LOM and HOM damping study in a superconducting deflecting cavity for ALS at LBNL

Superconducting deflecting cavities can be used in synchrotron light source to generate subpicosecond X-ray pulses while the impedance of the lower order modes (LOM) and higher order modes (HOM) in the cavity should be kept below an accepted level to avoid beam instability. These modes can be damped by adding waveguide on beam pipe. Detailed simulation of Q in CST Microwave Studio is introduced and experiment results on an aluminum model cavity with damping waveguide are reported to make a comparison.     

Monte-Carlo simulation of a compact gamma-ray detector using wavelength-shifting fibers coupled to a YAP scintillation crystal

The production and transportation of fluorescent light produced in wavelength-shifting fibers (WSFs) coupled to YAP scintillation crystal is simulated using the GEANT4 codes.An advantage of the wavelength-shifting fiber readout technique over a direct readout with a position-sensitive photo-sensor is the reduced requirement for position sensitive photomultiplier tube photocathode area.With this gamma-ray detector,the gamma camera is small and flexible and has larger effective field of view and low cost.Simulation results show that a) a mean 12 of photons per 59.5 keV gamma ray interaction is produced in the WSF located nearest to the incident gamma ray,and a spatial resolution of 3.6 mm FWHM is obtained,b)a mean 27 of photons per 140 keV gamma ray interaction is produced and a spatial resolution of 3.1 mm FWHM is obtained.Results demonstrate the feasibility of this concept of a compact gamma-ray detector based on wavelength-shifting fibers readout.However,since the very low photoelectron levels,it is very important to use a photon counting device with good single photo-electron response to readout the WSFs.     

Thermal conductivity of nanofluids and size distribution of nanoparticles by Monte Carlo simulations

Nanofluids, a class of solid–liquid suspensions, have received an increasing attention and studied intensively because of their anomalously high thermal conductivites at low nanoparticle concentration. Based on the fractal character of nanoparticles in nanofluids, the probability model for nanoparticle’s sizes and the effective thermal conductivity model are derived, in which the effect of the microconvection due to the Brownian motion of nanoparticles in the fluids is taken into account. The proposed model is expressed as a function of the thermal conductivities of the base fluid and the nanoparticles, the volume fraction, fractal dimension for particles, the size of nanoparticles, and the temperature, as well as random number. This model has the characters of both analytical and numerical solutions. The Monte Carlo simulations combined with the fractal geometry theory are performed. The predictions by the present Monte Carlo simulations are shown in good accord with the existing experimental data.     

Centered -discrepancy of random sampling and Latin hypercube design, and construction of uniform designs

In this paper properties and construction of designs under a centered version of the -discrepancy are analyzed. The theoretic expectation and variance of this discrepancy are derived for random designs and Latin hypercube designs. The expectation and variance of Latin hypercube designs are significantly lower than that of random designs. While in dimension one the unique uniform design is also a set of equidistant points, low-discrepancy designs in higher dimension have to be generated by explicit optimization. Optimization is performed using the threshold accepting heuristic which produces low discrepancy designs compared to theoretic expectation and variance.

     

A Generalization of the Collatz Problem. Building Cycles and a Stochastic Approach

The (3x+1)/2 problem is generalized into the n-furcation problem (l _i x+m _i )/n where i[0, 1, ..., n–1]. It is shown that, under some constraints on l _i and m _i , the main bijection property between the k less significant digits of the seed, written in base n, and the sequence of generalized parities of the k first iterates is preserved. This property is used to investigate a stochastic treatment of ensemble of large value seeds. The bijection property predicts stochasticity for a number of iterations equals to the number of significant digits of the seed. In fact, the stochastic approach is valid for much larger numbers, a property which is more easily shown by using increasing sequences than decreasing ones. Finally, we extend the stochastic approach to cases where the bijection theorem does not hold, introducing the matrix giving the probability that a j number (where j is the last significant digit of this number written in base n) gives a i number iterate.     

Stable conformations of 12-crown-O3N and its Li+ complex in aqueous solution

Stable conformations of 12-crown-O3N and its Li complexes in aqueous solution were investigated. To calculate the free energy differences of conformers of 12-crown-O3N and its Li+ complex, our procedure was to make use of two programs, CONFLEX and BOSS. The former generates conformers, and the latter calculates the differences in free energy of solvation between two conformers in aqueous solution. It was confirmed that the present procedure is applicable in solving the question of what is the most stable conformation of 12-crown-O3N in aqueous solution. Results of the calculations suggest that the order of stability for conformers in a vacuum is different from that in aqueous solution. It was also confirmed that the coordination geometry of solvent waters to Li+ changes depending on the distance between the cation and the crown ring.     

Design and synthesis of artificial ribonucleases based on 1,4-diazabicyclo[2.2.2]octane and imidazole

The review surveys the results of our studies devoted to the design of highly efficient catalysts of hydrolysis of the phosphodiester bonds in RNA. These catalysts contain the imidazole residue in the catalytic domain, one or several bis-quaternized rings of 1,4-diazabicyclo[2.2.2]octane as a polycationic RNA-binding domain, and a lipophilic radical. A versatile approach to artificial ribonucleases of this type was proposed, which allows one to vary not only the number of positive charges in the RNA-binding domain, the structure of the catalytic site, and their mutual arrangement but also the domain structure of the molecule as a whole. Analysis of the catalytic properties of the synthesized constructs makes it possible to optimize the domain structure and the geometry of the molecule ensuring its maximum ribonuclease activity.     

Scaling concepts for polymer brushes and their test with computer simulation

After a brief review of the scaling concepts for static and dynamic properties of polymer brushes in good solvents and Theta solvents, the Monte Carlo evidence is discussed. It is shown that under typical conditions the diameter of the last blob is of the order of 10-20% of the brush height, and therefore pronounced deviations from the self-consistent field predictions occur. In bad solvents, lateral microphase separation occurs leading to an irregular pattern of “dimples”. Particularly interesting is the response of brushes to shear deformation, and the interaction between two interpenetrating brushes. Recent attempts to understand the resulting shear forces via molecular-dynamics simulations are briefly described, and an outlook on related experiments is given. Dedicated to Prof. H.E. Stanley on the occasion of his 60th birthday Received 11 March 2002 and Received in final form 3 June 2002     

热容激光器激光输出特性的数值模拟

热容激光器在激射过程中，激光介质的温度快速升高，导致了热容激光器具有特殊的激光输出特性。通过理论分析、数值模拟和实验研究等方法，对热容激光器的激光输出特性进行研究，掌握热容激光器工作的基本规律，为热容激光器的设计、实验提供了重要的参考。建立了描述热容激光输出特性的理论模型，给出了输出功率随激光介质温升及工作时间的变化关系。数值模拟研究结果表明。激光介质的温度从300K升高到400K过程中，阈值功率密度从2．6W／cm^3增加到33．6W／cm^3，输出功率下降7％。利用建立的热容激光器实验装置．测量了不同占空比情况下的输出功率，占空比越高输出功率随时间下降得越快，与数值模拟结果相符。在低占空比时，数值模拟和测量结果吻合较好；在高占空比时出现误差，而且占空比越高误差越大。     

NCl(a~1Δ)自猝灭对NCl(a~1Δ)/I激光能量提取的影响

利用连续流平面型光腔动力学模型对NCl(a1Δ)/I 激光体系进行了模拟计算,探讨了温度在300 K 时NCl(a1Δ)自猝灭对NCl(a1Δ)/I激光能量提取的影响。计算结果表明,NCl(a1Δ)自猝灭反应对光腔位置的选取、功率密度沿流动方向的分布和总输出功率都有较大的影响。其中,光腔位置的可选范围大大缩短,在较小的初始HI粒子数密度和适当的输出镜反射率下总输出功率大幅度降低,而随着HI粒子数密度的增加,NCl(a1Δ)自猝灭对总输出功率的影响逐渐减小。