Acousto-optic modulation in magnetised diffusive semiconductors

The modulation of an intense electromagnetic beam induced by the acousto-optic (AO) effect has been analysed in a transversely magnetised semiconductor-plasma medium. The effect of carrier diffusion on the threshold field and gain profile of the modulated wave has been extremely investigated using coupled mode theory. The origin of the AO interaction is assumed to lie in the induced nonlinear diffusion current density of the medium. By considering the modulation process as a four wave parametric interaction an expression for effective third-order AO susceptibility describing the phenomena has been deduced. The modulation is greatly modified by propagation characteristics such as dispersion and diffraction due to dielectric relaxation of the acoustic mode. The threshold pump field and the steady state growth rates are estimated from the effective third-order polarisation in the plasma medium. Analytical estimation reveals that in the presence of enhanced diffusion due to excess charge carriers the modulated beam can be effectively amplified in a dispersionless acoustic wave regime. The presence of an external dc magnetic field is found to be favourable for the onset of diffusion induced modulational amplification of the modulated wave in heavily doped regime. Received 5 November 2001     

空心束虚阴极激励微波的主模式的研究

把虚阴极振荡激励微波的过程看成是一个时空振荡电流元激励微波的过程,在薄电荷层近似下,求得空心束虚阴极激励微波的主模式关系,并讨论了束几何位形与微波输出功率之间的关系。我们所确定的主模式与实验和粒子模拟的结果相符。     

用涨落模型解释有关主被动锁模Nd:YAG激光器的若干问题

应用锁模理论的涨落模型分析研究了已报道的若干主被动锁模Nd:YAG激光器的实验现象.对锁模的阈值行为,锁模几率随调制器调制功率的变化规律和系统的稳定性随可饱和吸收染料的相对浓度的变化规律等实验结果从理论上进行了比较成功的解释.     

结构体系的非概率可靠性分析方法

结构体系的可靠性与结构的失效模式有关。在非概率条件下 ,结构体系的可靠性取决于非概率可靠性指标最小的最危险失效模式。最危险失效模式的识别及相应极限状态方程的建立是结构体系非概率可靠性计算的关键问题。文中考虑了结构参数及强度、外载荷等参量的不确定性 ,基于随机可靠性方法中常用的优化准则法 ,提出了非概率结构体系主要失效模式的枚举准则。可在只需枚举出少量主要失效模式的情况下 ,不漏掉最危险失效模式。并提出用区间增量载荷法生成主要失效模式的极限状态方程。算例分析表明文中方法是实用和有效的。     

复模态理论在近海平台中的应用

运用复模态理论对考虑海洋平台结构与流体相互作用而引起的非经典阻尼问题进行了理论分析。水中耦合阻尼矩阵运用复模态解法，可以不必近似对角化。推导了随机波浪荷载作用下结构各种反应的能量谱密度函数及结构反应的统计特性，得到了结构反应的均方差和峰值的表达式。针对某典型平台在随机波浪荷载作用下动力反应用复模态法进行了计算并与运用实模态方法多重叠代方法的结果进行了比较。     

三维不可压缩边界层分离流的反解法

本文提出了计算具有分离区的三维层流和湍流边界层的一种反解法。计算中采用了半正交曲线坐标系和涡粘性各向异性湍流模型。数值分析中采用了变换域技术和 Keller分块差分格式。用本文方法计算了一个三维边界层分离流实例,计算结果同实验数据吻合得很好。     

Gradient reversed-phase high-performance chromatography of ionogenic analytes

This review highlights the basis of gradient reversed-phase high-performance liquid chromatography (RP-HPLC) of ionogenic analytes.We describe pH-gradient RP-HPLC in strict theoretical terms, with examples of experiments to provide improvements in analyte separations and peak shapes, and an original method of pK_a determination.Finally, we present the concept of the combined pH/organic-modifier gradient mode of RP-HPLC and illustrate it with applications. It allows optimization of separation of ionogenic analytes, along with a method for determining their biorelevant physico-chemical parameters [e.g., hydrophobicity (log k_w) and acidity (pK_a)]. The method is applicable to drugs and other xenobiotic mixtures, including individual analytes of interest assayed in biological fluids.     

Numerical model for the investigation of countercurrent chromatography

A numerical model is developed to describe the separation process of countercurrent chromatography (CCC) in this work. The theory of countercurrent extraction table (TCCET) is first proposed to calculate concentration distributions of chemical components in the CCC, which is essential for a numerical model to describe the dynamic equilibrium of mass transfer. According to the theory of countercurrent extraction, the concentration in chromatography obeys binomial distribution, while the outflow from the n-th stage is a negative binomial distribution. As a result of the central limit theorem, they will obey normal distribution for sufficiently large n. Row-stage ratio (R(RS)) is then defined to determine the K value or retention time because it has a linear relationship to K value and retention time. The stage for a certain K value can be subsequently obtained with a very simple form, n(k)=1/(2piq(k)X(2)(k, max)), which can be calculated from the peak height obtained from experiments. Finally, the actual stage for a separation chromatogram can be acquired with using this simple expression. The agreement between theoretic and experimental results is quite satisfactory in the normal-phase and reversed-phase elution mode.     

Retention mechanism divergence of a mixed mode stationary phase for high performance liquid chromatography

Mixed mode stationary phases utilize secondary retention mechanisms to add a dimensionality to the surface of high performance liquid chromatography (HPLC) adsorbents. This approach was used by several authors to improve the separation performance of single dimension separations. We explored the magnitude of these secondary interactions by performing an off-line two-dimensional (2D)-HPLC separation with a Scherzo SM-C18 column of a β-lactoglobulin tryptic digest with a mobile phase pH of 7 in the first dimension and 2 in the second. Mechanism divergence was determined using the peak capacity and a geometric approach to factor analysis, to measure the correlation. This separation was repeated with a C18 stationary phase as a control. It was found that the C18 column had a correlation coefficient of 0.784, smaller than the mixed mode column, 0.884. This indicated that the retention mechanisms of the C18 column were more divergent under these two pH environments than the mixed mode column. However, the SM-C18 still provided alternative selectivity of the peptides to that of the C18 and could be considered as a good alternative for further 2D-HPLC separations.     

An analytical study of the non-linear vibrations of cylindrical shells

The primary resonance response of simply supported circular cylindrical shells is investigated using the perturbation method. Donnell's non-linear shallow-shell theory is used to derive the governing partial differential equations of motion. The Galerkin technique is then employed to transform the equations of motion into a set of temporal ordinary differential equations. Considering only the primary resonance case, the method of multiple scales is used to study the periodic solutions and their stability. The necessary and sufficient conditions for appearance of the so-called companion mode are also discussed. To this end, a range of the possible multi-mode solution is obtained for response and excitation amplitudes and also excitation frequency as a function of damping, geometry and material properties of the shell. Parametric studies are performed to illustrate the effect of different values of thickness, length and material composition on the possibility of the companion mode participation in primary resonance response.