The suppression of selected acoustic frequencies in MRI

A study is made of certain dominant frequencies in the acoustic noise spectrum of the magnetic resonance imaging system. Motivated by both spring and string ideas, we investigate whether the contributions to the sound from certain frequencies can be canceled by the appropriate gradient pulse sequence design. From both simulations and experiments, vibrations resulting from an impulsive force associated with a ramping up of a gradient pulse are shown to be cancelled immediately upon the application of another impulsive force coming from the subsequent appropriately timed ramping down of that pulse. A general approach to suppression of multiple-frequency contributions involving a series of gradient pulses with variable timings is given for the cancellations between pairs of impulsive forces. Various examples are confirmed through string simulations, MRI experiments, and linear response theory. This also provides a foundation to explain some results in previous papers on this subject. The method suggests that a variety of pulse profiles and timing combinations can be used to attenuate important contributions to the acoustic spectrum.     

The equation of the p-adic closed strings for the scalar tachyon field

We investigate the structure of solutions of boundary value problems for a non-linear pseudodifferential equation describing the dynamics(rolling)of p-adic closed strings for a scalar tachyon field.     

On the multiplicity of eigenvalues of a vectorial Sturm-Liouville differential equation and some related spectral problems

We prove that under certain conditions, a vectorial Sturm-Liouville differential equation of dimension can only possess finitely many eigenvalues which have multiplicity . For the case , we find a sufficient condition on the potential function , and a bound depending on , such that the eigenvalues of the equation with index exceeding are all simple. These results are applied to find some sufficient conditions which imply that the spectra of two potential equations, or two string equations, have finitely many elements in common, and an estimate of the number of elements in the intersection of two spectra is provided.

     

Letter: A Model of Dark Energy for the Accelerating Universe

Recent observations of large scale structure of the Universe, especially that of Type Ia supernovae, indicate that the Universe is flat and is accelerating, and that the dominant energy density in the Universe is the cosmic dark energy. We propose a model in which the cosmic effective Yang-Mills condensate familiar in particle physics plays the role of the dark energy that causes the acceleration of the Universe. Since the quantum effective Yang-Mills field in certain states has the equation of state p _y = – _y , when employed as the cosmic matter source, it naturally results in an accelerating expansion of the Universe. With the matter components ( _m 1/3) being added into the model, the composition of YM condensate and matter components can give rise to the desired equation of state w –2/3 for the Universe.     

Optically active poly(amide–imide)s containing axially dissymmetric 1,1′-binaphthyl moieties

Novel optically active aromatic poly(amide–imide)s (PAIs) were prepared from newly synthesized 2,2′-bis(3,4-dicarboxybenzamido)-1,1′-binaphthyl dianhydride ((+ )-, (S)-, and (R)-BNDADA). PAIs based on dianhydride monomers with different ee % were investigated with respect to their structures and chiroptical properties. These polymers were highly soluble in polar aprotic solvents such as N,N-dimethylacetamide, N-methyl-2-pyrrolidone, pyridine, etc., and showed high glass transition temperatures of 287–290°C and 5% weight loss temperatures of 450–465°C in nitrogen. Optically active PAIs exhibited high specific rotations, excellent optical stabilities, and a dependence of optical activities on temperature. Investigations on chiroptical properties indicated that chiral conformation was possessed by optically active PAIs. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3147–3154, 1999     

Chain coherence length of rigid‐rod poly(p‐phenylene benzobisthiazole) and derivatives

Chain coherence length of rigid‐rod poly(p‐phenylene benzobisthiazole) (PBZT) and its derivatives in the solid state was determined from the wide‐angle X‐ray diffraction patterns of axially disordered crystal. The degree of the PBZT main chain extension was estimated from the coherence lengths and was compared to investigate the effects of side chain, orientation, heat treatment, and polymer solution concentration. Extremely small coherence length obtained from both highly oriented fibers and powder or bulk PBZT homopolymer suggested that a chain conformation deviated from the fully extended conceptual rigid‐rod, supporting the ribbon‐like conformation, as was previously predicted by molecular dynamic simulation. The deviation was also found to be highly dependent on the processing conditions. Fibers stretched during spinning exhibited much greater chain extension than the isotropic powder, the bulk, and fibers spun without tension. The chain extension was also dependent on the solution concentration prior to the processing. The PBZT produced from solution above the critical concentration exhibited higher chain extension than those from below the critical concentration. However, side chain attachment to the PBZT main chain or post‐heat treatments showed a minimal effect on the extension of the PBZT backbone. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 661–666, 1999     

内共振条件下轴向运动梁磁弹性超谐共振研究

以载流导线激发的磁场中轴向运动梁为研究对象,同时考虑外激励力作用,推导出梁的磁弹性非线性振动方程．通过位移函数的设定和伽辽金积分法,将非线性振动方程离散为常微分方程组．采用多尺度法得到系统的近似解析解．应用Matlab 和Mathematica 软件求解幅频响应方程,并对稳态解进行稳定性判定．通过具体算例得到前两阶假设模态的响应幅值随不同参数的变化规律．结果发现：系统在内共振条件下发生超谐波共振时,二阶假设模态幅值明显小于一阶;随着外激励的增大,多值解区域范围明显缩小;随着电流强度增加,振动幅值减小,表明载流导线能够起到控制共振的作用．     

感应加速腔横向阻抗测量

 描述了采用脉冲双线法测量感应加速腔横向阻抗的原理、方法和步骤。测量结果显示,新设计的真空腔和绝缘腔的横向阻抗特性优于以前设计的原形腔,而且加速间隙由于采用了类同轴结构,绝缘腔的低频模式被有效抑制,在753MHZ测得其横向阻抗为820W /m。     

Chiral invariant phase space event generator

An event generator based on the CHIPS model is implemented in the GEANT4 simulation toolkit. Nuclear fragment production in the process of pion capture on nuclei is used to tune the parameters of the CHIPS model describing clusterization of nucleons in nuclei. The spectra of nucleons and nuclear fragments in pion capture and in photonuclear reactions at 60 MeV are compared with experimental data. Received: 30 May 2000 / Accepted: 2 November 2000     

无限维关联系统的弦稳定性

对一类无限维关联系统引入弦稳定概念。系统弦稳定意谓着,当关联系统的初始状态为有界时,对任意时刻系统的状态也是有界的。本文将向量V函数法推广到无限维系统中,得到了关联系统渐近弦稳定的充分条件,克服了以前的方法在处理非线性系统的稳定性问题上的困难,扩大了系统稳定的参数范围。