Calculation of hydrogen and helium concentrations for CSNS target

The China Spallation Neutron Source(CSNS) is driven by protons whose energies are about 1.6 GeV.At such high energies, the spallation neutrons lead to the formation of large amounts of helium, hydrogen and new heavier species in the form of transmutation products. These hydrogen, helium and transmutation products have a critical effect on the mechanical properties on the one hand and exacerbate the displacement radiation damage on the other hand. In this paper, the background hydrogen/helium concentrations and the maximum hydrogen/helium concentrations near cracks in a tungsten target for CSNS have been calculated at temperatures of 100 and 300 by applying a theoretical model. For the CSNS tungsten target plate, we find the maximum hydrogen concentration near the tips of cracks ranges from 3.0×10~(-2)–2×10~(-1), which exceeds the hydrogen background concentration by 1.2–1.8 times; the maximum helium concentration near the tips of cracks ranges from 3.0×10~(-4)- 1.2×10~(-3), which exceeds the helium background concentration by 2- 4 times; the maximum hydrogen/helium concentration increases with the increase of the transfer length across the surfaces of the target and it decreases with the increase of temperature.     

Entanglement in a two-spin system with long-range interactions

The quantum entanglement between two spins in the Ising model with an added Dzyaloshinsky–Moriya(DM) interaction and in the presence of the transverse magnetic field is studied. The exchange interaction is considered as a function of the distance between spins. The negativity as a function of magnetic field, exchange and DM interaction is calculated.The effect of the distance between spins is studied based on the negativity. In addition, the effect of the thermal fluctuation on the negativity is also investigated.     

Analysis of the time-domain spectrum of hydrogen in electric field near helium surface

The Ryderberg electronic wave packet dynamics of hydrogen atom near helium surface in an electric field is investigated using the semiclassical method.The autocorrelation function is calculated when the photoionized electron is excited by a short laser pulse for different atom-surface separations.The results show that new recurrences appear because of the helium surface,and the number of recurrent peaks increases with the decrease in atom-surface distance.The new feature is ascribed to the bifurcation of new closed orbits in the classical dynamics of the photoionized electron.Therefore,surface properties have a significant effect on the spectrum of nearby atoms or ions.     

Molecular dynamics study of helium bubble pressure in titanium

In this paper,the pressure state of the helium bubble in titanium is simulated by a molecular dynamics(MD) method.First,the possible helium/vacancy ratio is determined according to therelation between the bubble pressure and helium/vacancy ratio;then the dependences of the helium bubble pressure on the bubble radius at different temperatures are studied.It is shown that the product of the bubble pressure and the radius is approximately a constant,a result justifying the pressure-radius relation predicted by thermodynamics-based theory for gas bubble.Furthermore,a state equation of the helium bubble is established based on the MD calculations.Comparison between the results obtained by the state equation and corresponding experimental data shows that the state equation can describe reasonably the state of helium bubble and thus could be used for Monte Carlo simulations of the evolution of helium bubble in metals.     

Quantum discord of thermal states of a spin chain with Calogero-Moser type interaction

In this paper,we have investigated the effect of Calogero-Moser type interaction on the quantum discord of thermal states of a spin chain.Our results imply that the quantum discord depends on the relative distance between the spins,the external magnetic field,and the temperature.By a comparison between the quantum discord and the entanglement of formation,the quantum discord is more robust than the entanglement of formation in the sense that the latter takes a zero value in a large range of the parameters,while the former takes a nonzero value.     

An Origin of Dzyaloshinskii–Moriya Interaction at Graphene-Ferromagnet Interfaces Due to the Intralayer RKKY/BR Interaction

We present a theory of both the itinerant carrier-mediated RKKY interaction and the virtual excitations-mediated Bloembergen–Rowland(BR) interaction between magnetic moments in graphene induced by proximity effect with a ferromagnetic film. It is shown that the RKKY/BR interaction consists of the Heisenberg, Ising, and Dzyaloshinskii–Moriya(DM) terms. In the case of the nearest distance, we estimate the DM term from the RKKY/BR interaction is about 0.13 meV for the graphene/Co interface, which i...     

Correlation between randomly varying birefringence and random dispersion map in a dispersion-managed soliton system

The correlation between perturbations caused by randomly varying birefringence and a random dispersion map is considered in a dispersion-managed soliton system, and their effects on soliton propagation and interaction are investigated numerically. These perturbations lead to the disintegration of a soliton, and enhance the interaction between solitons. The correlation plays an important role, and reinforces these effects. Furthermore, there is a stochastic resonance between two perturbations in the system; here the effect is the largest, and the corresponding distance until disintegration is the shortest. Finally, nonlinear gain and a filter are introduced to effectively suppress these effects.     

Molecular Dynamics Simulations of Helium Behaviour in Titanium Crystals

Molecular dynamics simulations are performed to investigate the behaviour of helium atoms in titanium at a temperature of 30OK. The nucleation and growth of helium bubble has been simulated up to 50 helium atoms. The approach to simulate the bubble growth is to add helium atoms one by one to the bubble and let the system evolve. The titanium cohesion is based on the tight binding scheme derived from the embedded atom method, and the helium-titanium interaction is characterized by fitted potential in the form of a Lennard-Jones function. The pressure in small helium bubbles is approximately calculated. The simulation results show that the pressure will decrease with the increasing bubble size, while increase with the increasing helium atoms. An analytic function about the quantitative relationship of the pressure with the bubble size and number of helium atoms is also fitted.     

Metal-Element-Incorporation Induced Superconducting Hydrogen Clathrate Structure at High Pressure

The recent observation of high critical temperature T_c in lanthanum and Yttrium hydrides confirms the key role of hydrogen cage(H-cage)in determining high superconductivity.Here,we present a new class of metastable H₁₂ clathrate structures based on the icosahedral cI 24-Na that can be stabilized by incorporation of metal elements.Analysis shows that the charge transfer from metal atoms to H atoms contributes to forming the H₁₂ clathrate.Nine dynamically stable structures are identified to exhibit superconductivity,and a maximum T_c of 28K is found in voids-doped Mo₆H₂₄.Calculations reveal that the low T_c is attributed to the weak interaction between H atoms in each cage due to the long H–H distance.The current results provide a possible route to design H-cage containing superconductors.     

Influence of Randomness and Localization on RKKY Interactions in Diluted Magnetic Semiconductors

We show that the spatially random distribution of magnetic moments of dopants in diluted magnetic semiconductors can partially localize the itinerant carriers and change the carrier-mediated indirect RKKY interaction. From numerical calculations of the electron states taking into account the interaction with magnetic impurities which are random both in spatial positions and in orientations of magnetic moments, we obtain the electron states and the RKKY interaction as a function of the distance between magnetic dopants L and of the sp - d exchange integral J. With the increase of disorder, the localization of itinerant electrons become stronger and the long-range regular oscillatory behaviour of the RKKY interaction gradually disappears and is replaced by severe fluctuations. The randomness and localization may enhance the RKKY interaction between dopants with short and middle distances and in favour of the ferromagnetism.     

Effects of Cr on H and He trapping and vacancy complexes in V in a fusion environment: a first-principles study

First-principles density functional theory was used to investigate the interaction between hydrogen (H) and helium (He) in V–Cr alloy, which is a potential structural material for use in fusion reactors. When vacancies are present in the V–Cr alloy, a single He atom prefers to occupy the octahedral site near the Cr atom rather than vacancy centre, which differs from the cases of iron and tungsten. Because of the decrease of the electron density around the He atom, there was a strong interaction between He and H. In the vicinity of He-vac complexes, H atoms tend to stay in the tetrahedral site rather than occupy the octahedral-interstitial site. A single He-vac complex can trap as many as six H atoms, which is more than can be trapped by an empty vacancy in the V–Cr alloy because of the electronic density redistribution of vacancy vicinity. This strong attraction explains the enhanced retention of H and He observed near the surface of V and V-based alloys under both sequential and simultaneous bombardments. The results provide useful insight into the application of the V-based alloys as candidate structural materials in fusion Tokamaks.     

氦离子显微镜对钨中氦行为的实验研究

针对热核聚变面向等离子体钨材料中氦泡形成、演变以及机理研究的需求,克服目前常用离子注入、电子扫描显微镜和透射电子显微镜等离线研究手段存在的不足,提出氦离子显微镜对钨中氦的上述行为原位实时在线研究方法.借助氦离子显微镜的离子注入、显微成像和聚焦离子束纳米加工功能,它可以提供能量为0.5—35 ke V、束流密度可达10~(25) ions/(m~2·s)以上的氦离子束,在该设备上进行钨中氦的注入实验.同时在注入过程,实时在线监测钨中氦泡形成、演变过程以及钨材料表面形貌的变化,原位在线分析钨材料表面氦泡的大小、迁移合并以及其诱发的钨表面和近表面的微观损伤.实验结果表明:氦离子显微镜是研究钨中氦行为演变过程及其微观机理研究的新的研究手段和强有力的实验工具.     

Influence of film-mediated interactions on the microwave and radio frequency spectrum of spin-polarized hydrogen on helium films

We argue that helium film-mediated hydrogen-hydrogen interactions strongly reduce the magnitude of cold collision shifts in spin-polarized hydrogen adsorbed on a helium film. With plausible assumptions about experimental parameters this can explain (i) the 2 orders of magnitude discrepancy between previous theory and recent experiments and (ii) the anomalous dependence of the cold collision frequency shifts on the film's 3He covering. The mediated interaction is attractive, suggesting that in current experiments the gas will become unstable before reaching the Kosterlitz-Thouless transition.     

Gauge vortices on a square lattice

The effective interaction between gauge vortices on a square lattice is revealed to be attractive at short distance and oscillate between repulsion and attraction at long distance. It is analyzed that the strong attraction at short distance makes the gauge vortices cluster at a given flux concentration; while the oscillatory nature in the interaction potential is the consequence of the existence of the electron Fermi surface in the nonuniform flux phases states.     

Gauge vortices on a square lattice

The effective interaction between gauge vortices on a square lattice is revealed to be attractive at short distance and oscillate between repulsion and attraction at long distance. It is analyzed that the strong attraction at short distance makes the gauge vortices cluster at a given flux concentration; while the oscillatory nature in the interaction potential is the consequence of the existence of the electron Fermi surface in the nonuniform flux phases states.     

Effects of Zr and V additions on the stability and migration of He in bcc W: A first-principles study

The effects of zirconium (Zr) and vanadium (V) on the dissolution, diffusion and trapping behaviors of helium (He) in body-centered cubic (bcc) tungsten (W) have been investigated using first-principles simulations. We found that Zr and V atoms usually prefer to disperse into W-rich region rather than aggregate in W host lattice. Energetically, the relatively stable position for an interstitial He remains the tetrahedral interstitial site in W with the presence of Zr/V atom. The solution energy of the He atom is reduced because of the attractive interaction between He and Zr/V atom. Compared to W with Zr, the stronger attractive interaction between V and He results in the lower He solution energy in W with V. Kinetically, the He atom diffusion to Zr/V has a lower energy barrier than that of He diffusion to W along the optimal diffusion path TIS–TIS. In addition, the existing Zr/V atom improves the ability of Zr/V-Vacancy complex to capture He atom. These results provide incontrovertible evidence that the existing Zr/V atom has great influence on the behavior of He atom in bulk W.     

Experimental study of cyclic action of plasma on tungsten

We report on experimental results on multiple action of hydrogen, deuterium, and helium plasmas produced by a plasma gun and the Globus-M tokamak on tungsten. The surface temperature in the course of irradiation is measured with a bichromatic pyrometer with a time resolution of ?1 μs. The morphology of the surface layer is investigated and X-ray structure analysis of tungsten exposed to multiple radiations by the plasma under various conditions is carried out. A slight decrease in the lattice parameter in the sample subjected to the maximal number of irradiation cycles is detected. It is shown that the morphology of the tungsten surface irradiated by the hydrogen plasma from the gun and by the deuterium plasma from the Globus-M tokamak changes (the structure becomes smoother). The characteristic depth of the layer in which impurities have been accumulated exceeds 0.5 μm. This depth was the largest for the sample exposed to 1000 shots from the gun and 2370 shots from the tokamak. It is shown that the helium jet from the plasma gun makes it possible to simulate the action of helium ions on the International Thermonuclear Experimental Reactor (ITER) diverter, producing a layer of submicrometer particles (bubbles).     

级联弧放电氦等离子体特性研究

采用另加偏压的单阴极弧氦放电直线等离子体装置对氦等离子体的基本特性进行了研究。对氦轴向输运规律做了描述并与光谱测量数据做了定性地比较。实验结果表明,氦等离子体的电子温度与电子密度均随放电电流、约束磁场的增加而增加。氦原子与氦离子的辐射光谱随放电电流、偏压、磁场的变化规律进行了测量分析,同时氦离子对钨靶积分辐照效应进行了观察。这些结果不但提供了氦等离子体的基本特性,对于研究氦离子与面向等离子材料相互作用导致产生气泡、肿胀、脆化损伤等的评估,特别是对将来伴有(n, α)反应时具有一定的参考价值。     

Elastic interaction and the phase transition in coherent metal-hydrogen systems

We study the statistical mechanics of hydrogen dissolved in metals. The underlying model is based on the assumption that the dominant attractive interaction between the protons in the metal is of an elastic nature.

In the first part of the paper we review some general properties of the elastic interaction. We then discuss the importance of boundary conditions for the form of the elastic interaction, which turns out to be of the Curie-Weiss type with macroscopic range.

In the second part we investigate the a-a' (‘gas-liquid’) phase transition in the hydrogen lattice fluid. The long-range part of the elastic interaction is treated in mean field approximation. In the canonical ensemble as opposed to the grand canonical ensemble one finds no co-existing phases near the critical point. Instead there is a continuous transition which changes into a first-order transition at tricritical points. In the temperature-density region which normally corresponds to the two-phase co-existence region the hydrogen density is inhomogeneous and varies on a macroscopic scale.

The peculiar nature of the a-a' phase transition is due to the long-range character of the elastic interaction, which ultimately results from the requirement of coherency of the host crystal. We argue that coherent metal-hydrogen systems offer examples of real systems where the classical theory of phase transitions applies.     

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采用另加偏压的单阴极弧氦放电直线等离子体装置对氦等离子体的基本特性进行了研究.对氦轴向输运规律做了描述并与光谱测量数据做了定性地比较.实验结果表明,氦等离子体的电子温度与电子密度均随放电电流、约束磁场的增加而增加.氦原子与氦离子的辐射光谱随放电电流、偏压、磁场的变化规律进行了测量分析,同时氦离子对钨靶积分辐照效应进行了观察.这些结果不但提供了氦等离子体的基本特性,对于研究氦离子与面向等离子材料相互作用导致产生气泡、肿胀、脆化损伤等的评估,特别是对将来伴有(n,α)反应时具有一定的参考价值.