Propagation of wave pulses in the regimes of single-particle and collective cherenkov effects during convective and absolute instabilities

Asymptotic solutions to the problem of the time evolution of delta-shaped wave pulses excited during resonant instabilities of electron beams in slowing-down electromagnetic media are found and investigated analytically. Convective and absolute instabilities developing under the conditions of collective and single-particle Cherenkov effects are considered. The results obtained apply to an arbitrary linear nonequilibrium dispersive medium that can be described by a set of first-or second-order differential transport equations.     

Effect of flux surface curvature on the linear coupling of electron cyclotron waves in tokamak plasmas

Specific features of the linear interaction of ordinary and extraordinary electromagnetic waves in the electron cyclotron frequency range in a nonuniform plasma confined in a toroidal magnetic trap are considered. Reduced wave equations taking into account the curvature of the cut-off surfaces in toroidal geometry are formulated. Using these equations, the distributions of the wave fields in the coupling region are analyzed. A method for calculating quasi-optical beams passed through the region of linear wave interaction is proposed.     

Electromagnetic oscillations near the critical surface in a plasma: Methodological note

Agreement between different approaches to studying the propagation of electromagnetic oscillations near the critical surface is elucidated. The propagation of plane waves, electromagnetic rays, and wave beams are analyzed. The results obtained are valid when the angles between the magnetic field and the plasma density gradient are not too small.     

Mathematical modeling of the excitation process in myocardial tissue: influence of fiber rotation on wavefront propagation and potential field

In our macroscopic model the heart tissue is represented as a bidomain coupling the intra- and extracellular media. Owing to the fiber structure of the myocardium, these media are anisotropic, and their conductivity tensors have a principal axis parallel to the local fiber direction. A reaction-diffusion system is derived that governs the distribution and evolution of the extracellular and transmembrane potentials during the depolarization phase of the heart beat. To investigate frontlike solutions, the system is rescaled and transformed into a system dependent on a small parameter. Subsequently a perturbation analysis is carried out that yields zero- and first-order approximations called eikonal equations. The effects of the transmural fiber rotation on wavefront propagation and the corresponding potential field, elicited by point stimulations, are investigated by means of numerical simulations.     

Long-term microcarrier suspension cultures of human embryonic stem cells

The conventional method of culturing human embryonic stem cells (hESC) is on two-dimensional (2D) surfaces, which is not amenable for scale up to therapeutic quantities in bioreactors. We have developed a facile and robust method for maintaining undifferentiated hESC in three-dimensional (3D) suspension cultures on matrigel-coated microcarriers achieving 2- to 4-fold higher cell densities than those in 2D colony cultures. Stable, continuous propagation of two hESC lines on microcarriers has been demonstrated in conditioned media for 6 months. Microcarrier cultures (MC) were also demonstrated in two serum-free defined media (StemPro and mTeSR1). MC achieved even higher cell concentrations in suspension spinner flasks, thus opening the prospect of propagation in controlled bioreactors.     

Evanescent interference patterns for fluorescence microscopy.

The increasing experimental use of total internal reflection/fluorescence photobleaching recovery has motivated a theoretical study of the spatial intensity profiles generated by two interfering evanescent waves. The interference patterns generated by evanescent waves differ considerably from those generated by plane waves in a homogenous medium because evanescent waves are not transverse and because the evanescent propagation number depends on the incidence angle of the totally internally reflected light. The periodicity and contrast of the evanescent interference patterns under various conditions are calculated; these parameters depend on the intensities, polarizations, and incidence angles of the two incident beams, as well as the refractive indices of the two media that form the planar interface where total internal reflection occurs. The derived intensity profiles are used to develop expressions for the shapes of fluorescence photobleaching recovery curves when evanescent interference patterns are used for fluorescence excitation and bleaching. The calculations also suggest that colliding beam experiments may confirm theoretically predicted evanescent field polarizations.     

A multiscale approach for modelling wave propagation in an arterial segment

A mathematical model of blood flow through an arterial vessel is presented and the wave propagation in it is studied numerically. Based on the assumption of long wavelength and small amplitude of the pressure waves, a quasi-one-dimensional (1D) differential model is adopted. It describes the non-linear fluid-wall interaction and includes wall deformation in both radial and axial directions. The 1D model is coupled with a six compartment lumped parameter model, which accounts for the global circulatory features and provides boundary conditions. The differential equations are first linearized to investigate the nature of the propagation phenomena. The full non-linear equations are then approximated with a numerical finite difference method on a staggered grid. Some numerical simulations show the characteristics of the wave propagation. The dependence of the flow, of the wall deformation and of the wave velocity on the elasticity parameter has been highlighted. The importance of the axial deformation is evidenced by its variation in correspondence of the pressure peaks. The wave disturbances consequent to a local stiffening of the vessel and to a compliance jump due to prosthetic implantations are finally studied.     

Obtaining high pressures and metastable states in condensed media by using high-current relativistic electron beams

Experimental studies performed at the Russian Research Centre Kurchatov Institute on the interaction of high-current relativistic electron beams with various condensed media, including highly porous materials, are reviewed. The experiments on obtaining high pressures and accomplishing the structural and chemical conversions in the focal spot of a high-current beam are described. The principles of imitating an ultra-highspeed impact and other energetic actions on an obstacle with the help of high-current relativistic beams are discussed. The possibility of using such beams for surface modification is considered. Experimental data on the induced electric conductivity in highly porous SiO₂ aerogels in the region of the beam energy deposition are presented.     

Conditions for Loss Compensation of Surface Plasmon Polaritons Propagation on a Metal/Gain Medium Boundary

One way to compensate for the surface plasmon polariton (SPP) propagation losses is to use a gain medium. However, simply ensuring high enough gain is not sufficient because it may violate the bounded character of the wave. Therefore, a detailed theoretical analysis is needed for the determination of the conditions for lossless or amplified SPP propagation. Here presented is an exact theoretical analysis of the SPP propagation in the case of an infinite metal/gain medium boundary. It is shown that the conditions for lossless/amplified SPP propagation can be conveniently examined and presented as a simply connected region in the complex plane of the gain medium dielectric function. Effective and minimum gain parameters are introduced, which facilitates the simultaneous analyses of different gain media/metals combinations. The practical application of these results is illustrated for several gain media/metal (silver, gold and aluminium) systems.     

ON THE THEORY OF NERVOUS CONDUCTION

Assuming that the propagation of the nervous impulse consists in the excitation of adjacent regions of the nerve by the action current of the already excited region, exact equations for the velocity of such a propagation are established and integrated. The result depends on the assumptions which we make about the laws of excitation. If Hoorweg''s law is accepted, it is found that the velocity of propagation decreases exponentially with time, and that there is a limiting distance which the impulse will travel and which cannot be exceeded. If however a set of equations proposed by L. Lapique is assumed to govern the process of excitation, we find that the velocity of propagation asymptotically reaches a constant value.     

重离子束生物工程中的一些基本物理问题

本文介绍了重离子束的基本特性和在生命科学应用中的优势,以及它的能区划分,传能线密度LET,相对生物效率RBE等,分别叙述了中、高能和低能离子束与介质作用的特点、在农学中应用的模式及其吸收剂量的计算,最后还对一些机理问题进行了简单讨论。     

A Novel Culture Model for Human Pluripotent Stem Cell Propagation on Gelatin in Placenta-conditioned Media

The propagation of human pluripotent stem cells (hPSCs) in conditioned medium derived from human cells in feeder-free culture conditions has been of interest. Nevertheless, an ideal humanized ex vivo feeder-free propagation method for hPSCs has not been developed; currently, additional exogenous substrates including basic fibroblast growth factor (bFGF), a master hPSC-sustaining factor, is added to all of culture media and synthetic substrata such as Matrigel or laminin are used in all feeder-free cultures. Recently, our group developed a simple and efficient protocol for the propagation of hPSCs using only conditioned media derived from the human placenta on a gelatin-coated dish without additional exogenous supplementation or synthetic substrata specific to hPSCs. This protocol has not been reported previously and might enable researchers to propagate hPSCs efficiently in humanized culture conditions. Additionally, this model obviates hPSC contamination risks by animal products such as viruses or unknown proteins. Furthermore, this system facilitates easy mass production of hPSCs using the gelatin coating, which is simple to handle, dramatically decreases the overall costs of ex vivo hPSC maintenance.     

Traveling Wave Solutions of a Nerve Conduction Equation

We consider a pair of differential equations whose solutions exhibit the qualitative properties of nerve conduction, yet which are simple enough to be solved exactly and explicitly. The equations are of the FitzHugh-Nagumo type, with a piecewise linear nonlinearity, and they contain two parameters. All the pulse and periodic solutions, and their propagation speeds, are found for these equations, and the stability of the solutions is analyzed. For certain parameter values, there are two different pulse-shaped waves with different propagation speeds. The slower pulse is shown to be unstable and the faster one to be stable, confirming conjectures which have been made before for other nerve conduction equations. Two periodic waves, representing trains of propagated impulses, are also found for each period greater than some minimum which depends on the parameters. The slower train is unstable and the faster one is usually stable, although in some cases both are unstable.     

Refinement of Strut-and-Tie Model for Reinforced Concrete Deep Beams

Deep beams are commonly used in tall buildings, offshore structures, and foundations. According to many codes and standards, strut-and-tie model (STM) is recommended as a rational approach for deep beam analyses. This research focuses on the STM recommended by ACI 318-11 and AASHTO LRFD and uses experimental results to modify the strut effectiveness factor in STM for reinforced concrete (RC) deep beams. This study aims to refine STM through the strut effectiveness factor and increase result accuracy. Six RC deep beams with different shear span to effective-depth ratios (a/d) of 0.75, 1.00, 1.25, 1.50, 1.75, and 2.00 were experimentally tested under a four-point bending set-up. The ultimate shear strength of deep beams obtained from non-linear finite element modeling and STM recommended by ACI 318-11 as well as AASHTO LRFD (2012) were compared with the experimental results. An empirical equation was proposed to modify the principal tensile strain value in the bottle-shaped strut of deep beams. The equation of the strut effectiveness factor from AASHTTO LRFD was then modified through the aforementioned empirical equation. An investigation on the failure mode and crack propagation in RC deep beams subjected to load was also conducted.     

Simulation of Sound Waves Using the Lattice Boltzmann Method for Fluid Flow: Benchmark Cases for Outdoor Sound Propagation

Propagation of sound waves in air can be considered as a special case of fluid dynamics. Consequently, the lattice Boltzmann method (LBM) for fluid flow can be used for simulating sound propagation. In this article application of the LBM to sound propagation is illustrated for various cases: free-field propagation, propagation over porous and non-porous ground, propagation over a noise barrier, and propagation in an atmosphere with wind. LBM results are compared with solutions of the equations of acoustics. It is found that the LBM works well for sound waves, but dissipation of sound waves with the LBM is generally much larger than real dissipation of sound waves in air. To circumvent this problem it is proposed here to use the LBM for assessing the excess sound level, i.e. the difference between the sound level and the free-field sound level. The effect of dissipation on the excess sound level is much smaller than the effect on the sound level, so the LBM can be used to estimate the excess sound level for a non-dissipative atmosphere, which is a useful quantity in atmospheric acoustics. To reduce dissipation in an LBM simulation two approaches are considered: i) reduction of the kinematic viscosity and ii) reduction of the lattice spacing.     

Dielectric permittivity of quantum plasma. Part I

Collisionless quantum plasma models based on the Schröbinger, Klein-Gordon, Dirac, and Pauli equations are considered. The transverse and longitudinal dielectric permittivities of isotropic quantum plasma are calculated in the frameworks of the models based on the Schröbinger and Klein-Gordon equations without allowance for the particle spin. Dispersion relations for transverse-longitudinal waves in beams of spinless quantum particles are derived, and the simplest quantum waves are analyzed.     

Asymptotic results for a multi-type contact birth-death process and related SIS epidemic

Exact results concerning the asymptotic speed of propagation of infection have recently been obtained for the multi-type SIS epidemic in continuous space when the contact distributions are assumed to be symmetric with the Laplace transforms finite for all entries. There is a link between the equations for this epidemic and the equations for a multi-type contact birth-death process. This enables methods developed for the epidemic to be used to obtain the asymptotic speed of translation for the contact birth-death process. Symmetry of the contact distributions is required but no existence constraint is placed on their Laplace transforms. The method for removing this constraint may also be used for the SIS epidemic. Results are given for both processes when the basic reproduction ratio is at most one.     

Enhancement of Photosynthesis by Alternated Light Beams and a Kinetic Model

Using the alga Chlorella and 2 light beams of defined wavelength, we have found that short period (1 sec) alternation gives all characteristics of enhancement of net oxygen evolution observed when the beams are superimposed. With increasing period of alternation (1-20 see) decay in enhancement is more rapid at higher intensities.We have developed a kinetic model for the current hypothesis of 2 photoacts operating in series and separated by electron transport reactions. The model is applicable at low light intensity such that rate is governed only by intensities and the fractions of reaction centers open for each photoact. Potentials and pool sizes of intermediates were taken from current estimates. Wavelength dependency of the 2 photoacts was taken from enhancement spectra for superimposed light beams. Analog computer treatment of the kinetic equations gave predictions of chromatic transients and alternated enhancement which are in reasonable but not complete agreement with experimental observations.     

A model of propagating calcium-induced calcium release mediated by calcium diffusion

The effect of sudden local fluctuations of the free sarcoplasmic [Ca++]i in cardiac cells on calcium release and calcium uptake by the sarcoplasmic reticulum (SR) was calculated with the aid of a simplified model of SR calcium handling. The model was used to evaluate whether propagation of calcium transients and the range of propagation velocities observed experimentally (0.05-15 mm s(-1)) could be predicted. Calcium fluctuations propagate by virtue of focal calcium release from the SR, diffusion through the cytosol (which is modulated by binding to troponin and calmodulin and sequestration by the SR), and subsequently induce calcium release from adjacent release sites of the SR. The minimal and maximal velocities derived from the simulation were 0.09 and 15 mm s(-1) respectively. The method of solution involved writing the diffusion equation as a difference equation in the spatial coordinates. Thus, coupled ordinary differential equations in time with banded coefficients were generated. The coupled equations were solved using Gear's sixth order predictor-corrector algorithm for stiff equations with reflective boundaries. The most important determinants of the velocity of propagation of the calcium waves were the diastolic [Ca++]i, the rate of rise of the release, and the amount of calcium released from the SR. The results are consistent with the assumptions that calcium loading causes an increase in intracellular calcium and calcium in the SR, and an increase in the amount and rate of calcium released. These two effects combine to increase the propagation velocity at higher levels of calcium loading.     

Deconvolution-free Subcellular Imaging with Axially Swept Light Sheet Microscopy

The use of propagation invariant Bessel beams has enabled high-resolution subcellular light sheet fluorescence microscopy. However, the energy within the concentric side lobe structure of Bessel beams increases significantly with propagation length, generating unwanted out-of-focus fluorescence that enforces practical limits on the imaging field of view size. Here, we present a light sheet fluorescence microscope that achieves 390 nm isotropic resolution and high optical sectioning strength (i.e., out-of-focus blur is strongly suppressed) over large field of views, without the need for structured illumination or deconvolution-based postprocessing. We demonstrate simultaneous dual-color, high-contrast, and high-dynamic-range time-lapse imaging of migrating cells in complex three-dimensional microenvironments, three-dimensional tracking of clathrin-coated pits, and long-term imaging spanning >10 h and encompassing >2600 time points.