Weak Nonlinearity of Ablative Rayleigh--Taylor Instability

The weakly nonlinear regime of single mode ablative Rayleigh-Taylor instability is studied, with consideration of preheat effect and the width of the ablation front. The Rayleigh-Taylor linear growth rate agrees well with the direct numerical simulation. For the density perturbation, the amplitude distribution of the fundamental mode has one peak value whereas those of the second and third harmonics have two and three peak values, respectively. Harmonics generation versus wave number is also given and it is close to the result of direct numerical simulation.     

Palaeodose underestimates caused by extended-duration preheats in the optical dating of quartz

An optical dating study of some quartzose sediments from northern and southern Australia, north Africa and north-western Europe has revealed that palaeodose (P) underestimates of 10–40% are obtained when an extended-duration preheat of 160°C for 16 h is used. For these samples in the 0–60 ka age range, a preheat of 220°C for 5 min produces the correct P, as inferred from the concordance with thermoluminescence (TL) palaeodose determinations on the same sedimentary sample or on burnt flint. Independent support for use of the 220°C preheat is given by the agreement between optical ages younger than 30 ka and ¹⁴C age determinations on associated charcoal. The deleterious effect of the 160°C preheat is illustrated by growth curves of optically stimulated luminescence (OSL) vs added dose. These curves show that the 160°C preheat induces a dose-dependent increase in sample sensitivity, which may be related to activation of the pre-dose mechanism. The 160°C preheat also causes low-dose supralinearity, although the supralinearity correction is insufficient to reduce significantly the degree of P underestimation. A normalization method based on preheat-induced sensitivity changes of the 110°C TL peak, albeit limited by saturation effects, warrants further investigation.     

Highly efficient frequency doubling of high-energy Nd:YAG laser using a CsB3O5 crystal

We have demonstrated efficient frequency doubling of high-energy fundamental Nd:YAG laser pulse energy of the multi-joule (J) level at a high repetition rate using high optical-quality top-seeded solution growth CsB₃O₅(TSSG-CBO) crystal for the first time. Second-harmonic (532 nm) generation (SHG) output energy of 1.2 J at 10 Hz is obtained with a conversion efficiency of 60%. This result has been obtained at the multi-J level by the growth of high optical-quality TSSG-CBO crystal with the large effective nonlinear coefficient and high damage threshold. These results indicate that TSSG-CBO is a good candidate material for high-energy SHG of Nd-doped lasers at the several J level or more with high repetition rate.     

Imaging of the reaction zone in a 100 kW oil-burning furnace by use of a broad-band excimer laser

The reaction zone in the hostile combustion environment of a 100 kW oil-burning furnace has been imaged by laser-induced fluorescence using a broad-band XeCl-excimer laser. Upon excitation, the averaged images obtained by using an interference filter around 320 nm (FWHM of 10 nm) show three distinct areas along the direction of the gas flow. An intense emission spreads around the spray axis and is attributed to the fluorescence of large hydrocarbons in the unburned fuel. Approximately 12 cm downstream of the nozzle, a narrow dark region is displayed suggesting the preheat zone of the combustion process where large hydrocarbons are considerably degraded. The third distinct region is characterized by a strong onset of the fluorescence intensity localized downstream of the dark region. This feature is strongly suppressed by replacing the interference filter by a broad-band transmission filter passing light from 350 to 500 nm. Since OH strongly absorbs at the laser wavelength and its fluorescence is significantly lower above 345 nm, the findings imply that the major contribution to the observed intensity in this region originates from the OH radical. This molecule reaches its maximum concentration immediately downstream of the flame front. However, a contribution from other flame species fluorescing around 320 nm cannot be ruled out. Nevertheless, the combined spatial and spectral information obtained imply that the reaction zone of the combustion process can be localized accurately. The results are compared with simultaneously performed numerical simulations of the burner and are in reasonable agreement.     

Dependence of Limited Growth Rate of High-Quality Gem Diamond on Growth Conditions

The growth rate of diamond has been investigated for a long time and researchers have been attempting to enhance the growth rate of high-quality gem diamond infinitely. However, it has been found according to previous research results that the quality of diamond is debased with the increase of growth rate. Thus, under specific conditions, the growth rate of high-quality diamond cannot exceed a limited value that is called the limited growth rate of diamond. We synthesize a series of type Ib gem diamonds by temperature gradient method under high pressure and high temperature （HPHT） using the as-grown {100} face. The dependence of limited growth rate on growth conditions is studied. The results show that the limited growth rate increases when synthetic temperature decreases, also when growth time is prolonged.     

氢气对薄反应区湍流火焰详细火焰结构影响研究

氢是一种非常有前景的清洁可再生能源载体.掺氢燃料预混稀燃是当前开发清洁高效的低排放燃气轮机最重要的能源转化方式之一。本文基于预混CH₄/H₂/air本生灯火焰,对氢气掺混影响下的湍流火焰详细火焰结构进行了测量和表征。实验采用CH₂O和OH基平面激光诱导荧光(PLIF,Planar Laser Induced Fluorescence)同步测量技术,获得了火焰预热区、反应区以及已燃区的详细火焰结构信息。本文对反应区和预热区火焰厚度进行了提取和统计。研究表明,氢气对火焰反应区、预热区均有明显作用。结果表明,掺氢小幅度增厚反应区厚度,但能够比较明显地降低预热区厚度。     

A computational model for cancer growth by using complex networks

In this work we propose a computational model to investigate the proliferation of cancerous cell by using complex networks. In our model the network represents the structure of available space in the cancer propagation. The computational scheme considers a cancerous cell randomly included in the complex network. When the system evolves the cells can assume three states: proliferative, non-proliferative, and necrotic. Our results were compared with experimental data obtained from three human lung carcinoma cell lines. The computational simulations show that the cancerous cells have a Gompertzian growth. Also, our model simulates the formation of necrosis, increase of density, and resources diffusion to regions of lower nutrient concentration. We obtain that the cancer growth is very similar in random and small-world networks. On the other hand, the topological structure of the small-world network is more affected. The scale-free network has the largest rates of cancer growth due to hub formation. Finally, our results indicate that for different average degrees the rate of cancer growth is related to the available space in the network.     

Modulation instability by intense laser beam in magnetized plasma

Modulation instability of an intense right-hand elliptically polarized laser beam propagating through magnetized plasma is investigated by a new method. The nonlinear dispersion relation, in which the relativistic and ponderomotive nonlinearities are taken into account, is obtained for the laser radiation in magnetized plasma by the Lorentz transformation. The Karpman equation is firstly generalized to the case of three dimensions with three field components. When the nonlinear frequency shift of the electromagnetic field in plasma is involved, the nonlinear evolution equation for the slowly varying envelope of the laser field is obtained. Thus, modulation instability of the intense laser beam in magnetized plasma is studied and the temporal growth rate of the instability is derived. The analysis shows that the peak growth rate of self-modulation instability is increased due to the axial magnetization of plasma. It is also shown that the growth rate of modulation instability is increased significantly near the critical surface in a laser-plasma.     

The effect of pre-annealing of sputtered ZnO seed layers on growth of ZnO nanorods through a hydrothermal method

Oriented ZnO nanorods were grown on ion-beam-sputtered ZnO seed layers through a hydrothermal approach without any metal catalyst. The sputtered ZnO seed layers were pre-annealed at different temperatures before the growth of ZnO nanorods. The effects of pre-annealing of the ZnO seed layers on the growth rate, crystallinity and optical properties of ZnO nanorods thereon were studied. The obtained ZnO nanorods had a wurtzite structure and grew along the preferential [0001] orientation with a normal direction to the substrates. Results show that the growth rate and density of the ZnO nanorods strongly depend on the pre-treatment conditions of the ZnO seed layer. With higher pre-treatment temperature, the crystallinity and surface characteristics of the ZnO seed layer were improved and thereafter the growth rate of ZnO nanorods thereon increased. Photoluminescence spectroscopy results show that the UV emission also becomes stronger and sharper with increasing annealing temperature of the ZnO seed layer.     

辐射驱动下主动式高精度冲击波速度精密诊断技术

激光加载可以产生比气泡加载更高的压力,是高压状态方程研究中一种新的加载方式。在激光加载高压状态方程(EOS)研究中,台阶靶是常用的靶型。针对传统台阶靶存在的预热膨胀问题,提出了使用主动式任意反射面速度干涉仪(VISAR)进行台阶靶预热和冲击波测量的方法。该技术利用条纹图移动量计算厚度增加量,利用冲击波到达自由面产生条纹跳变的时刻来获得精确的冲击波到达时刻。通过修正已测量台阶厚度与膨胀量,可以获得更加精确的台阶厚度值。通过精确的时间间隔可以得到冲击波传输的时间。在匀速传输的条件下可以获得高精度的冲击波传输速度。该方法在辐射驱动超高压条件下具有很好的适用性,可以为状态方程实验提供高精度的冲击波速度数据。     

磁化套筒惯性聚变中端面损失效应的一维唯象模型与影响分析

得益于激光预加热和轴向磁场的作用,磁化套筒惯性聚变(magnetized liner inertial fusion,MagLIF)构型理论上能有效降低聚变实现的难度,具有极大的应用潜力.本文选择MagLIF过程中伴随激光预加热所必然存在端面损失效应作为研究目标,搭建了能够描述几何参数与腊肠不稳定性等高维效应的一维唯象物理模型,并分别通过与二维流体动力学程序和国外同类程序的计算对比完成参数拟合校验;在此基础上,获得端面损失效应对MagLIF内爆过程及预加热效果的影响规律.计算结果表明:不同喷射半径下MagLIF负载在内爆过程的绝大多数时间内保持了相近的流体动力学演化过程,并在迟滞阶段经历了相同的质量损失比例,且考虑端面效应后得到的预加热和内爆产额相对变差,但却不改变规律性的趋势.所建立的模型与结论有助于加深对MagLIF预加热和端面损失过程中物理图像的认知和理解.     

A comparative study of the luminescence characteristics of polymineral fine grains and coarse-grained K- and Na-rich feldspars

The IRSL and post-IR IRSL (pIRIR) signal characteristics of polymineral fine grains are investigated and compared with those of K- and Na-rich feldspar extracts. TL signal loss after IR and pIRIR stimulations occurs mainly at around 320 °C for polymineral and Na-feldspar samples and around 410 °C for K-feldspar samples, when a preheat temperature of 250 °C for 60 s is used. After preheating to a higher temperature (320 °C for 60 s) all samples show a TL reduction around 410 °C in the blue detection window. Pulse annealing experiments for IRSL and pIRIR signals for preheats between 320 °C and 500 °C indicate that the signal stabilities are similar among the different feldspar types, when a higher preheat temperature (>320 °C) is used. Thermal activation energies for IRSL and pIRIR signals are largest in K-feldspar and smallest in polymineral fine grains, in both blue and UV detection windows for both fast time-resolved (TR) and continuous wave (CW) signals. These results suggest that IRSL and pIRIR signals in polymineral fine grains originate mainly from Na-feldspar grains; these signals are less thermally stable than those from K-feldspar, but a more stable signal (presumably from K-feldspar grains) can be obtained using a higher preheat temperature.     

温度对Si衬底上低压MOCVD外延生长ZnS薄膜质量的影响

用低压MOCVD系统在(111)Si衬底上,用两步生长方法(改变/流量比)在300～400℃时外延生长了ZnS单晶薄膜。随着衬底温度的降低,ZnS薄膜结晶质量提高,并在300℃生长时获得结晶完整性较好的(111)ZnS单晶薄膜。文中讨论了衬底温度对薄膜质量的影响。     

Numerical fluid solutions for nonlocal electron transport in hot plasmas: Equivalent diffusion versus nonlocal source

Flux limitation and preheat are important processes in electron transport occurring in laser produced plasmas. The proper calculation of both of these has been a subject receiving much attention over the entire lifetime of the laser fusion project. Where nonlocal transport (instead of simple single flux limit) has been modeled, it has always been with what we denote the equivalent diffusion solution, namely treating the transport as only a diffusion process. We introduce here a new approach called the nonlocal source solution and show it is numerically viable for laser produced plasmas. It turns out that the equivalent diffusion solution generally underestimates preheat. Furthermore, the advance of the temperature front, and especially the preheat, can be held up by artificial ‘thermal barriers’. The nonlocal source method of solution, on the other hand more accurately describes preheat and can stably calculate the solution for the temperature even if the heat flux is up the gradient.     

Modulation instability of intense laser beam in an electron-positron plasma

Modulation instability of an intense right-hand elliptically polarized laser beam propagating through an electron-positron plasma is investigated by a new method. The nonlinear dispersion relation, in which the relativistic and ponderomotive nonlinearities are taken into account, is obtained for the laser radiation in electron-positron plasma by the Lorentz transformation. The Karpman equation is generalized to the case of three dimensions with three field components. When the nonlinear frequency shift of the electromagnetic field in plasma is involved, the nonlinear evolution equation for the slowly varying envelope of the laser field is obtained. Thus, modulation instability of the intense laser beam in electron-positron plasma is studied and the temporal growth rate of the instability is derived. The analysis shows that the growth rate of modulation instability is increased significantly near the critical surface in a laser-plasma.     

Preheat effects on shock propagation in indirect-drive inertial confinement fusion ablator materials

The velocities and temperatures of shock waves generated by laser-driven hohlraum radiation fields have been measured for several indirect-drive inertial confinement fusion capsule ablator materials. For the first time, a time-resolved measurement of the preheat temperature ahead of the shock front has been performed and included in the analysis. It is found that preheat ahead of the shock front can cause significant shock propagation variations in the ignition capsule ablator materials being considered for the National Ignition Facility (NIF). If unaccounted for, these preheat effects could potentially preclude ignition at the NIF.     

COIL动态平衡式供碘系统的研制

 建立了动态平衡式COIL供碘系统的理论模型。根据理论计算结果研制了一套供碘系统。该系统体积为前供碘系统的八分之一，质量为三分之一，预热时间缩短了一半。实验结果表明：该供碘系统碘流量完全能满足碘量优化实验需要，碘量调节便捷，有较宽的调节范围，且具有长时间工作更稳定的特点。     

Correlating flame geometry in opposed-flow flame spread over thin fuels

Numerical analysis and scale analysis are combined in a novel manner in this work to develop closed-form expressions for flame geometry in opposed-flow flame spread over condensed fuels. A scale analysis is used to relate different geometric attributes to appropriate non-dimensional parameters. A comprehensive numerical model is then used to generate a large set of numerical data for flame height, flame length, and pyrolysis length as functions of different fuel and oxidizer parameters for flame spread in the thermal, kinetic, and radiative regimes. The numerical data is then correlated to scaled expressions and the unknown coefficients are numerically determined. It is shown that flame length, flame height, and pyrolysis length can be expressed in terms of the preheat length in different regimes of flame spread. An experimental approach is outlined to measure the preheat length necessary for accurately predicting the flame structure. Experimental images obtained from interferometry in two different regimes – downward spreading configuration and quiescent microgravity environment – are consistent with the proposed flame height correlation.     

Enhanced light output of InGaN LEDs with a roughened p-GaN surface using different TMGa flow rates in p-AlGaN layer

We have demonstrated the enhancement of light output of InGaN-based blue light-emitting diodes (LEDs) using different trimethylgallium (TMGa) flow rates in the growth of p-AlGaN epilayer to facilitate a rougher p-GaN surface. It is found that higher output power can be achieved from the LEDs with rougher surface morphologies when the TMGa flow rate (R_TMGa) is increased up to 60 sccm during p-Al_0.05Ga_0.95N epilayer growth. Such a rough surface obtained at higher R_TMGa is attributed to the fact that the vertical growth rate is faster than the lateral growth rate, thus, leading to the facet of crystal growth focuses mainly in the vertical direction. The output power of devices biased at 20 mA is 15.4, 15.9, 17.5, and 18.9 mW for TMGa flow rates of 10, 20, 40, and 60 sccm, respectively.     

A qualitative model for the growth mechanism of silver clusters in polymer solution

A simple and high-reproducible method for the synthesis of polymer-protected silver cluster of controlled size is described. UV-visible spectroscopy has been used for investigating the influence of the aging of the protective poly(vinylpyrrolidone) layer on the cluster growth rate at different reaction temperatures and poly(vinylpyrrolidone)/ethylene glycol weight ratios. The obtained results show that the aging time of the polymeric stabilizer solution plays a fundamental role in the reproducibility of the cluster growth process. A model for the metal cluster formation-grow process is also proposed. Received 18 July 2001 and Received in final form 3 October 2001