Interaction between dust grains near a conducting wall

The effect of a conducting electrode on the interaction of dust grains in an ion flow is discussed. It is shown that two grains levitating above the electrode at the same height may attract one another. This results in the instability of a dust layer.     

Calculation of the electric field in space plasma near a conducting sphere injecting an electron beam

The problem is considered of determining the electric field induced in the vicinity of a conducting spherical body that is at rest in a collisionless plasma and at the surface of which there is a prescribed sink of negative charge. The problem is solved for the general relativistic case under the assumption that the electron velocity in the neutralizing current is comparable with the speed of light. An integrodifferential equation is derived that describes the radial behavior of the electric field potential in the vicinity of the injector. A simplified method for determining the potential in the perturbed region is developed. The method implies that the problems of the potentials in a space charge region of radius R* (with a prescribed boundary potential ?*) adjacent the body and in the outer region r>R* are solved separately and then the solutions obtained are matched at boundary between these regions.     

Feedback stabilization of resistive wall modes in a tokamak with a double resistive wall

A study is made of the problem of active stabilization of the resistive wall modes in a tokamak with two conducting walls between the plasma column and the stabilizing system. The problem is analyzed for a steady-state configuration (without rotation) in the standard cylindrical approximation under the assumption that the stabilizing system responds instantaneously to a magnetic field perturbation by generating an in-phase signal with the required amplitude.     

Normal modes of a two-dimensional lattice of interacting dipoles

We derive the normal mode frequency spectra for a rectangular lattice of electric dipoles oscillating about configurations corresponding to relative minima in their mutual potential energy. For two such configurations —a) all dipoles parallel and b) adjacent rows antiparallel —we obtain numerical solutions for for various choices of the lattice parameters.     

Stochastic amplification of spatial modes in a system with one diffusing species

The problem of pattern formation in a generic two species reaction–diffusion model is studied, under the hypothesis that only one species can diffuse. For such a system, the classical Turing instability cannot take place. At variance, by working in the generalized setting of a stochastic formulation to the inspected problem, spatially organized patterns can develop, seeded by finite size corrections. General conditions are given for the stochastic patterns to occur. The predictions of the theory are tested for a specific case study.     

Energy approach to stability analysis of the locked and rotating resistive wall modes in tokamaks

A method is proposed for stability analysis of the locked and rotating resistive wall modes (RWMs) in tokamaks. The method is based on the relations describing the balance of energy permeating through the vessel wall. This is a natural extension of the traditional energy approach to the plasma stability tasks which allows incorporation of the energy outflow (absent in the classical energy principle) and its dissipation in the wall. The proposed method covers the locked and rotating modes with a complex growth rate. Its efficiency is proved by derivation of a general dispersion relation for such modes with further reduction to particular consequences for slow and fast RWMs. It is shown that in the latter case, when the skin depth becomes smaller than the wall thickness, the mode rotation essentially amplifies its damping, weakening and even suppressing the instability. This effect was earlier found in the frame of the slab model [V. D. Pustovitov, Phys. Plasmas 19, 062503 (2012)]. Here, it is confirmed with equations valid for toroidal geometry, which are obtained as a supplement to the standard energy principle. The presented results predict strong rotational stabilization of the fast RWMs, which occurs at the mode rotation frequency above a critical level. The estimates are given to allow comparison of these predictions with experimental results.     

Feedback suppression of resistive wall modes in a tokamak

The possibility of feedback suppression of the external kink modes in a tokamak with a resistive wall is studied theoretically, assuming that the stabilizing conductors are located at a certain distance from the wall and without making any assumptions regarding the locations of the magnetic sensors that close the feedback circuit and the parameters (i.e., the particular components of the perturbed magnetic field or magnetic fluxes) measured by the sensors. It is shown that the efficiency of the stabilizing system can generally be analyzed within a two-parameter model. The parameters of the problem are the jump in the logarithmic derivative of the radial magnetic field in the region where the stabilizing conductors are positioned and the ratio of the minor radius of the torus on which the conductors are wound to the radius of the wall. However, specific calculations should be carried out with at least a three-parameter model: the final results should depend on the currents in the conductors and the locations of the conductors and magnetic sensors. The relation between the magnetic parameter in the criterion for the suppression of the resistive wall modes and the currents in the stabilizing conductors is clarified, and the current magnitudes required for the suppression are estimated.     

Modeling of the feedback stabilization of the resistive wall modes in a tokamak

The problem of feedback stabilization of the resistive wall modes (RWMs) in a tokamak is discussed. An equilibrium configuration with the parameters accepted for the stationary ITER scenario 4A is considered as the main scenario. The effect of the vacuum chamber's shape on the plasma stability is studied. Ideal MHD stability is analyzed numerically by using the KINX code. It is shown that, in a tokamak with the parameters of the designed T-15M tokamak, RWMs can be stabilized by a conventional stabilizing system made of framelike coils. However, the maximum possible gain in β in such a tokamak is found to be smaller than that in ITER. It is shown that, in this case, a reduction in the plasma—wall gap width by 10 cm allows one to substantially increase the β limit, provided that RWMs are stabilized by active feedback.     

On the stability of Suydam modes in a nonuniformly rotating plasma

A simplified wave equation is derived that describes both Suydam modes in a nonuniformly rotating plasma column in a helical magnetic field and related flute modes. A study is made of a low-pressure plasma under the assumption that the azimuthal component of the magnetic field is much weaker than the axial component. It is shown that, when the monotonic radial variation of the plasma rotation velocity is sufficiently sharp, the plasma core becomes stable against short-wavelength Suydam modes. The instabilities that can develop in a nonuniformly rotating plasma are classified.     

The acoustic near field of a dancing honeybee

Summary The acoustic near field close to honeybees performing the wagging dance was investigated with pairs of small, matched microphones placed in various positions around the dancing bees. The dance sounds are produced by the wings, which act as an asymmetrical dipole emitter. Close to the abdomen, the sound pressures in the air spaces above and below the plane of the wings are totally out of phase. A zone of very intense acoustical short-circuiting exists close to the edges of the wings, where pressure gradients of about 1 Pa/mm are observed in the dorso-ventral direction (perpendicular to the plane of the wings). The pressure gradients drive air movements with velocity amplitudes up to about 1 m/s. The pressure gradients are much smaller in directions radially away from the bee and decrease rapidly with increasing distance from the wings. The sound pressure detected by a stationary probe at one side of the bee is strongly modulated at 12–13 Hz as a result of the bee's side-to-side wagging. Surprisingly little sound is found near the dancer's head. The positions of the follower bees reflect the properties of the acoustic field: The follower bees place their antennae in the zone of maximum acoustical short-circuiting where the air particle movements are most intense. These observations suggest 1) how follower bees can avoid mixing up the messages carried by the dance sounds when two or more bees are dancing only a few cm apart and 2) how the followers might extract information about a dancer's spatial orientation from the acoustic near field she produces. The observations also provide clues regarding the nature of the putative sound receivers.Dedicated to Professor Dr. Drs. h.c. mult. H. Autrum on the occasion of his 80th birthday     

Theory of nondiffusive penetration of a magnetic field into a conducting medium

The penetration of a current and, accordingly, a magnetic field into the plasma of pulsed systems characterized by short temporal and spatial scales can be investigated in electron magnetohydrodynamics. A study is made of the rapid penetration of the magnetic field of an injected high-current ion beam into a plasma.     

On the boundary of an equilibrium plasma near the magnetic separatrix in a tokamak

It is shown that plasma rotation near the tokamak the wall can result in a shift of the isobaric separatrix with respect to the magnetic one. This shift is calculated analytically, and this effect is exemplified by simple plasma equilibrium states. The plasma rotation that causes the shift of the separatrices can be driven either by a nonzero radial electric field at the plasma edge or by the Hall effect, which may take place even in the absence of the electric field.     

Surface waves on the boundary of a conducting medium and their excitation by a relativistic electron beam

Excitation of surface waves by a relativistic electron beam propagating over a conducting cylindrical medium (metal or highly ionized plasma) is investigated theoretically. Dispersion relations describing the linear interaction of surface electromagnetic waves with a monoenergetic electron beam are derived, and the growth rates and spatial amplification factors of excited waves are determined. Condition for the nonlinear trapping of the beam electrons by a surface wave is used to determine the maximum amplitude of the excited wave and the optimal radiator length. The electric field of a surface wave excited by an electron beam is estimated for a particular case.     

Energy deposition in a model of man in the near field

The spatial distribution of the specific absorption rate (SAR) was measured in a full-scale model of man using implantable electric field probes. The model was exposed in the near-field of linear and aperture antennas at 350 MHz. Effects of the wave polarization, antenna position and antenna gain on the SAR distribution and the average SAR in the whole-body and body parts are reported.     

Visualising individual green fluorescent proteins with a near field optical microscope.

The use of the green fluorescence protein (GFP) as an individual marker for applications in molecular biology requires detailed understanding of its photophysical and photodynamical properties. We investigated individual S65T mutants of GFP both on a glass surface and embedded in a water-pore gel. An aperture-type near field scanning optical microscope (NSOM) with two polarisation detection channels was applied to afford high spatial (approximately 70 nm) and temporal (0.5 ms) resolution. Shear-force and near field fluorescence imaging were performed simultaneously, allowing direct correlation between topographic and optical features. Polarisation data showed that the emission dipole moment of the proteins is fixed in space within both the barrel structure of the protein and the gel matrix used for spatial confinement of the proteins. The photophysical behaviour of the S65T-GFP mutants was monitored in time, with 500-micros real-time resolution and continuous imaging for periods of more than 2 h. Our results show the reversible on-off behaviour on a time scale that spans from 10(-4) to 10(3) s. Even a process generally identified as "bleaching" turns out to be reversible if a sufficient long observation time is allowed. As such, the photodynamics of individual GFPs appear to be much more complex than the properties deduced from ensemble-averaged measurements.     

Numerical analysis of the near field of ICRH antennas in a tokamak

Results are presented from numerical calculations of the near fields of ICRH antennas in the quasisteady current approximation in two-dimensional geometry. The distributions of the vacuum electric and magnetic fields as well as of the surface current density in the antenna elements and inside the tokamak chamber are obtained. The electrotechnical characteristics of the antennas are analyzed numerically as functions of their geometric parameters.     

Evolution and extinction of Afro-Arabian primates near the Eocene-Oligocene boundary

Revised age estimates for the primate-bearing localities of the Jebel Qatrani Formation (Fayum area, northern Egypt) have provided a new perspective on primate response to early Oligocene climate change in North Africa. Environmental changes associated with early Oligocene cooling might have driven the local extinction of at least 4 strepsirrhine primate clades (adapids, djebelemurines, plesiopithecids and galagids). Contrary to previous suggestions, oligopithecid (and possibly proteopithecid) anthropoids persisted beyond the Eocene-Oligocene boundary (EOB) in the Fayum area, and the former group evidently continued to diversify through the early Oligocene at lower latitudes. Propliopithecids and parapithecine parapithecids first appear in the Jebel Qatrani Formation millions of years after the EOB, so their derived dental and gnathic features can no longer be interpreted as sudden adaptive morphological responses to earliest Oligocene climatic events. Evidence for latitudinal contraction of Afro-Arabian primate distribution through the early Oligocene suggests that the profound late Oligocene restructuring of Afro-Arabian primate communities is most likely to have occurred in equatorial and low-latitude tropical Africa.     

Contribution of salt bridges near the surface of a protein to the conformational stability

Salt bridges play important roles in the conformational stability of proteins. However, the effect of a surface salt bridge on the stability remains controversial even today; some reports have shown little contribution of a surface salt bridge to stability, whereas others have shown a favorable contribution. In this study, to elucidate the net contribution of a surface salt bridge to the conformational stability of a protein, systematic mutant human lysozymes, containing one Glu to Gln (E7Q) and five Asp to Asn mutations (D18N, D49N, D67N, D102N, and D120N) at residues where a salt bridge is formed near the surface in the wild-type structure, were examined. The thermodynamic parameters for denaturation between pH 2.0 and 4.8 were determined by use of a differential scanning calorimeter, and the crystal structures were analyzed by X-ray crystallography. The denaturation Gibbs energy (DeltaG) of all mutant proteins was lower than that of the wild-type protein at pH 4, whereas there was little difference between them near pH 2. This is caused by the fact that the Glu and Asp residues are ionized at pH 4 but protonated at pH 2, indicating a favorable contribution of salt bridges to the wild-type structure at pH 4. Each contribution was not equivalent, but we found that the contributions correlate with the solvent inaccessibility of the salt bridges; the salt bridge contribution was small when 100% accessible, while it was about 9 kJ/mol if 100% inaccessible. This conclusion indicates how to reconcile a number of conflicting reports about role of surface salt bridges in protein stability. Furthermore, the effect of salts on surface salt bridges was also examined. In the presence of 0.2 M KCl, the stability at pH 4 decreased, and the differences in stability between the wild-type and mutant proteins were smaller than those in the absence of salts, indicating the compensation to the contribution of salt bridges with salts. Salt bridges with more than 50% accessibility did not contribute to the stability in the presence of 0.2 M KCl.     

Coupled field analysis of heat flow in the near field of a microwave applicator for tumor ablation

Microwave tumor ablation (MTA) offers a new approach for the treatment of hepatic neoplastic disease. Reliable and accurate information regarding the heat distribution inside biological tissue subjected to microwave thermal ablation is important for the efficient design of microwave applicators and for optimizing experiments, which aim to assess the effects of therapeutic treatments. Currently there are a variety of computational methods based on different vascular structures in tissue, which aim to model heat distribution during ablation. This paper presents results obtained from two such computational models for temperature distributions produced by a clinical 2.45 GHz MTA applicator immersed in unperfused ex vivo bovine liver, and compares them with measured results from a corresponding ex vivo experiment. The computational methods used to model the temperature distribution in tissue caused by the insertion of a 5.6 mm diameter "wandlike" microwave applicator are the Green's function method and the finite element method (FEM), both of which provide solutions of the heat diffusion partial differential equation. The results obtained from the coupled field simulations are shown to be in good agreement with a simplified analysis based on the bio-heat equation and with ex vivo measurements of the heat distribution produced by the clinical MTA applicator.