Supersonic gas jets generated via a conical nozzle are widely applied in the laser wakefield acceleration of electrons. The stability of the gas jet is critical to the electron injection and the reproducibility of the wakefield acceleration. Here we discussed the role of the stilling chamber in a modified converging–diverging nozzle to dissipate the turbulence and to stabilize the gas jets. By the fluid dynamics simulations and the Mach–Zehnder interferometer measurements, the instability originating from the nonlinear turbulence is studied and the mechanism to suppress the instability is proposed. Both the numerical and experimental results prove that the carefully designed nozzle with a stilling chamber is able to reduce the perturbation by more than 10% compared with a simple-conical nozzle.

A deep understanding of two-phase displacement in porous media with permeability contrast is essential for the design and optimisation of enhanced oil recovery processes. In this paper, we investigate the forced imbibition behaviour in two dual-permeability geometries that are of equal permeability contrast. First, a mathematical model is developed for the imbibition in a pore doublet, which shows that the imbibition dynamics can be fully described by the viscosity ratio $\lambda$ and capillary number $Ca_m$ which creatively incorporates the influence of channel width and length. Through the finite difference solution of the mathematical model, a $\lambda$–$Ca_m$ phase diagram is established to characterise the imbibition preference in the pore doublet. We then investigate the imbibition process in a dual-permeability pore network using a well-established lattice Boltzmann method, focusing on the competition between the viscous and capillary forces. Like in the pore doublet, the preferential imbibition occurs in high-permeability zone at high $Ca_{m}$ but in low-permeability zone at low $Ca_{m}$. When $Ca_m$ is not sufficiently high, an oblique advancing pattern is observed which is attributed to non-trivial interfacial tension. Using the newly defined capillary number, the critical $Ca_{m}$ curve on which the breakthrough simultaneously occurs in both permeability zones is found to match well with that from the pore doublet and it is the optimal condition for maximising the imbibition efficiency in the entire pore network.

High inductive helical support provides a solution to controlling the alignment error of inner electrodes in magnetically insulated transmission lines (MITLs). Three-dimensional particle-in-cell simulations were performed to examine the current loss mechanism and the effects of structural parameters on electron flow in an MITL with a helical inductor. An empirical expression related to the ratio of electron current loss to anode current and the ratio of anode current to self-limited current was obtained. Electron current loss caused by helical inductor with different structures was displayed. The results indicate that the current loss in an MITL, near an inductive helical support, comprises both the inductor current and the electron current loss. The non-uniform structure and current of a helical inductor cause an abrupt change in the magnetic field near the helical support, which leads to anomalous behavior and current loss of electron flow. In addition, current loss in the inductive helical-supported MITL is negligible when the inductance of the support is sufficiently high. This work facilitates the estimation of electron current loss caused by the inductive helical support in MITLs.

Compared with end pumping fiber combiner, one of the advantages for side pumping combiner is the unlimited pumping points, which means multi-point or cascaded side pumping can be realized. However, the loss mechanism of the cascaded structure is rarely discussed. In this paper, we present the numerical and experimental investigation about the loss mechanism of a two-stage-cascaded side pumping combiner based on tapered-fused technique. The influence of loss mechanism on the coupling efficiency and thermal load of the fiber coating is analyzed according to simulations and experiments with different tapering ratios for the first stage. Based on the analysis, a cascaded component with total pump coupling efficiency of 96.4% handling a pump power of 1088 W is achieved by employing 1018 nm fiber laser as the pump source. Future work to further improve the performance of a cascaded side pumping combiner is discussed and prospected.

The apparent gas permeability of a porous medium is an important parameter in the prediction of unconventional gas production, which was first investigated systematically by Klinkenberg in 1941 and found to increase with the reciprocal mean gas pressure (or equivalently, the Knudsen number). Although the underlying rarefaction effects are well known, the reason that the correction factor in Klinkenberg’s famous equation decreases when the Knudsen number increases has not been fully understood. Most of the studies idealize the porous medium as a bundle of straight cylindrical tubes; however, according to the gas kinetic theory, this only results in an increase of the correction factor with the Knudsen number, which clearly contradicts Klinkenberg’s experimental observations. Here, by solving the Bhatnagar–Gross–Krook equation in simplified (but not simple) porous media, we identify, for the first time, two key factors that can explain Klinkenberg’s experimental results: the tortuous flow path and the non-unitary tangential momentum accommodation coefficient for the gas–surface interaction. Moreover, we find that Klinkenberg’s results can only be observed when the ratio between the apparent and intrinsic permeabilities is ${\lesssim}30$; at large ratios (or Knudsen numbers) the correction factor increases with the Knudsen number. Our numerical results could also serve as benchmarking cases to assess the accuracy of macroscopic models and/or numerical schemes for the modelling/simulation of rarefied gas flows in complex geometries over a wide range of gas rarefaction. Specifically, we point out that the Navier–Stokes equations with the first-order velocity-slip boundary condition are often misused to predict the apparent gas permeability of the porous medium; that is, any nonlinear dependence of the apparent gas permeability with the Knudsen number, predicted from the Navier–Stokes equations, is not reliable. Worse still, for some types of gas–surface interactions, even the ‘filtered’ linear dependence of the apparent gas permeability with the Knudsen number is of no practical use since, compared to the numerical solution of the Bhatnagar–Gross–Krook equation, it is only accurate when the ratio between the apparent and intrinsic permeabilities is ${\lesssim}1.5$.

The discovery of the ancient city of Loulan in Xinjiang, China, at the beginning of the 20th century was of great significance for understanding the evolution of culture and civilization in Inner Asia. However, due to the lack of systematic chronological studies, the history of this ancient city remains unclear, particularly the date of its construction and abandonment. Here, we present the results of the first systematic radiocarbon (14C) dating carried out on artifacts from ancient Loulan. Our results show that human activity began as early as 350 cal BC, flourished during the interval from the 1st to 4th centuries AD, and completely disappeared around 600 AD. Most of the buildings in the city were constructed during the Eastern Han Dynasty rather than in Wei/Jin Dynasty, as previously indicated by excavated documents and letters (Hedin 1898; Xiao 2006). The development and flourishing of Loulan coincided with the interval of high ice accumulation and meltwater supply from surrounding mountains. The city began to decline and was finally abandoned following an abrupt decrease in ice accumulation and meltwater supply (Yao et al. 1996; Lauterbach et al. 2014), suggesting that natural climate change was the major factor responsible for the abandonment of Loulan.

Cryomilling combined with laser induction hybrid cladding (LIHC) was adopted to produce NiCrAlY coatings on Ni-based superalloy. The characteristics, oxidation resistance, and mechanical properties of the cryomilled NiCrAlY coatings by LIHC were investigated. By increasing the cryomilling time, the as-received spherical powder experienced a transition from flake-shaped to polygonal structure. The particle size increased firstly and then decreased. Moreover, increasing the cryomilling time induced the columnar growth in the NiCrAlY coatings. This in turn improved the oxidation resistance and the mechanical properties of the coatings. Especially, when the cryomilling time was increased to 15 h, the oxidation resistance of the coating at 1423 K was approximately nine times than that of GH4169 superalloy. The tensile strength of the cryomilled (15 h) coating increased to 1085 MPa and the ductility was 20.7%.

Morphogenesis and identification of embryonic differentiation in porcine embryos are crucial issues for developmental biology and laboratory animal science. The current paper presents a study on the asynchronous development of hatched porcine embryos from days 7 to 13 post-insemination. Examination of semi-thin sections of the hypoblast showed that it had characteristics similar to those of the mouse anterior visceral endoderm during embryonic disc formation. Also, a cavity appeared in the epiblast, which was similar to a mouse proamniotic cavity. With the gradual disappearance of Rauber's layer, the cavity opened and contacted the external environment directly, all of which formed the embryonic disc. To confirm the differentiation characteristics, we performed immunohistochemical analyses and showed that GATA6 was detected clearly in parietal endoderm cells during embryonic disc establishment. OCT4 was expressed in the inner cell mass (ICM) and trophoblast of hatched blastocysts and in the epiblast during formation of the embryonic disc. However, OCT4 showed comparatively decreased expression in the posterior embryonic disc, primitive streak and migrating cells. SOX2 was present in the ICM and epiblast. Therefore, both SOX2 and OCT4 can be used as markers of pluripotent cells in the porcine embryonic disc. At the start of gastrulation, staining revealed VIMENTIN in the posterior of the embryonic disc, primitive streak and in migrating cells that underlay the embryonic disc and was also expressed in epiblast cells located in the anterior primitive streak. Together with serial sections of embryos stained by whole mount immunohistochemistry, the mesoderm differentiation pattern was shown as an ingression movement that took place at the posterior of the embryonic disc and with bilateral migration along the embryonic disc borders.

ZnO is a highly efficient photon emitter, has optical and piezoelectric properties that are attractive for a variety of applications. Due to its stacking order and close lattice to GaN, it is also considered as a substrate material for GaN epitaxy. In the past the poor preparation of ZnO surface has been a major handicap to GaN epitaxy. However, proper treatment we developed recently can make both faces of ZnO smooth with atomic level terraces. Epitaxy of GaN on O-face and Zn-face ZnO by reactive molecular beam epitaxy was performed. We used low-temperature RF growth of GaN buffer layer on ZnO surface to protect it from both ammonia and Ga. No Ga2ZnO4, an oxide with the spinel structure formed due to reaction of ZnO with Ga, was found, in contrast to earlier reports. The low-temperature photoluminescence (PL) indicates that both faces of ZnO can provide a high quality GaN with high radiative efficiency. In previous research it has been reported that O-face ZnO is slightly better for GaN epitaxy. Our new finding demonstrates that high-quality GaN epilayers can be grown on Zn face of ZnO.

We have analyzed a non-randomly frustrated spin model which exhibits behavior remarkably similar to the phenomenology of structural glasses. The high-temperature disordered phase undergoes a strong first-order transition to a long-range ordered structure. Using Monte Carlo simulations, we have studied the behavior of the supercooled state by quenching to temperatures below this transition temperature. For a range of supercooling, the system remains ergodic and exhibits dynamics characteristic of supercooled liquids. Below a certain characteristic temperature, however, the system freezes into a “glassy” phase. In this phase, the system is non-ergodic and evolves through a distribution of traps characterized by a power-law distribution of trapping times. This change in the dynamic behavior is concurrent with the appearance of a shear instability.