Streamer self-focusing in an external longitudinal magnetic field.

The numerical simulation of the development of a streamer discharge in a gap with an external longitudinal magnetic field was used to demonstrate the self-focusing of such discharges. Self-focusing is caused by a sharp deceleration of the radial ionization wave due to a change in the electron energy distribution function, a decrease in the average electron energy, the rate of gas ionization, and the electron mobility in crossed electric and magnetic fields as compared to the case of the discharge development without a magnetic field. The self-focusing effect of a streamer discharge in an external longitudinal magnetic field is observed for both positive and negative pulse polarities. The paper proposes an estimate of the critical value of the magnetic field, which makes it possible to control the development of pulsed high-voltage discharges at various gas pressures.

Growth of nanoparticles in dynamic plasma.

Coagulation growth kinetics of nanoparticles in plasma is affected by interparticle electrostatic forces due to the charging phenomenon. In stationary plasmas, unipolar charging of particles results in retardation of particle growth and may result in a limitation on particle size. We demonstrate the opposite effect of enhanced particle growth in atmospheric pressure nonstationary arc discharge. Modeling of the nanoparticle growth kinetics reveals the formation of a bipolar charge distribution. As a result, reversed (attractive) Coulomb forces promote the formation of micrometer-size particles in a millisecond timescale as observed in experiment.

Counting the electrons in a multiphoton ionization by elastic scattering of microwaves.

Multiphoton ionization (MPI) is a fundamental first step in high-energy laser-matter interaction and is important for understanding the mechanism of plasma formation. With the discovery of MPI more than 50 years ago, there were numerous attempts to determine the basic physical constants of this process in direct experiments, namely photoionization rates and cross-sections of the MPI; however, no reliable data was available until now, and the spread in the literature values often reaches 2-3 orders of magnitude. This is due to the inability to conduct absolute measurements of plasma electron numbers generated by MPI, which leads to uncertainties and, sometimes, contradictions between MPI cross-section values utilized by different researchers across the field. Here, we report the first direct measurement of absolute plasma electron numbers generated at MPI of air, and subsequently we precisely determine the ionization rate and cross-section of eight-photon ionization of oxygen molecule by 800 nm photons σ8 = (3.3 ± 0.3)×10-130 W-8m16s-1. The method, based on the absolute measurement of the electron number created by MPI using elastic scattering of microwaves off the plasma volume in Rayleigh regime, establishes a general approach to directly measure and tabulate basic constants of the MPI process for various gases and photon energies.

Rayleigh scattering on the cavitation region emerging in liquids.

It is shown that the scattering of laser radiation off cavitation ruptures in fluids is similar to scattering by gas particles. When the characteristic dimensions of microscopic voids and bubbles are considerably smaller than the laser wavelength, the scattered light is in the Rayleigh regime, which allows for the detection of early stage cavitation. Simple estimates of the scattered radiation intensity and the dynamics of its changes in connection with the generation of cavitation in the test volume are obtained, allowing us to find the critical conditions for cavitation inception.

Correlation of action potentials in adjacent neurons.

A possible mechanism for the synchronization of action potential propagation along a bundle of neurons (ephaptic coupling) is considered. It is shown that this mechanism is similar to the salutatory conduction of the action potential between the nodes of Ranvier in myelinated axons. The proposed model allows us to estimate the scale of the correlation, i.e., the distance between neurons in the nervous tissue, wherein their synchronization becomes possible. The possibility for experimental verification of the proposed model of synchronization is discussed.

Initiation and blocking of the action potential in an axon in weak ultrasonic or microwave fields.

In this paper, we analyze the effect of the redistribution of the transmembrane ion channels in an axon caused by longitudinal acoustic vibrations of the membrane. These oscillations can be excited by an external source of ultrasound and weak microwave radiation interacting with the charges sitting on the surface of the lipid membrane. It is shown, using the Hodgkin-Huxley model of the axon, that the density redistribution of transmembrane sodium channels may reduce the threshold of the action potential, up to its spontaneous initiation. At the significant redistribution of sodium channels in the membrane, the rarefaction zones of the transmembrane channel density are formed, blocking the propagation of the action potential. Blocking the action potential propagation along the axon is shown to cause anesthesia in the example case of a squid axon. Various approaches to experimental observation of the effects considered in this paper are discussed.

Single-shot coherent Rayleigh-Brillouin scattering using a chirped optical lattice.

We present a method for obtaining coherent Rayleigh-Brillouin scattering (CRBS) spectra on timescales of hundreds of nanoseconds using rapidly chirped, pulsed, optical lattices. This enables us to transfer the spectral profile to a temporal profile which can be easily recorded on a single shot of an oscilloscope. These spectra are demonstrated to have sufficient signal-to-noise ratio to study CRBS models over a wide range of gas densities.

Dielectric fluid in inhomogeneous pulsed electric field.

We consider the dynamics of a compressible fluid under the influence of electrostrictive ponderomotive forces in strong inhomogeneous nonstationary electric fields. It is shown that if the fronts of the voltage rise at a sharp, needlelike electrode are rather steep (less than or about nanoseconds), the region of negative pressure arises, which can reach values at which the fluid loses its continuity with the formation of cavitation ruptures. If the voltage on the electrode is not large enough or the front is flatter, the cavitation in the liquid does not occur. However, a sudden shutdown of the field results in a reverse flow of liquid from the electrode, which leads to appearance of negative pressure, and, possibly, cavitation.

Hydrodynamic flow in a synaptic cleft during exocytosis.

It is shown that exocytosis in a chemical synapse may be accompanied by "microjet" formation due to the overpressure that exists in the vesicles. This mechanism may take place either at complete fusion of a vesicle with the presynaptic membrane or in the so-called kiss-and-run mode of neurotransmitter release. A simple hydrodynamic model of the viscous incompressible flow arising in the synaptic cleft is suggested. The occurrence of hydrodynamic flow (microjet) leads to more efficient transport of neurotransmitter than in the case of classical diffusive transport.

Measurements of streamer head potential and conductivity of streamer column in the cold nonequilibrium atmospheric plasmas.

This work presents a simple method for the characterization of streamers developing in cold atmospheric plasma jets. The method is based upon stopping ("scattering") of streamer by means of external DC potential in order to determine the potential of the streamer head. The experimental evidence presented in this work does not support the model of the electrically insulated streamer head. On the contrary, it is shown that the electrode potential is transferred to the streamer head along the streamer column to which it is attached with no significant voltage drop. Based on the proposed method, we determine various streamer parameters such as head charge (1-2×108 electrons), electrical field in the head vicinity (about 100 kV/cm), average conductivity (10-2 Ω-1cm-1) and plasma density of the streamer column (2×1013 cm-3).

Coherent Brillouin scattering.

We measure the spectrum of coherent Brillouin scattering (CBS) in a gas as a function of time and observe for the first time additional spectral sidebands and line shape narrowing of the Brillouin peak. We find that both effects result from the interference of the density modulation induced by the moving dipole force of the pump beams with the acoustic waves induced by their fast thermalization and are predicted by a hydrodynamic-light scattering model. These line shapes differ from both spontaneous and stimulated Brillouin scattering spectra and also from previous coherent Rayleigh-Brillouin measurements.

Action-potential-encoded second-harmonic generation as an ultrafast local probe for nonintrusive membrane diagnostics.

The Hodgkin-Huxley treatment of the dynamics of a nerve impulse on a cell membrane is combined with a phenomenological description of molecular hyperpolarizabilities to develop a closed-form model of an action-potential-sensitive second-harmonic response of membrane-bound chromophores. This model is employed to understand the key properties of the map between the action potential and modulation of the second harmonic from a cell membrane stained with hyperpolarizable chromophore molecules.

Spectral narrowing in coherent rayleigh scattering.

Spectral narrowing of the coherent Rayleigh scattering line shape in a room temperature CO(2) gas (2.5 x 10(23) m(-3)) with intense fields in the 10(15) W m(-2) range is observed. The line shape saturates to a width of approximately half that observed at low pump intensities and indicates a transition from scattering primarily from untrapped molecules to that from both trapped and untrapped molecules that are localized by the deep (60 K) optical potentials produced by the pump beams. At higher densities (5 x 10(24) m(-3)), collisions between the trapped and untrapped molecules broaden the spectral profile.

Switching intense laser pulses guided by Kerr-effect-modified modes of a hollow-core photonic-crystal fiber.

A Kerr-nonlinearity-induced profile of the refractive index in the hollow core of a photonic-crystal fiber (PCF) changes the spectrum of propagation constants of air-guided modes, effectively shifting the passbands in fiber transmission, controlled by the photonic band gaps (PBGs) of the cladding. This effect is shown to allow the creation of fiber switches for high-intensity laser pulses. The Kerr-nonlinearity control of air-guided modes in PCFs and the performance of a PCF switch are quantified by solving the propagation equation for the slowly varying envelope of a laser pulse guided in Kerr-effect-modified PCF modes. The spatial dynamics of the light field in a PBG waveguide switch is analyzed with the use of the slowly varying envelope approximation, demonstrating high contrasts of optical switching with PBG waveguides and hollow PCFs.