low to induce over kill effect in the yeast cells.     

Study of large gap 200 kV breakdowns in vacuum with high stored energy for ITER neutral beam accelerator

In the context of the ITER contract “ITER/CT/07/219–200 kV Stored Energy Tests”, electrical breakdown tests have been performed in vacuum with a stored energy of up to 425 J. The experiments have been conceived and performed with the collaboration of Consorzio RFX. The tests are being performed in the 1 MV test facility at IRFM, CEA-Cadarache. They should simulate the conditions that will be found in the ITER Neutral Beam accelerator, at 200 kV. This paper presents the set-up of the test bed, the choice of critical components, the diagnostic equipments and the results obtained with 200 kV applied on the anode electrode.     

Analysis of surface morphological change in PET films induced by tungsten ion implantation

Surface morphological changes and metal nanograin formation of polyethylene terephthalate films with tungsten ion implantation were studied. Tungsten ions were accelerated with a voltage of 40 kV and implanted at fluences from 5 × 10¹⁶ to 2 × 10¹⁷ cm^?2 using a metal vapor vacuum arc implanter. Scanning electron micrographs at the highest fluence show semi-spherical hills, indicating formation of tungsten nanograins on the polymer. The tungsten nanograin formation in the polymer film is confirmed by cross-sectional observation using transmission electron microscopy. Depth profiles of tungsten atoms obtained from energy dispersive X-ray spectra indicate densification and sputtering of the polymer surface layer during implantation. These results indicate that surface morphological change is related with the effects of a critical fluence and tungsten nanograin formation.