Tungsten Disk Research Articles

Lateral stress induced blistering of tungsten exposed to deuterium plasma

Surface blistering on tungsten under deuterium plasma exposure has been well known and investigated intensively in the last decade. However, the mechanism of the blistering is still unclear. There have been mainly two different models proposed: the gas driven model and lateral stress model. In this work, we designed an experiment to address this issue. Tungsten disc samples were prepared using twin-jet electro-polishing. The specimens were under deuterium plasma exposure to study the thickness effect on the surface blistering. The results showed that blistering was rarely observed on the surface around the inner edge of the central perforation with thickness of ∼100–500 nm. The blisters started to appear on the surface when the thickness was about 10 μm. Both the number and size of the blisters increased further on the outer surface with further increase in thickness. This trend was not obvious as the thickness increased up to above 180 μm. The diffusion depth of D in this work was calculated to be 8.5 μm. These results affirmed the lateral stress model as the surface blistering mechanism.

Fracture modes and grain growth of tungsten during extreme cyclic heating

To investigate the fracture mechanisms resulting from the thermomechanical loading in fusion reactors, cyclic heating experiments were carried out on tungsten discs (W). These experiments involved subjecting the samples to an arc-jet plasma gun, which can deliver a heat flux exceeding 20 MW m−2. The W discs had varying thicknesses and pulse durations. To gain a better understanding of the thermal environment, finite element (FE) simulations were performed to model heat transport through exposed specimens and the cooling assembly, and these were compared with experimental measurements. The temperature at the edge of the sample was found to reach a maximum of 1500 °C, and the temperature at the center of the upper surface of the disc fluctuated between a maximum of over 2800 °C and a minimum of 500 °C, depending on the thickness of the disc and the thermal contact between the sample and the cooling structures. Cracks were found to propagate through different modal regimes, starting with ductile growth, followed by a combination of ductile/intergranular growth, intergranular growth, and finally brittle transgranular fracture (cleavage fracture). Steep temperature gradients (∼1.6 × 106 °C/m) were found to induce grain growth, with a grain boundary mobility of 7.5 eV under moderate thermal stresses. Alternatively, crack formation and growth can occur due to residual stresses. Cumulative residual stress has been shown to correlate with the duration of exposure to elevated temperatures, leading to increased hardening, although with noticeable grain growth. Consequently, the strength of the sample is primarily driven by thermal stress rather than grain growth.

Updates on the measurement of the speed of gravity using LDVs and MEMS resonators

Five years ago, at the XIII AIVELA conference, a project was presented by the author for the laboratory determination of the speed of propagation of the gravitational interaction using a vibrating tungsten disc as source of a local gravitational perturbation and a high-Q silicon resonator as gravitational antenna. Using laser Doppler vibrometers to track the vibrations of the transmitter and of the receivers, the speed of propagation of gravity would have been calculated from their measured phase difference. Numerous developments happened in the project since then, from the construction and acquirement of important components of the generator of dynamic Newtonian fields, to the experimental measurement of the speed of VHF radio waves in the near-field region as a preparatory technical test for handling and elaboration of the corresponding gravitational data. This latter experiment produced unexpected superluminal results, making it worthy of further study. The results of the test also broadened the scope of expectations and possible interpretations of the results of the gravitational experiment, including the additional role of an indirect assessment of the existence of gravitons.

(Digital Presentation) Electrochemical Reduction of Pressed Tungsten (VI) Oxide on a Solid Electrode in a Melt of Calcium and Sodium Chlorides

This report presents the results of research on the electrochemical reduction of tungsten (VI) oxide in a molten electrolytic mixture of CaCl2-NaCl of eutectic composition. The electrochemical behavior of tableted WO3 on a solid tungsten electrode was studied by voltammetry. Analysis of the obtained products was carried out by X-Ray diffraction and electron microscopy.During the electrochemical reduction, pressed cylindrical samples were placed on a tungsten disk, which served as a cathode. A silica cloth impregnated with a CaCl2-NaCl eutectic melt was placed on top of the tableted sample in order to prevent a short circuit in the cell. The silica cloth and tableted tungsten oxide were pressed to the cathode by the anode. The anode was made of low-porous graphite, and the current was supplied to it with a molybdenum wire. The electrode assembly prepared in this way was placed in a crucible with molten chlorides of calcium and sodium.Analysis of the obtained results indicates that reduction processes occur in pressed tungsten oxide (Fig. 1, a). According to the results of X-Ray phase analysis, in the surface layer of pressed tungsten (VI) oxide, which was in contact with the cathode, when the amount of electricity (Q) twice greater than that required for the complete recovery of loaded tungsten trioxide was passed, the following phases were detected: W, CaWO4 and a small amount of NaxWO3. Having studied the entire volume of the tableted sample, the following phases were recorded: NaxWO3 (main phase) and CaWO4, W (Fig. 1, b). The formation of phases of sodium and calcium tungstates is explained by the interaction of tungsten trioxide with the components of the melt. Passing electricity through the pressed sample causes electrochemical reactions to occur with the formation of tungsten metal, which grows from the cathode/WO3/electrolyte interface into the sample volume.During galvanostatic electrolysis (i=0.4 A/cm2) of a pressed sample of tungsten (VI) oxide with a mass of 0.43 g (r=0.4 cm, h=0.174 cm) and a duration of 6 hours (Qpractical/Qtheoretical = 6), the degree of extraction of tungsten from tungsten trioxide is not less than 97.48%, and the current yield is about 29.06%. The X-Ray diffraction pattern of the obtained product under such conditions confirms the presence of only a monophase of crystalline tungsten (Fig. 1, b). Figure 1

Beam optics simulation of the ion injector for sodium Beam Emission Spectroscopy (Na-BES) on EAST

To upgrade the Li-BES to Na-BES on Experimental Advanced Superconducting Tokamak (EAST), a high-brightness and stable sodium (Na) ion source is developed, and the performance of Na ion beam optics is simulated by the COMSOL Multiphysics. The porous tungsten disk with 70% porosity and filled with albite Na2O·Al2O3·6SiO2 is successfully developed as the Na ion source. The extracted current density of the Na-beam increases with the extraction voltage, and its maximum current can reach 9 mA with the ion source diameter of 20 mm. The lifetime of the Na ion source is evaluated to be 63 mAh, which is much longer than that of the Li ion source. The simulation shows that the preferable distances among the ion source (or anode electrode), extractor and puller to get the well-focused beam are x puller - x extractor ∼ 0.06 m and x extractor - x ion source ∼ 0.02 m. Further simulation with these optimized distances determines the extraction voltage and ion source size for good beam performance. In addition, the simulation shows the beam divergence angle decreases by more than 40% by optimizing the Einzel voltage to 5 kV, suggesting an excellent focusing property of the Einzel lens. These works provide the basis and the design references for the Na-BES development on EAST.

Солнечный телескоп мягкого рентгеновского диапазона для наноспутника на основе камеры-обскуры

Pin-hole camera as a soft X-ray solar telescope is presented. The instrument is aimed on imaging of flares in the solar corona with angular resolution up to 40 angular seconds. It will allow to register temporal profiles of flares and determine spectra in soft X-ray. The telescope consist on pin hole 0.1 mm in diameter in tungsten disk, thin film Al/maylar filter and back illuminated CMOS based 2d detector. The telescope designed for 6U cubesat. The instrument itself has dimension of about 0.5U with extended tube from 20 to 50 cm length.

Pin-hole camera based soft X-ray solar telescope for nanosatellite

Pin-hole camera as a soft X-ray solar telescope is presented. The instrument is aimed on imaging of flares in the solar corona with angular resolution up to 40''. It will allow to register temporal profiles of flares and determine spectra in soft X-ray. The telescope consist on pin hole 0.1 mm in diameter in tungsten disk, thin film Al/maylar filter and back illuminated CMOS based 2d detector. The telescope designed for 6 U cubesat. The instrument itself has dimension of about 0.5 U with extended tube from 20 to 50 cm length. Keywords: Pin-hole camera, solar corona, soft X-ray, cubesat.

Evaluation of crucible for cold hearth transverse electron beam vapour generator

Electron beam guns are extensively used as concentrated energy source in various material processing applications. A transverse 270° bend e-beam evaporator is generally used as a vapour generator for laser-based isotope separation process. In such an evaporator a condition is imminent where direct impingement of the beam on crucible wall happens when charge level is less than a critical level during continuous operation. Hence, in absence of online level measurement or charge feeding system, electron guns are susceptible to safety issues related to direct impingement of electron beam on molten charge containment. Reported literature on these safety aspects of the vaporizer addressing the crucible materials is scanty. The objective of this study was to identify a crucible material and desired wall thickness to produce vapour of a metal which requires 1500 W to achieved desired vapour for laser-based isotope separation process. In this study tantalum and tungsten were investigated as candidate crucible materials experimentally. Experiments were carried out on Tantalum and Tungsten discs of different thickness to obtain the damaging power in case of direct impingement. Puncture power was estimated by a commercial software for the coupons used in the experiments. The simulation results are in good agreement with the experimental results with certain deviations. The deviations can be attributed to the uncertainties in the scatter in thermo-physical data at high temperature and experimental conditions. The use of a 2 mm thick tantalum crucible was found to be safe with the electron beam power of vaporizer up to 1500 W.

Design and Analysis of an Advanced Three-Point Bend Test Approach for Miniature Irradiated Disk Specimens

Candidate tungsten armor materials in a magnetic confinement fusion device must be able to withstand thermal variation that leads to internal stresses caused by the impinging heat load. In addition, the thermomechanical properties of these materials are degraded by irradiation-induced defect accumulation. Fission reactor–based irradiation data are used to predict the fusion neutron damage and property change. This study examines the motivation and design of a custom-designed three-point bend test for neutron-irradiated disk specimens that are 3 mm in diameter to be able to define the flexural strength of advanced tungsten materials, alloys, and composites—and to the extent that embrittlement occurs after neutron irradiation. The theory provided shows a calculation for the flexural deflection and shear deflection due to the small-geometry constraints. A finite element deformation analysis is performed to evaluate the mechanical stress field of disk bend specimens. The stress values above 80% of the maximum stress are concentrated in 2.4 mm of the 3.0-mm length of the centerline across the tungsten disk diameter. A bend test fixture has been designed and fabricated to enable testing of these specimens with precisely engineered tolerance and minimal machine compliance. This fixture will be able to be placed inside a universal testing frame at elevated temperatures for the mechanical property evaluation of future neutron-irradiated disk specimens.

The jet formation and penetration capability of hypervelocity shaped charges

The jet formation and penetration capacity of hypervelocity shaped charges (HSCs) against three steel targets was carefully examined using both experimental and numerical method. The HSCs with five different disc materials, i.e. silicon carbide, steel 45#, steel 4340, copper and tungsten, were manufactured to explore their penetration capability and compare with a classical conical shaped charge (CSC). The jet formation and penetration of both HSCs and a CSC was simulated using a numerical model that reproduces the results of experiments. It is found that the length and head velocity of the projectiles formed by HSCs with a tungsten disc are larger than those with other disc materials, and more importantly, they are even larger by 23.9% and 36.6% compared with that formed by the CSCs. However, the corresponding diameter of the entrance in the first steel target is much smaller than that of the CSC and HSCs with other disc materials. Overall, it is encouraging that the HSC with a tungsten disc yields a deeper penetration depth by 6.1% compared to a CSC, while other materials cannot compete with CSCs.

Characterisation of the attenuation properties of 3D-printed tungsten for use in gamma camera collimation

BackgroundThe aim of this work was to characterise the attenuation properties of 3D-printed tungsten and to assess the feasibility for its use in gamma camera collimator manufacture.Method3D-printed tungsten disks were produced using selective laser melting (SLM). Measurements of attenuation were made through increasing numbers of disks for a Tc-99m (140 keV) and I-131 (364 keV) source. The technique was validated by repeating the measurements with lead samples. Resolution measurements were also made with a SLM tungsten collimator and compared to Monte Carlo simulations of the experimental setup. Different collimator parameters were simulated and compared against the physical measurements to investigate the effect on image quality.ResultsThe measured disk thicknesses were on average 20% above the specified disk thicknesses. The measured attenuation for the tungsten samples were lower than the theoretical value determined from the National Institute of Standards and Technology (NIST) cross-sectional database (Berger and Hubbell, XCOM: photon cross-sections on a personal computer, 1987). The laser scan strategy had a significant influence on material attenuation (up to 40% difference). Results of these attenuation measurements indicate that the density of the SLM material is lower than the raw tungsten density. However, an improved performance compared to a lead collimator was observed. The SLM tungsten collimator was adequately simulated as 80% density and 110% septal thickness. Scatter and septal penetration were 17% less than a similar lead collimator and 33% greater than tungsten at 100% density.ConclusionsSLM manufacture of tungsten collimators is feasible. Attenuation properties of SLM tungsten are superior to the lead alternative and the opportunity for bespoke collimator design is appealing.

Measuring the speed of gravity with laser Doppler vibrometers and silicon mechanical resonators

The old question of the speed of propagation of gravitational interactions still needs a laboratory answer. An experimental setup for direct measurement of the propagation delay in the gravitational interaction between resonating gravitationally coupled oscillators is proposed. The oscillators are designed to act like transmitter and receiver in an AM radio communication. The signal, a dynamic Newtonian field, is generated by the precise vibrations of a massive tungsten disc, in resonance with its suspension system like a trampoline oscillator. The disc is driven by a shaker locked to a GPSDO frequency reference, while its movement is tracked with a laser Doppler vibrometer and a lock-in amplifier, locked to the same frequency reference. The gravitational signal is received by an antenna, a gold-coated silicon oscillator with resonance frequency of 2 kHz and a Q of 200 000 in thermal vacuum. The vibrations of the antenna are tracked by a second laser Doppler vibrometer with a noise level of 30 fm/√Hz and a lock-in amplifier using the output of the first lock-in amplifier as reference. High-vacuum chambers and multi-stage vibration and acoustic isolation systems are required for both the generator and the antenna. Cross-correlating the data of the two oscillators, their phase difference is then measured and, using the frequency of the signal and the distance between oscillators, the speed of gravity will be calculated. Numerous measurements at various distances spanning a 300 mm range are collected and used for the determination of other parameters, including G.

Formation of oxide layers on tungsten at low oxygen partial pressures

This work focuses on the oxidation of tungsten in inert gas atmospheres containing oxygen and moisture. It is particularly relevant for the European Spallation Source where the tungsten target is cooled by purified helium gas and the 5 MW proton beam can raise the maximum target temperature beyond the threshold for oxidation. Tungsten discs were oxidized isothermally at 400° to 900 °C for 2 h in pure helium and helium mixed with oxygen and water vapor, with varying partial pressures up to 500 Pa. Tungsten was oxidized even with a small amount of oxygen (≤5 ppm) present in industrially pure helium. Non-isothermal oxidation of tungsten foils was carried out in water vapor (∼100 Pa), in situ in an environmental scanning electron microscope. On specimens oxidized in inert gas containing water vapor (2 h, pH2O ∼790 Pa), Auger electron spectroscopy studies confirmed the presence of a thin oxide layer (40 nm) at 400 °C. At 500 °C the oxide layer was 10 times thicker. A dark, thin and adherent oxide layer was observed below 600 °C. Above this temperature, the growth rate increased substantially and the oxide layer was greenish, thick and porous. Oxide layers with varying stoichiometry were observed, ranging from WO3 at the surface to WO2 at the metal-oxide interface. For comparison, oxidation of tungsten alloys in He-5%O2 was studied. The implications of this work on the design and operation of the helium loop for cooling the target are discussed.

Study of filling material of dental composites: an analytical approach using radio-activation

Abstract The aim of this study is to carry out elemental analyses of dental composites acquired from different producers using photoactivation analysis (PAA). High energy electrons produced by an electron linear accelerator are absorbed by a tungsten disk (Bremsstrahlung converter) thereby producing high energy X-rays (bremsstrahlung). The dental composite materials under study were exposed to the bremsstrahlung radiation whereby radionuclides were produced through photonuclear reactions. Their radioactivities were measured using high resolution semiconductor spectrometers equipped with high purity germanium detectors (HPGe). The spectra were analysed using appropriate computer software. As a result, photonuclear reactions of 12 stable elements were detected in different dental composite species, and the elemental concentrations were calculated. For comparison, the dental composites were also investigated using scanning electron microscopy (SEM) and energy-dispersive X-ray fluorescence spectrometry (EDXRF). Various sizes and shapes of dental composites were observed using SEM. However, contents of dental composites, e.g. Mg, Ni, Ba and Sr were obtained by PAA whilst C, O, Al, S, Ba and Sr were detected by EDXRF spectrometry. The results for Ba and Sr obtained using the two techniques show considerable difference.

Effect of Base Oil Structure on Elastohydrodynamic Friction

The EHD friction properties of a wide range of base fluids have been measured and compared in mixed sliding–rolling conditions at three temperatures and two pressures. The use of tungsten carbide ball and disc specimens enabled high mean contact pressures of 1.5 and 2.0 GPa to be obtained, comparable to those present in many rolling bearings. The measurements confirm the importance of molecular structure of the base fluid in determining EHD friction. Liquids having linear-shaped molecules with flexible bonds give considerably lower friction than liquids based on molecules with bulky side groups or rings. EHD friction also increases with viscosity for liquids having similar molecular structures. Using pure ester fluids, it is shown that quite small differences in molecular structure can have considerable effects on EHD friction. The importance of temperature rise in reducing EHD friction at slide–roll ratios above about 5% has been shown. By measuring EHD friction at several temperatures and pressures as well as EHD film thickness, approximate corrections to measured EHD friction data have been made to obtain isothermal shear stress and thus EHD friction curves. These show that under the conditions tested most low molecular weight base fluids do not reach a limiting friction coefficient and thus shear stress. However, two high traction base fluids appear to reach limiting values, while three linear polymeric base fluids may also do so. Constants of best fit to a linear/logarithmic isothermal shear stress/strain rate relationship have been provided to enable reconstruction of isothermal EHD friction behaviour for most of the fluids tested.

Characterization of a compact ECR plasma source and its applications to studies of helium ion damage to tungsten

Exposure of tungsten to low energy (<100 eV) helium plasmas at temperatures between 900–1900 K in both laboratory experiments and tokamaks has been shown to cause severe nanoscale modification of the near surface resulting in the growth of tungsten tendrils. Tendril formation can lead to non-sputtered erosion and dust formation. Here we report on characterization of a compact electron cyclotron resonance (ECR) He plasma source with an ion flux of ∼2.5 × 1019 ions m−2 s−1, average fluence of 3 × 1024 ions m−2, and the surface morphology changes seen on the exposed tungsten surfaces. Exposures of polished tungsten disks at temperatures up to 1270 K have been performed and characterized using scanning electron microscopy and atomic force microscopy (AFM) scans. Bubbles and craters have been seen on the exposed tungsten surface growing to up to 150 nm in diameter. The ECR source has been tested for eventual use on a scanning tunneling microscopy experiment intended to study the early stages of surface morphology change due to He ion exposure.

High heat-flux self-rotating plasma-facing component: Concept and loading test in TEXTOR

This contribution reports on the concept of a circular self-rotating and temperature self-stabilising plasma-facing component (PFC), and test of a related prototype in TEXTOR tokamak.This PFC uses the Lorentz force induced by plasma current and magnet field (J×B) to create a torque applied on metallic discs which produce a rotational movement. Additional thermionic current, present at high operation temperatures, brings additional temperature stabilisation ability. This self-rotating disk limiter was exposed to plasma in the TEXTOR tokamak under different radial positions to vary the heat flux. This disk structure shows the interesting ability to stabilise its maximum temperature through the fact that the self-induced rotation is modulated by the thermal emission current. It was observed that the rotation speed increased following both the current collected by the limiter, and the temperature of the tungsten disks.

Two-dimensional to three-dimensional transition of tungsten clusters anchored on graphite surface

Experimental studies have been performed on unisize tungsten clusters constructed on a graphite surface by means of the scanning tunneling microscopy. It was found that the geometry of the clusters changes instantaneously from a monatomic-layer tungsten disk to a diatomic-layer structure between the cluster size of 10 and 11. We concluded that this transition is driven by a change in the dominant interaction from the attractive electrostatic interaction between the cluster and the surface to intracluster cohesive metallic interaction.

Shock testing accelerometers with a Hopkinson pressure bar

The electronic industry continues to dramatically reduce the size of electrical components. Many of these components are now small enough to allow shock testing with Hopkinson pressure bar techniques. However, conventional Hopkinson bar techniques must be modified to provide a broad array of shock pulse amplitudes and durations. For this study, we evaluate the shock response of accelerometers that measure large amplitude pulses, such as those experienced in projectile perforation and penetration tests. In particular, we modified the conventional Hopkinson bar apparatus to produce relatively long duration pulses. The modified apparatus consists of a steel striker bar, annealed copper pulse shapers, an aluminum incident bar, and a tungsten disk with mounted accelerometers. With these modifications, we obtained accelerations pulses that reached amplitudes of 10kG and durations of 0.5ms. To evaluate the performance of the accelerometers, acceleration-time responses are compared with a model that uses data from a quartz stress gage. Comparisons of data from both measurements are in good agreement.

A conventional positron source for international linear collider

A possible solution to realize a conventional positron source driven by a several-GeV electron beam for the International Linear Collider is proposed. A 300Hz electron linac is employed to create positrons with stretching pulse length in order to cure target thermal load. ILC requires about 2600 bunches in a train which pulse length is 1ms. Each pulse of the 300Hz linac creates about 130 bunches, then 2600 bunches are created in 63ms. Optimized parameters such as drive beam energy, beam size, and target thickness, are discussed assuming a L-band capture system to maximize the capture efficiency and to mitigate the target thermal load. A slow rotating tungsten disk is employed as positron generation target.