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• A method for inserting a well defined fatigue precrack in W is established. • Continuous in-situ observations on crack formation and extension during fatigue precracking are performed. • Reliable fracture toughness values of ITER specification W (ALMT-grade) are assessed 4. The RT fracture toughness values are ~10MPa√m for the T-L and 12~13MPa√m for the L-T orientation. • Orientational variation in RT fracture toughness is comparably small in the ALTM-grade plate. Fracture toughness of the divertor armor in ITER needs to be elucidated for prediction of the response of armor materials to intense heat loading. The present study has been performed to establish a method for inserting a well, defined fatigue precrack into the test specimens and measure the reliable fracture toughness values of tungsten (W) materials that exhibit low temperature brittleness. Sub-sized, single edge notched bend (SENB) specimens with two orientations, L -T and T-L, taken from a commercially available pure W plate manufactured in accordance with the ITER material specifications (ITER specification W, ALMT-grade), were precracked in two-step fatigue loading: fully uniaxial compression fatigue to provoke a crack from the notch tip, and subsequent 3-point bend (3 PB) fatigue to extend the crack length for the minimization of influence of the residual tensile stresses generated by compression fatigue. During compression and 3 PB fatigue loading the fatigue crack was closely monitored by continuous in-situ observations on the notch tip. Continuous tracing of the crack tip position has enabled capture of the fatigue cracking behavior as a kinematic response in a rolled W material, disclosing a marked difference in crack inducement and extension between the two orientations. 3 PB fracture testing of the precracked specimens at room temperature (RT) yielded ~10 MPa√m for the T-L and 12~13 MPa√m for the L -T orientation as fracture toughness: anisotropic variation in fracture toughness of the W plate is comparatively small. Fracture surface observations reveal that a crack path is entirely transgranular for the L -T, while crack propagation involves intergranular fracture to a large extent for the T-L orientation. These results obtained are discussed and compared with the literature data. [ABSTRACT FROM AUTHOR]

Hydrogen isotope retention of tungsten in nuclear fusion reactors is one of the hot research issues all along. In this paper, tungsten samples in different rolled surfaces were polished by mechanical processing, subsequently subjected to D 2 + irradiation and thermal desorption. To better understand the experimental observations, this study also performed molecular dynamics (MD) simulation and investigated the effects of temperature, grain number, grain boundary density, and crystal orientation on hydrogen retention. It is found that the grain number and grain boundary density of rolled tungsten increase successively in RD/TD, RD/ND, and TD/ND surfaces. The RD/ND surface exhibits the best hydrogen radiation resistance, whereas the TD/ND surface is unsatisfactory. MD simulations further indicate that hydrogen retention is more obvious with the increase of grain density in tungsten, and hydrogen atoms are more easily enriched at the grain boundaries. With the increase in temperature, the retention of hydrogen atoms in monocrystalline/polycrystalline tungsten decreases significantly. The average implantation depth of H atoms is deepest along the <111> and <112> crystalline directions, which reveals that hydrogen retention is dependent on the crystal orientations. The good agreement between the experimental data and simulation results reveals that grain boundaries play an important role in hydrogen retention. [Display omitted] • The hydrogen retention behavior is studied in W samples with different rolled surfaces. • RD/ND surface has the best hydrogen resistance, while the TD/ND surface is undesirable. • The hydrogen retention behavior depends on temperature, grain size, grain boundary density and orientation. [ABSTRACT FROM AUTHOR]

Highlights • Thermal shock behavior of samples was characterized after helium irradiation. • Combined helium irradiation and thermal shock has significant influence on crack propagation. • Observing fuzz was more likely to converge densely at the phase interface. • Y 2 O 3 doping can improve thermal shock resistance of the W-Y 2 O 3 composite. Abstract Tungsten can be the preferred plasma-facing material for nuclear fusion reaction. In this work, W-Y 2 O 3 composite powder was successfully fabricated through the wet chemical method, and bulk materials were obtained by rolling and high temperature annealing. W-2 vol% Y 2 O 3 was exposed to 80 eV helium ion irradiation with a flux of 1.5 × 1022 ions/(m2·s) at 1503–1553 K to assess its thermal shock resistance and helium ion irradiation damage behavior. The thermal shock behaviors of samples were studied under a laser beam. The energy densities of the laser thermal shock were 0.32, 0.48, and 0.64 GW/m2. W-Y 2 O 3 was compared with commercial pure tungsten. The hardness of pure tungsten and W-Y 2 O 3 before and after helium irradiation was measured by using a micro-hardness tester. The surface micrograph after helium ion irradiation and thermal shock was observed through scanning electron microscopy. Results show that W-Y 2 O 3 composite materials have better resistance to laser thermal shock than pure tungsten. [ABSTRACT FROM AUTHOR]

Abstract Spark plasma sintering (SPS) is one of the methods used to achieve the low-temperature densification of refractory metal materials. In this study, powder prepared through a wet chemical method was consolidated via SPS at 1100 °C, 1200 °C, 1350 °C, 1600 °C, and 1800 °C to obtain a high-performance W-TiC-Y 2 O 3 composite material. Densification was studied by analyzing the densification curve and changes in the microstructure of the samples. This process could be divided into three stages: the bonding stage, the sintering neck growth stage, and the shrinkage and spherification stage of closed pores. Surface diffusion and grain boundary diffusion played different roles in densification. The density, grain size, and Vickers hardness of the tungsten material increased significantly as temperature increased. This study evaluated the sintering process and provided a basis for obtaining high-performance tungsten materials through SPS. Highlights • Surface diffusion and grain boundary diffusion played different roles in densification. • The density, grain size, and Vickers hardness of tungsten increased as temperature increased. • The densification of the composite accelerated at the initial stage of sintering but decelerated after a certain period. • The densification of the powder at different sintering stages was influenced by different mechanisms. [ABSTRACT FROM AUTHOR]

Abstract Tungsten is one of the most promising candidates for plasma-facing material for fusion. In this experiment, W–TiC–Y 2 O 3 composites were successfully prepared by the wet chemical method. The properties of the W–TiC–Y 2 O 3 composites were studied through field-emission scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction analysis, thermal conductivity, and transient laser thermal shock. During the sintering of the composite powder, TiC and Y 2 O 3 precipitations converted to Y-Ti-O precipitations. After transient laser thermal shock, the composites underwent melting, cracking, and resolidification, resulting in changes in surface morphology and internal structure of the composites. Graphical abstract Image 1 Highlights • W–TiC–Y 2 O 3 composites possessed excellent mechanical property and thermal shock resistance. • Y 2 TiO 5 , Y 2 TiO 6 , and Y 2 TiO 7 improved the mechanical properties of the material at high temperature. • The effect of different power density on material surface was analyzed. • The change of surface morphologies and internal structure due to transient thermal shock were discussed. [ABSTRACT FROM AUTHOR]

Tungsten is a primary material in plasma-facing components (PFCs), but several disadvantages limit its application in fusion reactors. This work examined tungsten–niobium (W–Nb) alloys as potential PFC materials and investigated their response to low-energy helium ion irradiation. W–Nb powders were milled for 25 and 36 h, and the W–Nb samples were exposed to 50 and 80 eV helium ion irradiation with a flux of 1.5 × 10 22 ions/m 2  s at 1230 °C. The sample with a dense structure and obtained after a 25-h milling exhibited enhanced anti-irradiation performance. Sufficient energy can be provided to promote helium ion migration in the matrix, resulting in the formation of nanotendrils. Furthermore, in W–Nb alloys, the Nb-rich regions are more attractive to helium ions and thus provide protection to the tungsten matrix. [ABSTRACT FROM AUTHOR]

In tokamaks, the temperature of the plasma-facing wall is an important parameter for achieving particle balance and therefore steady-state operation. QUEST, which is a middle-sized spherical tokamak, has hot walls that act as plasma-facing walls. They can be actively heated with sheath heaters and actively cooled with water. To control the wall temperature, heating and cooling systems have been developed. These systems adjust the power of the sheath heaters and the motor valves of the cooling system, respectively. The two systems communicate via Ethernet through UDP and control the hot-wall temperature cooperatively. The plasma control system (PCS) in QUEST has also been modified, especially with respect to gas fueling, in order to enable long-duration plasma sustainment. A feedback controller has been installed in the PCS, together with a mass flow controller, allowing Hα emission from the plasma which is used as a reference signal, to be well controlled. Plasma density calculations using a field-programmable gate array are proposed for the feedback control system. [ABSTRACT FROM AUTHOR]

The transient thermal shock behaviors of W–ZrC/Sc 2 O 3 composites with different ZrC contents were evaluated using transient thermal shock test by electron and laser beams. The effects of different ZrC doping contents on the surface morphology and thermal shock resistance of W–ZrC/Sc 2 O 3 composites were then investigated. Similarity and difference between effects of electron and laser beam transient heat loading were also discussed in this study. Repeated heat loading resulted in thermal fatigue of the irradiated W–ZrC/Sc 2 O 3 samples by thermal stress, leading to the rough surface morphologies with cracks. After different transient thermal tests, significant surface roughening, cracks, surface melting, and droplet ejection occurred. W–2vol.%Sc 2 O 3 sample has superior thermal properties and greater resistance to surface modifications under transient thermal shock, and with the increasing ZrC content in W alloys, thermal shock resistance of W–Zr/Sc 2 O 3 sample tends to be unsatisfied. [ABSTRACT FROM AUTHOR]

Fatigue pre-cracking performance and fracture toughness in polycrystalline tungsten (W) and molybdenum (Mo) have been investigated in relation to grain boundary (GB) configuration with respect to the crack advance direction. Sub-sized, single edge notched bend (SENB) specimens with three different orientations, R-L (ASTM notation) for a forged Mo rod and L-S and T-S for a rolled W plate, were pre-cracked in two steps: fully uniaxial compression fatigue loading to provoke crack initiation and its stable growth from the notch root, and subsequent 3-point bend (3PB) fatigue loading to extend the crack. The latter step intends to minimize the influence of the residual tensile stresses generated during compression fatigue by moving the crack tip away from the plastic zone. It is shown that fatigue pre-cracking performance, especially pre-crack extension behavior, is significantly affected by the specimen orientation. The R-L orientation, giving the easiest cracking path, permitted crack extension completely beyond the plastic zone, while the L-S and T-S orientations with the thickness cracking direction of the rolled plate sustained the crack lengths around or possibly within the plastic zone size due to difficulty in crack advance through an aligned grain structure. Room temperature fracture toughness tests revealed that the 3PB fatigued specimens exhibited appreciably higher fracture toughness by about 30% for R-L, 40% for L-S and 60% for T-S than the specimens of each orientation pre-cracked by compression fatigue only. This indicates that 3PB fatigue provides the crack tip front out of the residual tensile stress zone by crack extension or leads to reduction in the residual stresses at the crack tip front. Strong dependence of fracture toughness on GB configuration was evident. The obtained fracture toughness values are compared with those in the literature and its strong GB configuration dependence is discussed in connection with the appearance of pop-in. [ABSTRACT FROM AUTHOR]

The plasma control system (PCS) of QUEST is developed according to the progress of QUEST project. Since one of the critical goals of the project is to achieve the steady-state operation with high temperature vacuum vessel wall, the PCS is also required to have the capability to control the plasma for a long period. For the increase of the loads to processing power of the PCS, the PCS is decentralized with the use of reflective memories (RFMs). The PCS controls the plasma edge position with the real-time identification of plasma current and its position. This identification is done with not only flux loops but also hall sensors. The gas fueling method by piezo valve with monitoring the Hα signal filtered by a digital low-pass filter are proposed and suitable for the steady-state operation on QUEST. The present status and prospect of the PCS are presented with recent topics. [ABSTRACT FROM AUTHOR]

Pure tungsten and tungsten alloys with 1, 2, and 3 wt.% Lu 2 O 3 were fabricated by mechanical alloying and spark plasma sintering at 1600 °C for 3 min. These samples were characterized by X-ray diffraction, field-emission scanning electron microscopy, energy-dispersive spectroscopy, Vickers microhardness, thermal conductivity analyses, thermal shock test, and deuterium retention. Results revealed that the microhardnesses of the samples have a similar trend with the relative density. With increased the doped Lu 2 O 3 content, the relative density and microhardness increased and the Lu 2 O 3 particles significantly increased the density and strengthened of tungsten alloy. With increased temperature from 300–1100 K, the thermal conductivity decreased with increased Lu 2 O 3 content. The doped Lu 2 O 3 particles revealed good enhancement effect in the thermal shock resistance and a good influence in the deuterium retention. [ABSTRACT FROM AUTHOR]

In the present RF-driven divertor plasma of QUEST, it has been observed that orbit-driven current flows in the open magnetic surfaces outside of the closed magnetic surfaces. To observe this phenomenon and the characteristics of the orbit-driven current, current density profiles have been calculated on two different equilibrium conditions. We calculated current density profiles from particle guiding center orbits both for the fundamental and the second harmonic resonances for the 8.2 GHz electron cyclotron current drive. From this calculation, hollow current density profiles have been obtained with significant characteristics on both conditions. Only positive current distribution has been observed in the open magnetic surfaces outside of the closed magnetic surfaces. [ABSTRACT FROM AUTHOR]

In order to evaluate the magnetic surface current density distributions, the electron guiding center orbits were calculated with the equilibrium fitting plasma equilibrium analysis. The current density distributions in electron cyclotron-heated plasma were estimated from the orbit analyses in the equilibrium plasma shaping for the fundamental and the second-harmonic resonant electrons separately. The current density distribution profiles on the equatorial plane were obtained for the electrons with initially positive and negative velocities parallel to the magnetic field direction as well as the trapped electrons. The surface averaged current density profiles of the closed flux surfaces were also evaluated. A significant amount of the positive current density distributions appeared outside the last closed flux surface (LCFS), while all the negative current density distributions were evaluated inside the LCFS. The trapped electrons being the second-harmonic electron cyclotron resonant contributed the negative current inside the LCFS. The positive current density distributions outside the LCFS were significant for the driven current density for both the fundamental and second-harmonic resonances. [ABSTRACT FROM AUTHOR]

The plasma control system (PCS) of QUEST was a centralized system, which lost its scalability because of the overload imposed on its central processing unit (CPU) of the PCS, making it impossible to add new functions. Thus, the PCS is distributed into a main workstation (WS) and subsystem (SS) with a reflective memory (RFM) in order to share data between these systems so as to mitigate the load on each system. As a result, 128 double-precision floating-point numbers (DBLs) can be transferred from the SS to the WS with a maximum latency of 250 μs. The WS and the SS each have quad-core CPUs, and tasks are executed in parallel. Although one of the four cores is intermittently occupied by up to 90% by this transaction, the occupation is normally 60%. A time correction procedure is used to map the recorded data sets on the WS and the SS to a common time base by referring to the time difference between two systems. [ABSTRACT FROM AUTHOR]

The effect of grain orientation, heat-treatment temperature and test temperature on the mechanical properties of tungsten (W), which vary depending on plastic working and fabrication process, was investigated by mechanical testing of tensile or bending. Heavily worked W samples (1.5–2.0 mm in the final thickness) exhibit degradation of fracture strength due to recrystallization embrittlement after heat-treatment at 1240 °C (temperature of diffusion bonding between W and a candidate material of the Fe base support structure). On the other hand, W samples with lower thickness reduction rates do not suffer degradation of fracture strength after heating up to around 1300 °C, and show somewhat higher fracture strength by heat-treatment below 1300 °C than the samples in the as-received state. The observed behavior is a reflection of recovery of dislocations introduced by plastic working. High temperature tensile testing of ITER grade W with an anisotropic grain structure and S-TUN with an equiaxed grain structure revealed that both W grades exhibit plastic elongation at temperatures higher than 200 °C with essentially the same temperature dependence of yield strength, which is relatively insensitive to grain orientation in the structure at 200–1300 °C. [ABSTRACT FROM AUTHOR]

In the present RF-driven plasma with a lot of high-energy electrons, there may be anisotropic plasma pressure, which makes the usual equilibrium analysis difficult, but the Cauchy condition surface method can reconstruct the plasma shape precisely regardless of the anisotropy. In addition, the plasma current effect in the open magnetic surfaces outside of the closed magnetic surfaces is considered in the RF-driven divertor plasma. In the reconstruction process, singular value (SV) decomposition is used and optimal criterion function for generalized cross validation is estimated concerning truncation or reduction of the small-SV components. [ABSTRACT FROM PUBLISHER]

W-2%vol Y 2 O 3 composite material were prepared by wet-chemical and rolling methods. A comparative study was conducted with commercially pure tungsten. The laser thermal shock and the damage behavior of the material after helium ion irradiation were studied. The damage behavior was analyzed through X-ray diffractometry, field emission scanning electron microscopy, focused ion beam, field emission transmission electron microscopy, and energy-dispersive X-ray spectroscopy. Results showed that W–Y 2 O 3 composite material have higher resistance to laser thermal shock than pure tungsten. Helium ion irradiation further illustrated that the cracking caused by laser thermal shock is dominated by intergranular cracks. In addition, tungsten matrix damage is associated with grain orientation deformation after the helium ion irradiation of pure tungsten and W–Y 2 O 3 composite material, and the grain orientation resistance to helium ion irradiation damage is higher than that to increasing grain boundary density. Several helium bubbles were found in the tungsten matrix, the fuzz structure, and the Y 2 O 3 particles, and all the helium bubbles were polyhedral rather than spherical. • W–Y 2 O 3 composite possess higher thermal shock resistance than pure tungsten. • Compared with increasing grain boundary density, grain orientation is more favorable to resist irradiation damage. • Helium bubbles were observed in the Y 2 O 3 particles. • The reported helium bubbles were polyhedral rather than spherical. [ABSTRACT FROM AUTHOR]

To study the D 2 retention behavior of W–ZrC/Sc 2 O 3 composites, mirror-polished samples prepared by spark plasma sintering (SPS) were exposed to different irradiation doses at room temperature (RT). TDS measurements were subsequently performed from RT to 900 K. Results revealed that Sc 2 O 3 particles did not benefit the D 2 + ion irradiation resistance, whereas W–1vol% ZrC/2 vol% Sc 2 O 3 composites showed superior irradiation resistance over other samples. D 2 implantation was also performed at 523 K with the irradiation dose of 1 × 1020 D 2 +/m2 to study the influence of temperature on D 2 retention during the capture process, which could apparently decrease D 2 retention by low-energy traps. Only small dislocation loops or precipitates were observed in W–1vol% ZrC/2 vol% Sc 2 O 3 sample after irradiation up to 1.0✕1021 D 2 +/m2. • The effect of ZrC/Sc 2 O 3 doping on irradiation resistance behavior of W materials was investigated. • 1 vol% ZrC/2 vol% Sc 2 O 3 composite possess better irradiation resistance than SPS/W. • Increasing implantation temperature decrease D 2 retention by low-energy traps. [ABSTRACT FROM AUTHOR]

W-2vol.%TiC composites were prepared through wet-chemical method combined with sintering and hot-rolling process. In this work, the evolution of damage micrograph in W-2vol.%TiC was studied by SEM in situ observation as functions of annealing temperature and implantation fluence. The results show that the surface morphology of the W matrix was slightly affected after D ions irradiation, while the dispersively distributed TiC particles were damaged severely. After annealing at 1173 K and 1573 K, the irradiation damage structure was repaired. [ABSTRACT FROM AUTHOR]