11th Annual BNL Accelerator Test Facility Program Committee and Users' Meeting

The Seventh Annual Users' Meeting of the Canadian Light Source was a great success, as the world's newest synchrotron welcomed close to 400 researchers and students from across Canada, the U.S., Europe and Australia to the University of Saskatchewan on November 17–21, 2004. The plenary session of the Users' Meeting, held November 20, was preceded by five workshops: “XAFS Analysis Using Ifeffit, Athena and Artemis,” “SR Applications in Environmental Science,” “Medical Imaging,” “Protein Crystallography,” and “Applications of Elliptically Polarized Synchrotron Radiation.” Meetings of the Beamline Advisory Committee and beamline teams were held on November 21.

After welcoming remarks by Dr. Alan Anderson, Chair of the CLS User Advisory Committee, the meeting opened with a briefing by CLS Executive Director William Thomlinson regarding the accomplishments made by the CLS over the preceding year.     

On a class of nonlocal nonlinear Schrödinger equations and wave collapse

A similar type of nonlocal nonlinear Schr?dinger (NLS) system arises in both water waves and nonlinear optics. The nonlocality is due to a coupling between the first harmonic and a mean term. These systems are termed nonlinear Schr?dinger with mean or NLSM systems. They were first derived in water waves by Benney-Roskes and later by Davey-Stewartson. Subsequently similar equations were derived and found to be fundamental systems in quadratically nonlinear optical media. Wave collapse can occur in these systems. The collapse structure and the role of the ground state in the collapse process are studied. There are similarities to the well-known collapse mechanism associated with classical NLS system. Numerical simulations show that NLSM collapse occurs with a quasi self-similar profile that is a modulation of the corresponding ground-state. Further, it is found that NLSM collapse can be arrested by adding small nonlinear saturation.     

Molecular dynamics study of the vibrational relaxation of OCl and OCl in the bulk and the surface of water and acetonitrile

The vibrational relaxation of OCl and OCl⁻ in the bulk and the liquid/vapor interface of water and acetonitrile is studied by molecular dynamics computer simulations. Both equilibrium calculations of the vibrational friction and non-equilibrium simulation of the energy relaxation are used to elucidate the factors that influence the rate of energy relaxation in systems that represent polar ionic and non-ionic solutes in polar protic and non-protic solvents. We find that, in agreement with previous experimental and theoretical studies, the relaxation of the ionic solute is much faster than that of the non-ionic solute in both the solvents. However, while the relaxation is slowed down considerably when the non-ionic solute is transferred from the bulk to the interface, no such surface effect is found in the case of the ionic solute. This behavior can be explained by noting that the ionic solute is able to keep its first solvation shell intact upon transfer to the interface and that the main contribution to the friction is due to the Lennard-Jones part of the intermolecular potential.     

Acoustic centering of sources measured by surrounding spherical microphone arrays

The radiation patterns of acoustic sources have great significance in a wide range of applications, such as measuring the directivity of loudspeakers and investigating the radiation of musical instruments for auralization. Recently, surrounding spherical microphone arrays have been studied for sound field analysis, facilitating measurement of the pressure around a sphere and the computation of the spherical harmonics spectrum of the sound source. However, the sound radiation pattern may be affected by the location of the source inside the microphone array, which is an undesirable property when aiming to characterize source radiation in a unique manner. This paper presents a theoretical analysis of the spherical harmonics spectrum of spatially translated sources and defines four measures for the misalignment of the acoustic center of a radiating source. Optimization is used to promote optimal alignment based on the proposed measures and the errors caused by numerical and array-order limitations are investigated. This methodology is examined using both simulated and experimental data in order to investigate the performance and limitations of the different alignment methods.     

Noise-induced linewidth in frequency combs

Frequency combs generated by trains of pulses emitted from mode-locked lasers are analyzed when the center time and phase of the pulses undergo noise-induced random walk, which broadens the comb lines. Asymptotic analysis and computation reveal that, when the standard deviation of the center-time jitter of the nth pulse scales as n(p/2) where p is a jitter exponent, the linewidth of the kth comb line scales as k(2/p). The linear-dispersionless (p=1) and pure-soliton (p=3) dynamics in lasers are derived as special cases of this time-frequency duality relation. In addition, the linewidth induced by phase jitter decreases with power P(out), as (P(out))(-1/p).     

Percolation in living neural networks

We study living neural networks by measuring the neurons' response to a global electrical stimulation. Neural connectivity is lowered by reducing the synaptic strength, chemically blocking neurotransmitter receptors. We use a graph-theoretic approach to show that the connectivity undergoes a percolation transition. This occurs as the giant component disintegrates, characterized by a power law with an exponent beta approximately or = 0.65. Beta is independent of the balance between excitatory and inhibitory neurons and indicates that the degree distribution is Gaussian rather than scale free.     

A Lower Bound on the Distortion of Embedding Planar Metrics into Euclidean Space

We exhibit a simple infinite family of series-parallel graphs that cannot be metrically embedded into Euclidean space with distortion smaller than