Molecular Dynamics Simulations of the Interactions Between Tungsten Dust and Beryllium Plasma-Facing Material

In the present research,molecular dynamics simulation is applied to study the interactions between tungsten dusts and a beryllium plasma-facing material surface.Calculation results show that it is quite difficult for nanometer-size dust particles to damage the plasma-facing material surface,which is different from the micrometer-size ones.The reason may be the size difference between dust and crystal grains.The depth of dust penetration into plasma-facing materials is closely related to the incident velocity,and the impacting angle also plays an important role.Dust and material surface damage is also investigated.Results show that both incident velocity and angle can significantly influence the damage.     

The effect of positive/negative ion on the dust grain charging process in a Vasyliunas-Cairns (VC)-distributed dusty plasma system

Charging mechanism of dust particles has been considered as a growing research area in dusty plasma physics because of its exciting results. In this paper, we consider a low-temperature non- equilibrium multispecies plasma model, which consists of Vasyliunas–Cairns (VC) distributed electrons, negative/positive streaming ions, and negatively-charged dust grains to explain the charging mechanism of dust grains. The main theme of this work is to derive expressions of currents for negatively-charged dust grains (considering an equilibrium state position) in the plasma environment comprised of electrons and positive/negative streaming ions using the VC distribution function. Our proposed model shows that the dust grain surface potential is significantly affected by different plasma parameters such as the negative ion streaming velocity (S_n), positive ion streaming velocity (S_i), spectral indices of VC distribution, negative ion charging state (Z_n), positive ion charging state (Z_i), and negative ion number density (ρ).     

Influence of the Collisions on the Dusty Plasma Sheath in the Presence of an Oblique Magnetic Field

The dust dynamics in a magnetized collisional plasma-sheath are numerically studied by using the fluid model. Isothermal electrons, cold fluid ions, cold fluid dust grains and immobile neutral particles are taken into account in the sheath. As dust can be created by detaching small pieces of the wall limiting plasma, naturally, these grains can have different sizes. Therefore, the influence of dust size on the sheath characteristics is considered. Assuming the dust–neutral collision cross section has a power law dependency on the dust velocity. The comparison of the effect of the dust radius in both specific collisional models shows that in the constant cross section model, dust size plays a more role with respect to the constant collision frequency. The effect of the dust size on dust velocity is investigated for different values of the power factor. It shows that dust velocity when reaching near the wall in constant cross section model is much less than constant mobility model, and the velocity of the smaller dust is lower on the wall. If dust density is very small, the kind of collisional model has no significant influence on the electric potential. But by increasing dust density, a little fall in the local electric potential and a little rise in the sheath thickness are seen in constant cross section model.     

应急排气管道对主烟囱中速度及气体混合的影响

应用计算流体力学(CFD)方法,模拟内部的应急排气管道存在与否对主烟道内气体组分混合的影响情况。以速度及示踪气体浓度的变异系数(COV)表征截面混合均匀性,对比研究了高风速(约13.1 m/s)、中风速(约8.2 m/s)、低风速(约0.56 m/s)三种典型工况。结果表明:主烟道内风速13.1 m/s,无内部管道可使...     

Diagnosis of the surface morphology of laser-ablated materials using the weighted- DCT approach in laser speckle interferometry for application to plasma facing materials

To implement on-line, real-time monitoring for the surface morphology of Plasma-Facing Materials (PFMs) in tokamak, we developed a Laser Speckle Interferometry measurement approach. A laser ablation method was used to simulate the erosion process during Plasma-Wall Interactions in a tokamak. In the present investigation, we evaluated the results of laser ablation morphology changes on the surface of Mo material reconstructed by four different approaches (Flood-fill, Quality-guided, Discrete Cosine Transform (DCT) and Weighted-DCT). The morphology results measured by the weighted-DCT approach are very close to the measurement results from confocal microscopy with an average error rate within 7%. It is verified that the weighted-DCT algorithm has high accuracy and can efficiently reduce the influence of noise pollution coming from laser ablation, which is used as a proxy for erosion from plasma wall interaction. Additionally, the CPU computer time has been shortened. This is of great significance for the real-time monitoring of PFMs’ morphology in the Experimental Advanced Superconducting Tokamak (EAST) in the future.     

竖直窄矩形通道内空气-水两相流中气弹运动速度研究

以空气和水为工质,应用高速摄像仪,对竖直窄矩形通道(3.25 mm×40 mm)内气液两相弹状流进行了可视化实验研究。气、液相表观速度分别为0.1~2.51 m/s和0.16~2.62 m/s,工作压力为常压。实验中发现窄矩形通道内弹状流与圆管中存在较大差别,气弹多发生变形,高液相流速时变形更为严重。窄边液膜含气量较高,在高液相流速时窄边液膜不下落,宽边液膜中含有由气弹头部进入和气弹尾部进入的气泡。气弹速度受气弹头部形状和宽度影响较大,受气弹长度影响较小。气弹速度可由Ishii & Jones-Zuber模型计算,但在低液相折算速度时偏差较大,其主要原因为漂移速度计算值较实验值偏小。     

Main Directions and Recent Test Modeling Results of Lithium Capillary-Pore Systems as Plasma Facing Components

At present the most promising principal solution of the divertor problem appears to be the use of liquid metals and primarily of lithium Capillary-Pore Systems (CPS) as of plasma facing materials. A solid CPS filled with liquid lithium will have a high resistance to surface and volume damage because of neutron radiation effects, melting, splashing and thermal stressinduced cracking in steady state and during plasma transitions to provide the normal operation of divertor target plates and first-wall protecting elements. These materials will not be the sources of impurities inducing an increase of Zeef and they will not be collected as dust in the divertor area and in ducts. Experiments with lithium CPS under simulating conditions of plasma disruption on a hydrogen plasma accelerator MK-200 [-(10 - 15) MJ/m^2, - 50 μs] have been performed. The formation of a shielding layer of lithium plasma and the high stability of these systems have been shown. The new lithium limiter tests on an up-graded T-11M tokamak (plasma current up to 100 kA, pulse length -0.3 s) have been performed. Sorption and desorption of plasma-forming gas, lithium emission into discharge, lithium erosion, deposited power of the limiter are investigated in these experiments. The first results of experiments are presented.     

Exposure of Equal-Channel Angular Extruded Tungsten to Deuterium Plasma

Surface morphology and deuterium retention in ultrafine-grained tungsten fabricated by equal-channel angular pressing(ECAP) have been examined after exposure to a low energy,high-flux deuterium(D) plasma at fluences of 3×10~(24) D/m~2 and 1×10~(25) D/m~2 in a temperature range of 100 ℃-150 ℃.The methods used were scanning electron microscopy(SEM) and thermal desorption spectroscopy(TDS).Sparse and small blisters(~0.1 μm) were observed by SEM after D plasma irradiation on every irradiated surface;yet they did not exhibit significant structure or plasma fluence dependence.Larger blisters or protrusions appeared after subsequent TDS heating up to 1000 ℃.The TDS results showed a single D desorption peak at ~220℃ for all samples and the D retention increased with increasing numbers of extrusion passes,i.e.,the decrease of grain sizes.The increased D retention in this low temperature range should be attributed to the faster diffusion of D along the larger volume fraction of grain boundaries introduced by ECAP.     

低能氦离子辐照诱导的钨材料结构演化

在温度为923 K、氦离子流强为7×10~(21) m~(-2)·s~(-1)的条件下,考察了低能氦离子辐照对钨材料表面结构的影响。采用扫描电子显微镜(Scanning Electron Microscope,SEM)、导电原子力显微镜(Conductive Atomic Force Microscope,CAFM)、称重法、X射线衍射(X-ray Diffraction,XRD)以及电子背散射衍射(Electron Back-Scattered Diffraction,EBSD)对辐照后钨材料的结构演化规律进行了分析。SEM和CAFM的研究表明,在辐照初期样品表面形成了纳米尺寸的氦泡,随着辐照剂量的增加,氦泡的尺寸和密度逐渐增加,最终引起钨表层的剥落。质量损失和溅射产额的分析结果表明,钨材料表层的剥落是钨损伤的主要形式。SEM、XRD和EBSD的分析证实了辐照后钨样品的表面形貌变化与晶体取向之间具有很强的关联性。研究结果表明,相对于(101),氦原子更容易在(111)和(001)等晶面上吸附、扩散和聚集,这些研究结果将为面向等离子体材料的优化设计提供有用的参考。     

Comparison of High-Energy He+ and D+ Irradiation Impact on Tungsten Surface in the IR-IECF Device

Polycrystalline tungsten specimens were irradiated in the Iranian Inertial Electrostatic Confinement Fusion device (IR-IECF) by high energy (~100 keV) and high fluency (~10¹⁹ ions/cm²) helium and deuterium plasma to investigate the implantation impact of high energetic ions on tungsten as a candidate for fusion first wall material. Comparison of the exposure by He and D₂ plasma and influence of high temperature (~1,100 °C) implantation of each ion has been examined. Scanning electron microscopy was used to investigate surface morphology changes for various ion fluencies. Results showed the onset of visible surface pores formation especially for helium implanted samples which increased with higher implant fluencies, eventually resulting in a rough and flaky surface structure, unlike deuterium implanted samples on which smoothening of the surface occurred. Microhardness measurements were used to evaluate mechanical properties of implanted tungsten. Each specimen sustained surface hardening after implantation which was observed to increase with greater ion dose. The phase formation and structural evolution were studied by X-ray diffractometry method.     

Local damage of reinforced concrete slabs by impact of deformable projectiles

This study is to get informations about the local damage of reinforced concrete slabs by the impact of deformable projectiles. Five types of projectiles with different magnitudes of axial strength were employed for the impact tests. The target specimen were 0.6 m square reinforced concrete slabs with different thickness ranging from 7.0 to 15.0 cm. The striking velocity was kept at 200 m/s in all tests. And the effects of the projectile nose shape on the extent of local damage were also investigated experimentally.     

Gas-solid Two-phase Flow Analysis and Structure Optimizationof Spent Fuel Shearing Machine

Mechanical shearing systems are commonly used in reprocessing plants to crack spent fuel. The radioactive dust generated by shearing needs to be contained to ensure the full utilization of spent fuel pellets and the maintainability of the hot chamber. The airflow pattern and particle distribution and movement in the machine are of great significance to ensure the normal operation of the shearing machine. In order to supplement the deficiencies of experimental research and theoretical research, and improve dust distribution, reduce dust levels and optimize design of the shearing machine, the flow field in bar shearing machine with injected virtual particles was simulated based on the discrete particle model. By analyzing the distribution of velocity, pressure and turbulent kinetic energy of the fluid in shearing machine and the destructive effect of dust particles on the inner wall of shearing machine, the structure and boundary conditions of the shear were optimized. The results show that the eddy current, turbulence and high impact force can be effectively avoided by improving the structure of the flow passage and the input conditions. The velocity field, pressure field and turbulent kinetic energy distribution of gas-solid two-phase flow in the box are more uniform and the gradient change is smaller, and the damage of box wall is effectively reduced.     

乏燃料剪切机气固两相流分析及结构优化

后处理厂普遍采用机械法的剪切机系统进行乏燃料破解，剪切产生的放射性粉尘需进行包容设计，确保乏燃料芯块的充分利用和热室的可维修性，研究乏燃料剪切机内气流形态和颗粒物分布运动规律对于保障剪切机的正常运行具有重要意义。为补充试验研究和理论研究的不足，改善剪切机内粉尘分布、降低粉尘水平和优化剪切机设计，本文以注入模拟颗粒的乏燃料剪切机为研究对象、离散颗粒模型为基础，对剪切机内气固两相流进行了计算流体力学模拟。通过分析剪切机内流体的速度、压力和湍动能的分布规律，以及粉尘颗粒对箱体内部的破坏作用，对剪切机的结构和边界条件设计进行了优化。结果表明：改进流道结构和输入条件可有效避免剪切机内涡流、湍流和高冲击力的产生，箱体内气固两相流的速度场、压力场和湍动能分布规律较为均匀，梯度变化较小，可有效降低箱壁损伤。     

The damage effects of a 1 micron proton beam on a single pollen grain

Very little work has been carried out on the damage caused by high energy focused ion beams incident on matter, in particular to the more sensitive biological materials. Here in a preliminary investigation we assess the damage caused by the impact of a one μm beam of 4 MeV protons on a single cell pollen grain of Impatiens sultanii. By utilising optical microscopy, electron microscopy and elemental loss monitoring using PIXE, three types of damage have been identified; a very rapid charring of the surface, localised damage at the point of proton bombardment, and shrinkage and distortion of the sample around the point of impact. The magnitudes of these effects are variable and may depend on the nature and character of the specimen surface.     

Effects of Low Energy and High Flux Helium／Hydrogen Plasma Irradiation on Tungsten as Plasma Facing Material

The High-Z material tungsten (W) has been considered as a plasma facing material in the divertor region of ITER (International Thermonuclear Experimental Reactor). In ITER, the divertor is expected to operate under high particle fluxes (> 1023 m-2s-1) from the plasma as well as from intrinsic impurities with a very low energy (< 200 eV). During the past dacade, the effects of plasma irradiation on tungsten have been studied extensively as functions of the ion energy, fluence and surface temperature in the burning plasma conditions. In this paper, recent results concerning blister and bubble formations on the tungsten surface under low energy (< 100 eV) and high flux (> 1021 m-2s-1) He/H plasma irradiation are reviewed to gain a better understanding of the performance of tungsten as a plasma facing material under the burning plasma conditions.     

Dust Charging and Dynamics in Tokamaks

Fundamental characteristics of charging of a dust particle and its dynamics in SOL/divertor plasma in tokamaks are studied. According to the OML (orbit motion limited) theory, the charging process is extremely faster, with a charging time of nanoseconds, than the dynamics process of the dust particle in SOL/divertor plasma, with a characteristic time of milliseconds, which means that the local charge state can be taken as the equilibrium charge state. It was clarified that the equilibrium charge Zd,eq can be determined in the form of Zd,eq / Rd Te, which is a function of both the normalized relative speed of plasma ion flow with respect to the velocity of the dust particle and the plasma temperature ratio. After the investigation of dominant forces acting on dust particles, the friction forces due to the plasma ion absorption and ion Coulomb scattering are found of the same order for the case of low relative speed. The critical radius of a dust particle, for which the gravity is larger than the friction forces due to plasma ions, is obtained.     

Theoretical studies of macroscopic erosion mechanisms of melt layers developed on plasma facing components

During plasma instabilities in tokamak devices, metallic plasma facing components (PFC) undergo surface vaporization and melting. Macroscopic losses of melt layers are of a serious concern to the lifetime of PFC, the damage of nearby components, and potential core plasma contamination. A normal or inclined plasma stream flowing at the melt layer surface of PFC at very high velocities (∼10⁵ m/s) can induce Kelvin-Helmholtz (K-H) instabilities. We present an extensive linear stability theory and capillary droplet ejection model adapted to the problem of melt layer erosion and splashing. Based on this linear analysis, the stability criterion is established accounting the influence of the thicknesses of both plasma stream and melt layer. The growth rate of the most unstable wave is investigated with respect to different parameters such as plasma density and velocity, material properties, and melt layer thickness. A capillary droplet ejection model is then developed and used to analytically estimate the erosion rate of the melt layer for tungsten and aluminum targets. The present work brings a detailed understanding of the onset of K-H instabilities developed in melt layers due to plasma stream impact and builds a theoretical basis to estimate a macroscopic erosion rate, material losses and lifetime for PFC.     

Application of ICCG Method to Numerical Solutions of Few-Group Neutron Diffusion Equations

Emergency core cooling (ECC) mater is carried up to the upper plenum and falls down again into the core during the reflood phase in PWR-LOCA. Therefore the quench front also propagates downward from the top of the core. The effect of upward steam flow rate on the top-down quench propagation was experimentally investigated. It was found that top-down quench velocity was delayed by upward steam flow. This effect is more significant when rod surface temperature is low and the falling water flow rate is small.

The effect of the flow rate and the rod temperature on the quench velocity was correlated based on the experimental results under the conditions of atmospheric pressure, saturation temperature for water and steam, rod surface temperature of 350–600°C, down-ward water velocity of 0.01–0.1 m/s and upward steam velocity of 0–20 m/s.     

The behaviour of WB 35 piping material under dynamic loading in the high speed tensile test

For the determination of the strength-, deformation- and fracture behaviour of the material 17 MnMoV 6 4 (WB 35) which is used for piping components, tensile tests were carried out at different loading rates (monotonic and impact-type) on smooth and notched pipe strip specimens over a temperature range extending from − 30°C to 250°C.For the conduct of the tests a hydraulic high speed tensile machine having a free motion device was used; the velocity of impact was preset at ca. 7 m/s.With impact-type (dynamically) loaded specimens in general higher strength and deformation values were obtained than with monotonic (statically) loaded ones. In all of the specimens having low deformation values which were investigated microfractographically, ductile portions were found adjacent to the notch on the fracture surface.     

Dust tracking techniques applied to the STARDUST facility: First results

An important issue related to future nuclear fusion reactors fueled with deuterium and tritium is the creation of large amounts of dust due to several mechanisms (disruptions, ELMs and VDEs). The dust size expected in nuclear fusion experiments (such as ITER) is in the order of microns (between 0.1 and 1000 μm). Almost the total amount of this dust remains in the vacuum vessel (VV). This radiological dust can re-suspend in case of LOVA (loss of vacuum accident) and these phenomena can cause explosions and serious damages to the health of the operators and to the integrity of the device. The authors have developed a facility, STARDUST, in order to reproduce the thermo fluid-dynamic conditions comparable to those expected inside the VV of the next generation of experiments such as ITER in case of LOVA. The dust used inside the STARDUST facility presents particle sizes and physical characteristics comparable with those that created inside the VV of nuclear fusion experiments. In this facility an experimental campaign has been conducted with the purpose of tracking the dust re-suspended at low pressurization rates (comparable to those expected in case of LOVA in ITER and suggested by the General Safety and Security Report ITER-GSSR) using a fast camera with a frame rate from 1000 to 10,000 images per second. The velocity fields of the mobilized dust are derived from the imaging of a two-dimensional slice of the flow illuminated by optically adapted laser beam. The aim of this work is to demonstrate the possibility of dust tracking by means of image processing with the objective of determining the velocity field values of dust re-suspended during a LOVA.