Successive Picket Drive for Mitigating the Ablative Richtmyer–Meshkov Instability

The ablative Richtmyer–Meshkov instability(ARMI) is crucial to the successful ignition implosion of the inertial confinement fusion(ICF) because of its action as the seed of the Rayleigh–Taylor instability. In usual ICF implosions, the first shock driven by various foots of the pulses plays a central role in the ARMI growth. We propose a new scheme for refraining from ARMI with a pulse of successive pickets. With the successive-picket pulse design, a rippled capsule surface is compressed by three successive shocks with sequentially strengthening intensities and ablated stabilization, and the ablative Richtmyer–Meshkov growth is mitigated quite effectively.Our numerical simulations and theoretical analyses identify the validity of this scheme.     

Preliminary investigation on electrothermal instabilities in early phases of cylindrical foil implosions on primary test stand facility

Recent experiments on the implosions of 15-mm long and 2-μm thick aluminum liners having a diameter of 12.8 mm have been performed on the primary test stand(PTS) facility. The stratified structures are observed as alternating dark and light transverse stripes in the laser shadowgraph images. These striations perpendicular to the current flow are formed early in the implosion, i.e., at the stage when the bulk of the material mass was almost at rest. A two-dimensional(2 D)magnetohydrodynamics(MHD) code is employed to simulate the behavior of liner dynamics in the early phases. It is found that the striations may be produced by the electrothermal instability(ETI) that results from non-uniform Joule heating due to the characteristic relation between the resistivity and the temperature. In 2 D simulations, the stratified structures can be seen obviously in both density and temperature contours as the liner expands rapidly. By analyzing instability spectrum, the dominant wavelengths of the perturbations are 8.33 μm–20.0 μm, which agree qualitatively with the theoretical predictions.It is also interesting to show that ETI provides a significant seed to the subsequent magneto Rayleigh–Taylor(MRT)instability.     

Nonlinear Saturation Amplitude in Classical Planar Richtmyer–Meshkov Instability

The classical planar Richtmyer–Meshkov instability(RMI) at a fluid interface supported by a constant pressure is investigated by a formal perturbation expansion up to the third order,and then according to definition of nonlinear saturation amplitude(NSA) in Rayleigh–Taylor instability(RTI),the NSA in planar RMI is obtained explicitly.It is found that the NSA in planar RMI is affected by the initial perturbation wavelength and the initial amplitude of the interface,while the effect of the initial amplitude of the interface on the NSA is less than that of the initial perturbation wavelength.Without marginal influence of the initial amplitude,the NSA increases linearly with wavelength.The NSA normalized by the wavelength in planar RMI is about 0.11,larger than that corresponding to RTI.     

Nonequilibrium kinetics effects in Richtmyer–Meshkov instability and reshock processes

Kinetic effects in the inertial confinement fusion ignition process are far from clear. In this work,we study the Richtmyer–Meshkov instability and reshock processes by using a two-fluid discrete Boltzmann method. The work begins by interpreting the experiment conducted by Collins and Jacobs(2002, J. Fluid Mech. 464, 113–136). It shows that the shock wave causes substances in close proximity to the substance interface to deviate more significantly from their thermodynamic equilibrium state. The ...     

RT instability in cylindrical implosion of jelly ring

The interfacial RT instability experiments on imploding jelly liners in cylindrically convergent geometry have been performed. The liner‘s instability growth was observed clearly with a high-speed framing camera. Jelly liners have different initial perturbation forms on their inner and outer interfaces, being either smooth or sinusoidal. The initial perturbations also have different magnitudes and spatial frequencies (for example, mode n=-5, 10, 20). The experimental results show that the growth and coupling of perturbations on inner and outer surfaces are remarkably different. Meanwhile, the relevant 2-D numerical simulation of hydrodynamics combined with Level Set method has been performed. Using the numerical code, we can design the parameters of imploding jelly liner, and predict the experimental results. The results of simulation are demonstrated to be in good agreement with the measured data in a series of experiments.     

Evolution of downsized crescent-shaped dune in wind tunnel experiment

The migration of a downsized crescent-shaped dune was investigated in a wind tunnel experiment.Quantified upwind influx and vertical oscillation of the sand bed were introduced to modulate the saturation level of the sand flux above the dune surface to affect dune evolution.The evolution was recorded by top-view photography and then abstracted as the evolution of self-defined characteristic quantities using a digital image processing algorithm.The results showed that,in contrast to the case for spanwise quantities,the evolution of streamwise quantities corresponds to a linear increase in the modulation magnitude more positively and in a monotonic and convergent manner.In contrast with quantities on the windward face,the changes in quantities with respect to the horns were nonmonotonic with time and almost uncorrelated with the variation in modulation strength,which reveals the distinctiveness of leeside evolution.     

Unusual softening behavior of yield strength in niobium at high pressures

In situ synchrotron angle-dispersive x-ray diffraction experiments on niobium powders have been conducted at pressures up to 61 GPa and room temperature using the diamond anvil cell technique. From the full width at half maximum of the measured diffraction lines, the yield strength was derived with the line-width analysis theory. The niobium powder sample was found to be compressed more packed firstly and then yielded at~14 GPa–18 GPa. Following an initial increase in the yield strength with pressure, an obvious decrease was observed occurring at ~42 GPa–47 GPa accompanying with a typical pressure dependence above 47 GPa. The experimentally observed anomalous softening of the yield strength in niobium surprisingly follows the trend of the predicted unusual softening in the shear modulus by the recent theoretical investigations. The possible mechanisms, applicable to interpret the yield strength softening of materials at high pressure,were also discussed in detail.     

Characteristics of nonlinear evolution of wavepackets in boundary layers

The nonlinear evolution of a finite-amplitude disturbance in a 3-D supersonic boundary layer over a cone was investigated recently by Liu et al.using direct numerical simulation(DNS).It was found that certain small-scale 3-D disturbances amplified rapidly.These disturbances exhibit the characteristics of second modes,and the most amplified components have a welldefined spanwise wavelength,indicating a clear selectivity of the amplification.In the case of a cone,the three-dimensionality of the base flow and the disturbances themselves may be responsible for the rapid amplification.In order to ascertain which of these two effects are essential,in this study we carried out DNS of the nonlinear evolution of a spanwise localized disturbance(wavepacket) in a flat-plate boundary layer.A similar amplification of small-scale disturbances was observed,suggesting that the direct reason for the rapid amplification is the three-dimensionality of the disturbances rather than the three-dimensional nature of the base flow,even though the latter does alter the spanwise distribution of the disturbance.The rapid growth of 3-D waves may be attributed to the secondary instability mechanism.Further simulations were performed for a wavepacket of first modes in a supersonic boundary layer and of Tollmien-Schlichting(T-S) waves in an incompressible boundary layer.The results show that the amplifying components are in the band centered at zero spanwise wavenumber rather than at a finite spanwise wavenumber.It is therefore concluded that the rapid growth of 3-D disturbances in a band centered at a preferred large spanwise wavenumber is the main characteristic of nonlinear evolution of second mode disturbances in supersonic boundary layers.     

Study on scattering correction in fast neutron tomography at NECTAR facility

Fast neutron tomography has been established as an inspection and detection tool at the NECTAR facility at the FRM-II reactor.Scattered neutrons from the object become a major disturbance and bring in artifacts and deviations in reconstruction results,especially for hydrogenous material object.In this article,an iterative scattering correction method for fast neutron tomography was proposed.In each loop of iteration the scattering component of the projections will be simulated by Monte-Carlo program MCNPX based on the previous reconstruction result and then it will be subtracted from original projections.The differences between scattering components at different perspectives were quantitatively evaluated and an average scattering component image was used for all projections finally.Smooth and uniform slices with more clear edges were obtained and the new reconstructed attenuation coefficients are quite close to the real one compared to the results without scattering correction,in which case the relative error of the reconstructed attenuation coefficients is about 10%–30%.     

Simulation of shock-induced instability using an essentially conservative adaptive CE/SE method

An essentially conservative adaptive space time conservation element and solution element(CE/SE)method is proposed for the effective simulation of shock-induced instability with low computational cost.Its implementation is based on redefined conservation elements(CEs)and solution elements(SEs),optimized interpolations and a Courant number insensitive CE/SE scheme.This approach is used in two applications,the Woodward double Mach reflection and a twocomponent Richtmyer–Meshkov instability experiment.This scheme reveals the essential features of the investigated cases,captures small unstable structures,and yields a solution that is consistent with the results from experiments or other high order methods.     

Application of Arnoldi method to boundary layer instability

The Arnoldi method is applied to boundary layer instability, and a finite difference method is employed to avoid the limit of the finite element method. This modus operandi is verified by three comparison cases, i.e., comparison with linear stability theory(LST) for two-dimensional(2D) disturbance on one-dimensional(1D) basic flow, comparison with LST for three-dimensional(3D) disturbance on 1D basic flow, and comparison with Floquet theory for 3D disturbance on 2D basic flow. Then it is applied to secondary instability analysis on the streaky boundary layer under spanwise-localized free-stream turbulence(FST). Three unstable modes are found, i.e., an inner mode at a high-speed center streak, a sinuous type outer mode at a low-speed center streak, and a sinuous type outer mode at low-speed side streaks. All these modes are much more unstable than Tollmien–Schlichting(TS) waves, implying the dominant contribution of secondary instability in bypass transition. The modes at strong center streak are more unstable than those at weak side streaks, so the center streak is ‘dangerous' in secondary instability.     

Rayleigh–Taylor instability at spherical interfaces of incompressible fluids

Rayleigh–Taylor instability(RTI) of three incompressible fluids with two interfaces in spherical geometry is derived analytically. The growth rate on the two interfaces and the perturbation feedthrough coefficients between two spherical interfaces are derived. For low-mode perturbation, the feedthrough effect from outer interface to inner interface is much more severe than the corresponding planar case, while the feedback from inner interface to the outer interface is smaller than that in planar geometry. The low-mode perturbations lead to the pronounced RTI growth on the inner interface of a spherical shell that are larger than the cylindrical and planar results. It is the low-mode perturbation that results in the difference between the RTI growth in spherical and cylindrical geometry. When the mode number of the perturbation is large enough, the results in cylindrical geometry are recovered.     

Magnetic and mechanical properties of Ni–Mn–Ga/Fe–Ga ferromagnetic shape memory composite

A ferromagnetic shape memory composite of Ni–Mn–Ga and Fe–Ga was fabricated by using spark plasma sintering method. The magnetic and mechanical properties of the composite were investigated. Compared to the Ni–Mn–Ga alloy,the threshold field for magnetic-field-induced strain in the composite is clearly reduced owing to the assistance of internal stress generated from Fe–Ga. Meanwhile, the ductility has been significantly improved in the composite. A fracture strain of 26% and a compressive strength of 1600 MPa were achieved.     

Construction and cosmic-ray test of the new inner drift chamber for BESIII

A new inner drift chamber has been built which can replace the aged part of the BESIII drift chamber when needed. The design of the new inner drift chamber can minimize the ineffective area in the very forward and backward region and hence reduce the background event rate. With this design, the new inner drift chamber is expected to have a longer lifetime and improved performance due to the lower occupancy. The endplates and the cylinder were machined with high precision. Wire stringing was performed after the mechanical structure was assembled, and good quality of wire stringing was ensured by measurement of the tension and leakage current of the wires. After completion of the physical construction of the new chamber, a cosmic-ray test was carried out to test its performance. The results of the cosmic-ray test show that the new inner chamber achieves a spatial resolution of127 μm and a d E/dx resolution of 6.4%, which satisfies the design specifications.     

Quantitative Single-Ion Irradiation by ASIPP Microbeam

A single-ion microbeam facility has been constructed by the microbeam research group in ASIPP (Institute of Plasma Physics, Chinese Academy of Science). The system was designed to deliver defined numbers of hydrogen ions produced by a van de Graaff accelerator, covering an energy range from 200 keV to 3 MeV, into living cells (5μm-20μm diameter) growing in culture on thin plastic films. The beam is collimated by a 1-μm inner diameter HPLC (high performance liquid chromatograph) capillary, which forms the micron-dimensional beam-line exit. A microbeam collimator, a scintillation ion counting system and a fast beam shutter, which constitute a precise dosage measuring and controlling system, jointly perform quantitative single-ion irradiation. With this facility, we can presently acquire ion-hitting efficiency close to 95%.     

Intrinsic stress analysis of sputtered carbon film

Intrinsic stresses of carbon films deposited by direct current (DC) magnetron sputtering were investigated. The bombardments of energetic particles during the growth of films were considered to be the main reason for compressive intrinsic stresses.The values of intrinsic stresses were determined by measuring the radius of curvature of substrates before and after film deposition.By varying argon pressure and target-substrate distance,energies of neutral carbon atoms impinging on the growing films were optimized to control the intrinsic stresses level.The stress evolution in carbon films as a function of film thickness was investigated and a void-related stress relief mechanism was proposed to interpret this evolution.     

Intrinsic stress analysis of sputtered carbon film

Intrinsic stresses of carbon films deposited by direct current （DC） magnetron sputtering were investigated. The bombardments of energetic particles during the growth of films were considered to be the main reason for compressive intrinsic stresses. The values of intrinsic stresses were determined by measuring the radius of curvature of substrates before and after film deposition. By varying argon pressure and target-substrate distance, energies of neutral carbon atoms impinging on the growing films were optimized to control the intrinsic stresses level. The stress evolution in carbon films as a function of film thickness was investigated and a void-related stress relief mechanism was proposed to interpret this evolution.     

Numerical study of contacts between a flat-ended punch and a half-space embedded with inhomogeneities

This paper presented a numerical approach to solving the problem of a flat-ended punch in contact with a half-space matrix embedded with multiple three dimensional arbitrary-shaped inhomogeneities.Based on the semi-analytical method(SAM)and the equivalent inclusion method,numerical procedures were developed and the effects of inclusion shape and distribution were analyzed.Fast Fourier transform technique was implemented to accelerate the calculation of surface deformation and subsurface stress.Interactions of inter-inclusions and inclusion-matrix were taken into account.Numerical results showed the presence of inhomogeneities(i.e.,microstructures in solids)indeed had a great effect on local contact pressure and a strong disturbance to the subsurface stress field in the vicinity of inclusions.The effects were dependent on the shape and distribution of inclusions and inter-inclusion interactions.The physical significance of this study is to provide an insight into the relation between the material microstructure and its response to the external load,and the solution approach and procedures may find useful applications,for example,the analysis of fatigue and crack propagation for composite materials,prediction of stress field in solids containing material defects,and study of the mechanism of chemical-mechanical polish(CMP)for inhomogeneous materials,etc.     

Micro-fabrication and hermeticity measurement of alkali-atom vapor cells based on anodic bonding

A vapor cell provides a well-controlled and stable inner atmosphere for atomic sensors, such as atomic gyroscopes, atomic magnetometers, and atomic clocks, and its hermeticity affects the stability and aging of atomic sensors. We present the micro-fabrication of a micro-electromechanical system wafer-level hermit vapor cell based on deep reactive ion etching and vacuum anodic-bonding technology. The anodic-bonding process with the voltage increasing in steps of 200 V had a critical influence on vapor cell hermeticity. Further, the silicon–glass bonding surface was experimentally investigated by a scanning electron microscope, which illustrated that there were no visual cracks and defects in the bonding surface. The leak rate was measured using a helium leak detector. The result shows that the vapor cells with different optical cavity lengths comply with the MIL-STD-883 E standard(5 × 10~(-8) mbar·L/s). Moreover, D2 absorption spectroscopy was characterized via optical absorption. The bonding strength was determined to be 13 MPa, which further verified the quality of the vapor cells.     

Structural Phase Transformations of ZnS Nanocrystalline Under High Pressure

In-situ energy dispersive x-ray diffraction on ZnS nanocrystalline was carried out under high pressure by using a diamond anvil cell. Phase transition of wurtzite of 10nm ZnS to rocksalt occurred at 16.0GPa, which was higher than that of the bulk materials. The structures of ZnS nanocrystalline at different pressures were built by using materials studio and the bulk modulus, and the pressure derivative of ZnS nanocrystalline were derived by fitting the equation of Birch-Murnaghan. The resulting modulus was higher than that of the corresponding bulk material, which indicates that the nanomaterial has higher hardness than its bulk materials.