Optical studies of plasma inhomogeneities in a high-current pulsed magnetron discharge

Results are presented for experimental studies of the plasma glow in a high-current pulsed magnetron discharge by using a high-speed optical frame camera. It is found that the discharge plasma is inhomogeneous in the azimuthal direction. The plasma bunches rotate with a linear velocity of ∼1 cm/μs in the direction of electron Hall drift, and their number is proportional to the discharge current. Plasma inhomogeneities in the form of plasma jets propagate in the form of plasma jets from the cathode region toward the anode. It is shown analytically that the formation of inhomogeneities is caused by the necessity to transfer high-density electron current across the magnetic field.     

Stimulated ionization scattering of a wave beam forming a discharge channel in a magnetic mirror trap

The stimulated scattering of a whistler wave beam forming an extended discharge channel in a magnetic mirror trap is discovered and investigated experimentally. It is shown that the beam is scattered by relaxaction oscillations of the lattice of plasma inhomogeneities excited by the beam field. The spectrum of the pump field in the RF discharge plasma is found to broaden considerably and to contain individual modulation peaks corresponding to lattice oscillations. The peaks are observed at working gas pressures at which the electron mean free path is close to the wavelength of the standing wave forming the discharge channel. A physical model describing the phenomena observed is developed.     

Freeze-fracture electron microscopy of lipid membranes on colloidal polyelectrolyte multilayer coated supports

Lipid membranes were assembled on polyelectrolyte (PE)-coated colloidal particles. The assembly was studied by means of confocal microscopy, flow cytometry, scanning force microscopy, and freeze-fracture electron microscopy. A homogeneous lipid coverage was established within the limits of optical resolution. Flow cytometry showed that the lipid coverage was uniform. Freeze-fracture electron microscopy revealed that the lipid was adsorbed as a bilayer, which closely followed the surface profile of the polyelectrolyte support. Additional adsorption of polyelectrolyte layers on top of the lipid bilayer introduced inhomogeneities as evident from jumps in the fracture plane. Characteristic lipid multilayers have not been seen with freeze-fracture electron microscopy.     

Stimulated and co-operative electron transfer in energy conversion and catalysis.

A new mechanism of electron transfer, stimulated electron transfer, is postulated, in which an electronic feedback is drastically increasing both the rate of electron transfer and the propagation of free energy along electron transferring molecular pathways. In principle, the idea of pushing a system far from equilibrium to achieve a high reaction rate and co-operative phenomena is applied to molecular electron transfer. The effect is calculated from a semiclassical kinetic model of a chain redox reaction with autocatalytic feedback on individual rate constants, where the steps have subsequently been minimized to obtain a continuous electron transfer pathway with electronic feedback. The influence of inhomogeneities and asymmetries in the electron transfer path and of vectorial components (electrical field, gradient of redox potential) are discussed as well as the acceleration of individual and multiple electron transfer as a function of feedback. Examples of autocatalytic feedback are provided including mechanisms involving electron transfer proteins and multi-centre electron transfer catalysts. Such a phenomenon can be described for molecular and interfacial electron transfer in analogy to stimulated and coherent light emission. The results suggest that autocatalytic or stimulated electron transfer may be a key to the understanding of efficient electron transfer and co-operative multi-electron transfer catalysis in biology and a challenge for fuel production mechanisms in artificial photosynthesis and fuel cycles.     

Effects of the substratum on the migration of primordial germ cells

It is now clear from work on defined cell types on artificial substrates that various chemical and physical inhomogeneities in the substrates can guide cell locomotion. It is also becoming clear that less well defined inhomogeneities in living cell substrates can guide the normal locomotion of embryonic migratory cells in vivo. The primordial germ cells (p.g.cs) of early anuran amphibian embryos are proving a useful model for the study of cell migration. When isolated from the embryo and cultured on living cellular substrate, p.g.cs become oriented by the shapes of the underlying cells or by their stress fibre cytoskeleton, or both. A combination of scanning and transmission electron microscopy in vivo shows a clearly aligned cellular substrate for p.g.c. migration along part of their route. Furthermore, we find that the glycoprotein fibronectin is involved in p.g.c. adhesion, which suggests a link between orientation of the substrate cells and p.g.c. guidance.     

Generation of plasma inhomogeneities and their total suppression in a volume self-sustained discharge

The principle types of inhomogeneities formed in a volume self-sustained discharge are analyzed. Possible mechanisms and conditions for suppression of their development are considered. An ultimately homogeneous volume self-sustained discharge in CO₂: N₂: He mixtures where local plasma inhomogeneities are absent is obtained. At the energy contribution of 170 J/l, the duration of steady discharge burning in the CO₂: N₂: He = 1: 2: 3 mixtures at atmospheric pressure is 10 μs.     

Channeling of high-power radio waves under conditions of strong anomalous absorption in the presence of an averaged electron heating source

Strong anomalous absorption of a high-power radio wave by small-scale plasma inhomogeneities in the Earth’s ionosphere can lead to the formation of self-consistent channels (solitons) in which the wave propagates along the magnetic field, but has a soliton-like intensity distribution across the field. The structure of a cylindrical soliton as a function of the wave intensity at the soliton axis is analyzed. Averaged density perturbations leading to wave focusing were calculated using the model proposed earlier by Vas’kov and Gurevich (Geomagn. Aéron. 16, 1112 (1976)), in which an averaged electron heating source was used. It is shown that, under conditions of strong electron recombination, the radii of individual solitons do not exceed 650 m.     

Structures of the plasma channels in a Besselian beam

A mechanism for the formation of the structure of an optical discharge in Besselian laser beams is proposed on the basis of analyzing numerous experiments. The discharge structure is determined by the periodicity of the field of a Besselian beam in the radial and longitudinal directions and also depends on the power and duration of the heating pulse. In the initial stage of the plasma channel formation, the configuration of the channel inhomogeneities follows the discharge structure. If the spatial scale of the discharge structure is small, then the developing channel evolves into a homogeneous state. The time required for the structural inhomogeneities of the plasma channel to be smoothed out is estimated as a function of their scale length. __________ Translated from Fizika Plazmy, Vol. 27, No. 9, 2001, pp. 846–858. Original Russian Text Copyright ￠ 2001 by Pyatnitsky.     

Picosecond runaway electron beams in air

Experimental data on the generation of picosecond runaway electron beams in an air gap with an inhomogeneous electric field at a cathode voltage of up to 500 kV are presented. The methods and equipment developed for these experiments made it possible to measure the beam characteristics with a time resolution of better than 10⁻¹¹ s, determine the voltage range and the beam formation time in the breakdown delay stage, and demonstrate the influence of the state of the cathode surface on the stability of runaway electron generation. It is demonstrated that the critical electron runaway field in air agrees with the classical concepts and that the accelerated beam can be compressed to ∼20 ps. It is unlikely that, under these conditions, the beam duration is limited due to the transition of field emission from the cathode to a microexplosion of inhomogeneities. The maximum energy acquired by runaway electrons in the course of acceleration does not exceed the value corresponding to the electrode voltage.     

Small-scale instability and nonlinear structures in current-carrying atmospheric plasma in the D region of the ionosphere

It is shown that the electric field may cause instability of low-frequency potential oscillations with a period of about 1 min in the D region of the ionosphere. This is related to the fact that the attachment rate of electrons to molecules is a nonmonotonic function of the temperature: it increases at low temperatures and decreases at high temperatures. The temperature corresponding to the maximum attachment rate is about 1000 K. The development of instability can result in the formation of quasi-steady layers of the electron density with a characteristic spatial period of several tens of meters. Such inhomogeneities can affect the propagation of radio waves with wavelengths on the order of or shorter than 10 m.     

Scroll-wave dynamics in human cardiac tissue: lessons from a mathematical model with inhomogeneities and fiber architecture

Cardiac arrhythmias, such as ventricular tachycardia (VT) and ventricular fibrillation (VF), are among the leading causes of death in the industrialized world. These are associated with the formation of spiral and scroll waves of electrical activation in cardiac tissue; single spiral and scroll waves are believed to be associated with VT whereas their turbulent analogs are associated with VF. Thus, the study of these waves is an important biophysical problem. We present a systematic study of the combined effects of muscle-fiber rotation and inhomogeneities on scroll-wave dynamics in the TNNP (ten Tusscher Noble Noble Panfilov) model for human cardiac tissue. In particular, we use the three-dimensional TNNP model with fiber rotation and consider both conduction and ionic inhomogeneities. We find that, in addition to displaying a sensitive dependence on the positions, sizes, and types of inhomogeneities, scroll-wave dynamics also depends delicately upon the degree of fiber rotation. We find that the tendency of scroll waves to anchor to cylindrical conduction inhomogeneities increases with the radius of the inhomogeneity. Furthermore, the filament of the scroll wave can exhibit drift or meandering, transmural bending, twisting, and break-up. If the scroll-wave filament exhibits weak meandering, then there is a fine balance between the anchoring of this wave at the inhomogeneity and a disruption of wave-pinning by fiber rotation. If this filament displays strong meandering, then again the anchoring is suppressed by fiber rotation; also, the scroll wave can be eliminated from most of the layers only to be regenerated by a seed wave. Ionic inhomogeneities can also lead to an anchoring of the scroll wave; scroll waves can now enter the region inside an ionic inhomogeneity and can display a coexistence of spatiotemporal chaos and quasi-periodic behavior in different parts of the simulation domain. We discuss the experimental implications of our study.     

Statistical thermodynamics of association colloids. IV. Inhomogeneous membrane systems

A previously developed self-consistent field theory to describe the equilibrium properties of lipid-like membranes is extended to allow the presence of inhomogeneities parallel to the membrane. Conformations of lipid molecules and foreign ('guest') molecules in the membrane are obtained by step-weighted random walk generation of the chains on a lattice, where the theory is made two-dimensional to account for the parallel inhomogeneities. No pre-assigned positions of the head groups, or other parts of the molecules, are introduced. Nearest neighbor interactions are accounted for through Flory-Huggins type interaction parameters. The theory is applied to the incorporation of trans-membrane molecules and conditions for channel formation are discussed. Lateral phase separation in membranes consisting of non-mixing lipids is also considered. Water only slightly enriches the boundary between the two lipid regions. The aliphatic chains are very well able to smoothly cover inhomogeneities in the bilayer. No indications of instability of the membrane due to the induced inhomogeneities are found.     

Theory of the anomalous resistivity of a randomly inhomogeneous plasma

The effect of random density inhomogeneities on the anomalous plasma resistivity caused by a current-driven ion acoustic instability is considered. It is shown that, under certain conditions, dissipation due to the plasma inhomogeneity can be more efficient than that due to nonlinear effects. The scenario under consideration can occur in the low-density corona of a high-density Z-pinch.     

Assessment of spatial inhomogeneities in intima media thickness along an arterial segment using its dynamic behavior

To assess locally deviating structural and mechanical properties of arterial walls, the spatial variance in end-diastolic intima media thickness (IMT) and the change in IMT during the cardiac cycle (DeltaIMT) were determined along a short segment of the common carotid artery (15.86 mm), at 16 positions simultaneously. Intrasubject spatial inhomogeneities along the artery were revealed by a spatial variance significantly larger than the temporal variance over several beats. If differences between positions were confirmed, the extent of the inhomogeneity was obtained by comparison of IMT and DeltaIMT at each position with their spatial medians +/- the least-significant difference. Because no intersubject comparisons were necessary, a single session of several measurements was sufficient to assess inhomogeneities in the arterial wall properties of a subject, making the method independent of biological variability between subjects. The method was evaluated on 47 presumed healthy subjects (age range 21-75 yr). In 22 subjects, spatial inhomogeneities in DeltaIMT occurred (P < 0.05). In young subjects, DeltaIMT was locally decreased, i.e., in systole inhomogeneities were less compressed than their surrounding tissue. In older subjects, DeltaIMT was locally increased, i.e., the inhomogeneity was locally more compressed than its surrounding wall tissue.     

Derivation of a field equation of brain activity

We present a nonlinear field theory of the brain under realistic anatomical connectivity conditions describing the interaction between functional units within the brain. This macroscopic field theory is derived from the quasi-microscopic conversion properties of neural populations occurring at synapses and somas. Functional units are treated as inhomogeneities within a nonlinear neural tissue.     

Structure of the single-stranded polyribonucleotide polycytidylic acid.

The Fourier transformation was obtained from the experimental diffuse X-ray scattering curve of adult human oxyhaemoglobin in an aqueous solution. The correlation function of the form of the molecule was calculated from known coordinates of horse haemoglobin atoms. The distribution function of inhomogeneities was deduced from the above Fourier transformation and the correlation function. This distribution function of inhomogeneities is described by a series of maxima providing evidence for a short and long-range order extending up to 5 nm which is close to the maximum dimension of the haemoglobin molecule.     

NMR imaging of roots: methods for reducing the soil signal and for obtaining a 3‐dimensional description of the roots

Studies of root morphology or root architecture are limited by the problems of observing roots in soil. NMR imaging can preferentially detect the water in roots rather than soil and can therefore produce a map of the distribution of the water in roots, even though there is much more water in the soil than in the roots. The gradient echo imaging technique was shown to detect less signal from water in soil than the more usual spin‐echo technique, probably because the signal loss due to magnetic field inhomogeneities was not refocused in the gradient echo experiments. The gradient echo sequence produced images of a water‐filled capillary tube in soil and of the main roots of a 31‐day‐old oat plant, images which had better contrast than those from the spin‐echo sequence. The improvement was particularly noticeable in the wetter soils. The finer roots could only be observed in the driest of soil and with the spin‐echo sequence. Magnetic field inhomogeneities have a greater effect on thinner roots than on thicker ones, and because the initial signal from thin roots was so small, the additional signal loss due to magnetic field inhomogeneities made the fine roots undetectable with the gradient echo sequence. The problem of loss of perspective associated with projection images was overcome by taking a number of images with different projection planes. By comparing images, a set of (x, y, z) coordinates was obtained for the 4 main roots of a 31‐day‐old oat plant. Computer graphics methods made it possible to compare the digitised root coordinate data set with the original images, thereby confirming the accuracy of the digitised data.     

Monte Carlo evaluation of the effect of inhomogeneities on dose calculation for low energy photons intra-operative radiation therapy in pelvic area

The aim of this study was to evaluate the effect of inhomogeneities on dose calculation for low energy photons intra-operative radiation therapy (IORT) in pelvic area. A GATE Monte Carlo model of the INTRABEAM® was adapted for the study. Simulations were performed in the CT scan of a cadaver considering a homogeneous segmentation (water) and an inhomogeneous segmentation (5 tissues from ICRU44). Measurements were performed in the cadaver using EBT3 Gafchromic® films. Impact of inhomogeneities on dose calculation in cadaver was 6% for soft tissues and greater than 300% for bone tissues. EBT3 measurements showed a better agreement with calculation for inhomogeneous media. However, dose discrepancy in soft tissues led to a sub-millimeter (0.65 mm) shift in the effective point dose in depth. Except for bone tissues, the effect of inhomogeneities on dose calculation for low energy photons intra-operative radiation therapy in pelvic area was not significant for the studied anatomy.     

Evaluation of viscoelastic property inhomogeneities in soft tissues exposed to low frequency perturbations

This work presents the methods of recognition of inhomogeneities of tissue shear viscoelastic properties using partial data on the internal displacements in an object exposed to low-frequency perturbation. An approach to detect tissue inhomogeneities using the single displacement component is presented.     

Self-organization in Balanced State Networks by STDP and Homeostatic Plasticity

Structural inhomogeneities in synaptic efficacies have a strong impact on population response dynamics of cortical networks and are believed to play an important role in their functioning. However, little is known about how such inhomogeneities could evolve by means of synaptic plasticity. Here we present an adaptive model of a balanced neuronal network that combines two different types of plasticity, STDP and synaptic scaling. The plasticity rules yield both long-tailed distributions of synaptic weights and firing rates. Simultaneously, a highly connected subnetwork of driver neurons with strong synapses emerges. Coincident spiking activity of several driver cells can evoke population bursts and driver cells have similar dynamical properties as leader neurons found experimentally. Our model allows us to observe the delicate interplay between structural and dynamical properties of the emergent inhomogeneities. It is simple, robust to parameter changes and able to explain a multitude of different experimental findings in one basic network.