相对论返波管频率特性研究

研究了等离子体填充下相对论返波管的频率特性。结果表明,随着等离子体浓度增加,返波管的振荡频率明显上移;对空波导情况,实验结果同理论分析结果是相符的。     

等离子体填充0.14 THz相对论返波管模拟

模拟了0.14 THz相对论返波管中电子束与氩气相互作用产生等离子体的过程, 研究了在不同气压条件下, 等离子体对相对论返波管的输出功率、频率以及起振时间的影响. 模拟结果表明, 等离子体背景能引起太赫兹波段真空电子器件脉冲缩短, 并出现新的频率分量; 适当地注入等离子体能减少0.14 THz相对论返波管的起振时间, 提高器件的输出功率. 关键词： 太赫兹 相对论返波管 粒子模拟 等离子体     

非磁化等离子体填充的相对论返波管的粒子模拟

 对非磁化等离子体填充的相对论返波管进行了粒子模拟分析，结果表明在一定的等离子密度范围内，观察到电子注的良好传输，得到了高功率电磁波输出。等离子体密度变化过程中，输出存在有峰值功率点。通过粒子模拟清晰地观察到注、等离子体及波相互作用的物理过程，并且模拟结果解释了部分实验现象。     

充中性气体相对论返波振荡器的粒子模拟研究

 用PIC粒子模拟方法研究了充中性气体相对论返波管的物理机制，成功模拟了电子束碰撞充入返波管中的中性气体电离产生等离子体的过程，在电子束传输的路径上形成离子通道，有效中和电子束径向空间电荷力，有利于电子束的传输及束波相互作用产生微波。增加中性气体密度，返波管的输出频率明显上移，其辐射的功率和效率比相同的真空器件也有明显的提高。     

充填不同气体相对论返波管特性的PIC-MCC模拟

讨论了PIC-MCC方法的基本原理以及在UNIPIC软件中的编程实现,并用该软件模拟了充填不同种类气体在不同气压下相对论返波管的气体电离过程,讨论了所产生的等离子体对电子束的传输以及波束相互作用的影响.给出了返波管输出功率、频率和起振时间随气体种类和气压等参数的变化情况.模拟结果表明,电离产生的阳离子是返波管性能改善的原因,而腔内滞留的低能电子是破坏返波管振荡而引起脉宽缩短的重要因素. 关键词： 蒙特卡罗碰撞 粒子模拟 高功率微波 相对论返波管     

非磁化等离子体填充的相对论返波管的粒子模拟

对非磁化等离子体填充的相对论返波管进行了粒子模拟分析,结果表明在一定的等离子密度范围内,观察到电子注的良好传输,得到了高功率电磁波输出。等离子体密度变化过程中,输出存在有峰值功率点。通过粒子模拟清晰地观察到注、等离子体及波相互作用的物理过程,并且模拟结果解释了部分实验现象。     

充中性气体相对论返波振荡器的粒子模拟研究

用PIC粒子模拟方法研究了充中性气体相对论返波管的物理机制,成功模拟了电子束碰撞充入返波管中的中性气体电离产生等离子体的过程,在电子束传输的路径上形成离子通道,有效中和电子束径向空间电荷力,有利于电子束的传输及束波相互作用产生微波。增加中性气体密度,返波管的输出频率明显上移,其辐射的功率和效率比相同的真空器件也有明显的提高。     

同轴相对论返波管中波束作用过程

根据同轴相对论返波管的特点建立了物理模型，采用时域有限差分法研究了同轴相对论返波管中波束作用过程。研究表明:同轴相对论返波管中波束作用的输出效率与两端反射系数密切相关，而且其内部场分布特点对于提高输出效率非常有利。经过优化设计，利用 500 keV, 4.0 kA 电子束流，微波起振时间为7 ns，输出效率大于38%，与以往数值模拟和实验结果符合较好。根据计算结果进一步分析得到，与空心相对论返波管相比，同轴相对论返波管中空间电荷效应的影响较小。     

HL-2A装置上的微波反射测量系统

微波反射技术是近几年来在大中型核聚变装置上发展起来的一项重要的等离子体诊断。它具有非常好的时空分辨率，不仅可以用于测量等离子体电子密度的空间分布，还可以测量等离子体旋转和湍流的空间分布。微波反射测量系统用二个返波管作为扫频振荡源，其扫频范围为26-0GHz和40-60GHz，周期为1ms。我们利用这套微波反射测量系统在HL-2A装置上进行了测量，经过1年的试验，这套系统工作正常。     

同轴相对论返波管中波束作用过程

根据同轴相对论返波管的特点建立了物理模型,采用时域有限差分法研究了同轴相对论返波管中波束作用过程。研究表明:同轴相对论返波管中波束作用的输出效率与两端反射系数密切相关,而且其内部场分布特点对于提高输出效率非常有利。经过优化设计,利用 500 keV, 4.0 kA 电子束流,微波起振时间为7 ns,输出效率大于38%,与以往数值模拟和实验结果符合较好。根据计算结果进一步分析得到,与空心相对论返波管相比,同轴相对论返波管中空间电荷效应的影响较小。     

Prediction of the sound reduction index: a modal approach

Different methods for the calculation of the sound reduction index using modal analysis are described. The calculations use two steps: a vibratory study to determine the transverse displacement of the plate and a study of radiation. Orthotropic plates are studied in particular and the resulting calculation algorithm has been programmed. Initial hypotheses are indicated, as well as results obtained for various plates or partitions. Modal analysis calculation results are then compared to results of the Cremer-Sewell approach.     

Compact closed-form micromechanical expressions for the effective uncoupled and coupled linear properties of layered composites

This work aims mainly to derive closed-form expressions for the effective properties of layered composites when linear uncoupled and coupled phenomena, such as conduction, elasticity, thermoelectricity and piezomagnetoelectricity, are concerned. In addition, properties may be graded in the direction normal to the layer plane. The remarkable feature of the obtained results is that they are expressed in a unified and very compact way for all uncoupled and coupled linear mechanical and physical phenomena. Our results include as special ones all the relevant results reported up to now in the literature. The key to obtaining our general compact coordinate-free results resides in extending Hill’s interfacial relations for a perfect elastic interface to the setting of an arbitrary finite number of uncoupled and coupled linear phenomena. The effects of the presence of stiffeners and softeners on the effective properties of layered composites are also investigated. In particular, four limiting cases for piezoelectricity are studied in detail. As illustrations, the general results are applied to several coupled phenomena and to functionally graded layered composites.     

Electron-electron double resonance in nitroxide radical solutions

All the pair interactions arising from the ground state H₂, N₂, CO₂, HF and LiH molecules are used as models for investigating the validity of the multipolar results (corresponding to the classical interaction of the permanent multipole moments of the interacting molecules) as a representation of the first-order Coulomb interaction energy. Charge overlap effects, the neglect of which are responsible for the misbehaviour of the multipolar results as the intermolecular distance R becomes small, are discussed as a function of the nature of the interacting molecules. Useful analytical formulae are presented which represent the numerical results. Some of the difficulties in using the multipolar results to represent the anisotropic part of a dimer potential are discussed in general and more specifically with respect to the evaluation of the mean square torque of nitrogen molecules.     

基于二代curvelet变换的图像融合研究

曲波(Curvelet)作为一种新的多尺度分析方法比小波更加适合分析二维图像中的曲线或直线状边缘特征,而且具有更高的逼近精度和更好的稀疏表达能力.将curvelet变换引入图像融合,能够更好地提取原始图像的特征,为融合图像提供更多的信息.第二代curvelet理论的提出也使得其理论更易理解和实现.因此,提出了一种基于第二代curvelet变换的图像融合方法,首先将图像进行curvelet变换,然后在相应尺度上利用融合规则将curvelet系数融合,最后进行重构得到融合结果.对多聚焦图像进行了实验,采用均方误差、偏差指数和相关系数对融合结果进行了客观评价,并与基于小波变换的融合进行了比较,实验结果表明该方法除分解2层时与小波性能相当,取其他分解层数时均获得更好的融合效果.     

Electron–phonon effect on the ground-state binding energy of hydrogenic impurity in quantum-well wire in presence of an electric field

The hydrogenic impurity binding energy in rectangular quantum well wire including both barriers of finite height and an applied electric field are studied. The polaron effects on the ground-state binding energy in electric field are investigated by means of Landau-Pekar variation technique. The results for the binding energy as well as polaronic correction are obtained as a function of the size of the wire, the applied electric field and the position of the impurity. Our calculations are compared with previous results in quantum wires of comparable dimensions.     

Two-Photon Excited Fluorescence and Molecular Reorientations in Liquid Solutions

Theoretical expressions are derived that relate the two-photon excited fluorescence depolarisation experiments to the molecular symmetry and the rotational motions of fluorescent molecules. Diffusive rotational motions in liquid solvents are considered, as well as the influence of fast unresolved motions (e.g. librations). The results obtained are compared with one-photon excited fluorescence depolarisation experiments. The derived theoretical expressions can be applied for detailed analyses of the molecular rotation in solvent. Several of the results are useful for determining and assigning the components of two-photon absorption tensors.     

Second-order Green's function theory of a Heisenberg paramagnet

The Heisenberg paramagnet in one, two, and three dimensions is analyzed by a second-order Green's function theory similar to that used by Knapp and ter Haar. This theory, which incorporates the exact values for the zero, first, and second moments of the relaxation function as boundary conditions, yields results satisfying the rotational symmetry of the paramagnetic region as well as the principle of detailed balance. We find that our predictions for equal time properties in the classical limit are identical with the RPA Green's function theory of Liu as well as the spherical model results of Lax. The quantum limit is analyzed, and our predictions for the 1/T series coefficients for both internal energy and susceptibility are compared with exact results.Supported by National Science Foundation.     

Stochastic modeling of coherent phenomena in strongly inhomogeneous media

A procedure of numerical simulation for coherent phenomena in multiply scattering media is developed on the basis of the juxtaposition of a Monte Carlo stochastic method with an iterative approach to the solution of the Bethe-Salpeter equation. The time correlation function and the interference component of coherent backscattering are calculated for scalar and electromagnetic fields. The results of simulation are in good agreement with experimental results, as well as with theoretical results obtained by generalizing the Milne solution.     

Assessing the effect of the matrix in resonant infrared MAPLE

The ablation yield of polyethylene glycol is measured as a function of laser pulse energy and polymer molecular weight for both free running and Q-switched Er:YAG lasers. The results show that the deposition rate is dependent on the absorption coefficient in a way that is consistent with the blow-off ablation model but that the linear dependence of the efficiency on laser pulse energy is also consistent with a steady-state ablation model. The results for both lasers are remarkably similar given that the pulse durations are 350 μs and 85 ns, respectively. Furthermore, polystyrene thin films are deposited as well and it is found that the morphology depends on the THF/MeOH ratios in the matrices. The significance of these results is discussed as well as future directions for this research.