Recapture of secondary electrons by the target in a DC planar magnetron discharge

In this paper we describe a simple two-dimensional model that allows the study of the individual secondary electron orbits in a DC planar magnetron discharge. Emphasis is on the recapture of secondary electrons by the target, which is enabled by their small initial energy, because this reduces the effective secondary electron yield as seen by the discharge. This reduction depends strongly on both the position along the race track and the gas pressure and it can be substantial for typical planar magnetron operating conditions. Our simple model allows to conclude that because of the sensitivity of the discharge on the secondary electron yield, the current-voltage characteristic, the spatial distribution as well as the pressure dependence of the planar magnetron discharge will be influenced by recapture.     

Coherent bremsstrahlung and parametric X-ray radiation from nonrelativistic electrons in a crystal

X-ray radiation generated by nonrelativistic electrons interacting with a crystal target exhibits several distinctive features in comparison to the relativistic case. The difference is related to the interference of the parametric X-ray radiation and coherent bremsstrahlung, which takes place for the nonrelativistic electrons. The characteristics of this radiation have been studied in the Bragg and Laue geometries in an electron microscope using a beam of electrons with energies in the 50–100 keV range. The necessary requirements on the target parameters, the measuring instrumentation, and the experimental geometry are established. Variation of the X-ray radiation frequency depending on the angle of electron beam incidence on the target in the region of non-relativistic electron energies has been observed for the first time. The X-ray radiation frequency has been also studied as a function of the primary electron beam energy. Tunable soft X-ray radiation with quantum energy in the range below 1 keV is obtained. The radiation quantum yield per electron within a unit solid angle amounts to ～10^?8.     

Ionisation cluster-size formation by electrons: from macroscopic to nanometric target sizes

An indispensable prerequisite for a deeper understanding of specified physical, chemical or biological changes initiated in matter when being exposed to ionising radiation is a detailed knowledge of particle track structure. Here, the structure of electron tracks is of particular interest since electrons are set in motion in large numbers as secondary particles during the slow down of any kind of ionising radiation in matter. From the point of view of radiation induced early damage to genes and cells, which starts with the early damage to segments of the DNA molecule, the most effective secondary electrons are those at energies of a few hundred eV since the yield of double-strand breaks induced by such electrons in the DNA shows a maximum. This can be explained by the fact that in water cylinders, 2 nm in diameter and height (as a substitute to small segments of the DNA), the probability of the electron-induced formation of ionisation cluster sizes greater than or equal to two is highest also at initial electron energies of a few hundred eV. In view of this promising feature of ionisation cluster-size distributions formed by low-energy electrons in nanometric targets of liquid water for explaining particular radio-biological endpoints, it is the aim of the present work to investigate the properties of cluster-size formation by electrons as a function of target size. Here, main emphasis is laid on the behaviour of cluster-size distributions if the target size is reduced from macroscopic to nanometric volumes.     

Bremsstrahlung radiation from high-energy electrons in wide gas-filled gaps

Electric discharge in wide interelectrode gaps filled with air at atmospheric pressure have been experimentally studied using applied voltage pulses with amplitudes up to 800 kV and a front width within 150–200 ns. The discharge was accompanied by the emission of a 10–20-ns-long pulse of radiation with photon energies above 5 keV. It is established that this emission is related to the generation of runaway electrons from the heads of anode-directed streamers. The calculated spectrum of the observed bremsstrahlung radiation exhibits a maximum at photon energies about 15 kV, which is due to the absorption of photons by the molecules of a gas in which the discharge takes place.     

Electron emission in a flat cathode-curved anode system in the space-charge-limited current regime

The process of electron emission in a vacuum diode with curved cathode in the space-charge-limited current regime has been theoretically studied. The space charge density, electric field, and velocity field distributions in the interelectrode gap are determined by means of expansion with respect to a small parameter representing the ratio of the characteristic transverse and longitudinal scales. Based on these results, the current density distribution over the cathode surface is analytically described. The analytical results are compared to the data of numerical calculations available in the literature.     

An analog of the vavilov-cherenkov radiation generated by a moving macroscopic source

The pulses of current formed when a pulse of gamma quanta travels over an extended absorbing region have been numerically simulated. Features of the electromagnetic field generated by these pulses are considered.     

Anomalous backscattering of high-power laser radiation by the flat surface of a solid

Results of measurements of the backscattering characteristics of high-power laser radiation at flat randomly rough surfaces of various materials are described for a wide range of radiation and surface parameters. It is observed that for probe radiation intensities in the range ∼10³–10⁷ W/cm² and a pulse duration of ∼10⁻⁸ s the scattering pattern becomes anomalous under various additional conditions. The results are analyzed in accordance with the laws of structural conditionality, the qualitative boundary, abnormality, and alternation of nonequilibrium. Possible practical applications of the observed effect are discussed. Pis’ma Zh. Tekh. Fiz. 25, 29–38 (August 26, 1999)     

Generation of terahertz radiation by hot electrons in carbon nanotubes

We demonstrate theoretically that quasi-metallic carbon nanotubes emit terahertz radiation induced by an applied voltage. It is shown that in the ballistic transport regime their spontaneous emission spectra have a universal frequency and bias voltage dependence, which raises the possibility of utilizing this effect for high-frequency nanoelectronic devices.     

Spectral properties of the nonspherically decaying radiation generated by a rotating superluminal source

The focusing of the radiation generated by a polarization current with a superluminally rotating distribution pattern is of a higher order in the plane of rotation than in other directions. Consequently, our previously published [J. Opt. Soc. Am. A24, 2443 (2007)] asymptotic approximation to the value of this field outside the equatorial plane breaks down as the line of sight approaches a direction normal to the rotation axis, i.e., is nonuniform with respect to the polar angle. Here we employ an alternative asymptotic expansion to show that, though having a rate of decay with frequency (mu) that is by a factor of order mu(2/3) slower, the equatorial radiation field has the same dependence on distance as the nonspherically decaying component of the generated field in other directions: It, too, diminishes as the inverse square root of the distance from its source. We also briefly discuss the relevance of these results to the giant pulses received from pulsars: The focused, nonspherically decaying pulses that arise from a superluminal polarization current in a highly magnetized plasma have a power-law spectrum (i.e., a flux density S infinity mu(alpha)) whose index (alpha) is given by one of the values -2/3, -2, -8/3, or -4.     

Morphology of the nonspherically decaying radiation beam generated by a rotating superluminal source

We consider the nonspherically decaying radiation field that is generated by a polarization current with a superluminally rotating distribution pattern in vacuum, a field that decays with the distance R(P) from its source as R(P)(-1/2), instead of R(P)(-1). It is shown (i) that the nonspherical decay of this emission remains in force at all distances from its source independently of the frequency of the radiation, (ii) that the part of the source that makes the main contribution toward the value of the nonspherically decaying field has a filamentary structure whose radial and azimuthal widths become narrower (as R(P)(-2) and R(P)(-3), respectively) the farther the observer is from the source, (iii) that the loci on which the waves emanating from this filament interfere constructively delineate a radiation subbeam that is nondiffracting in the polar direction, (iv) that the cross-sectional area of each nondiffracting subbeam increases as R(P), instead of R(P)(2), so that the requirements of conservation of energy are met by the nonspherically decaying radiation automatically, and (v) that the overall radiation beam within which the field decays nonspherically consists, in general, of the incoherent superposition of such coherent nondiffracting subbeams. These findings are related to the recent construction and use of superluminal sources in the laboratory and numerical models of the emission from them. We also briefly discuss the relevance of these results to the giant pulses received from pulsars.     

Quasi-stationary thermal regime in a film-substrate system during periodic-pulsed heating by laser radiation

The effect of thermal conductivity of the film-substrate contact boundary on the establishment of the quasi-stationary thermal regime during periodic-pulsed heating is studied.Moscow Institute of Electronics and Mathematics. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 64, No. 4, pp. 401–407, April, 1993.     

Effect of nitrogen pressure on the energy of runaway electrons generated in a gas diode

The energy spectra of runaway electrons generated in a gas diode under the action of voltage pulses with a front width of ∼300 ps and amplitude of ∼140 kV have been studied using a time-of-flight spectrometer at nitrogen pressures in a range of 0.1–760 Torr. The delay of runaway electron beam pulse relative to the driving voltage pulse has been determined. The electron energy depends in a complicated manner on the nitrogen pressure in the gas diode and on the cathode geometry. A minimum breakdown voltage for a gap between tubular cathode and flat anode has been observed at a nitrogen pressure of ∼100 Torr. A decrease in the nitrogen pressure below 100 Torr leads to an increase in the maximum of voltage drop on the gap and the energy of the main fraction of electrons.     

Regulation of the membrane wrapping transition of a cylindrical target by cytoskeleton adhesion

The adsorption of external objects to the cell membrane often triggers cellular responses involving large deformations. In phagocytosis, upon contact with the target, the cell creates large extensions that wrap around the target and ultimately lead to its engulfment. Although active force generation, in particular by actin polymerization, is required for completion of this process, the elastic deformation of the cell membrane upon adhesion to an external object might play an important part in its initiation. In this paper, the elastic deformation of a bilayer owing to the binding of a cylindrical object is studied, taking into account the membrane bending rigidity and the surface tension, the membrane adhesion to both the external target and inner cytoskeleton. The problem is studied within the framework of the Helfrich–Hamiltonian and using force balance relations and the proper boundary conditions that are related to the adhesion energy coefficients. It is shown that membrane wrapping around the target may be a continuous or abrupt transition upon increasing the target binding energy, depending on the value of the parameter. The degree of wrapping and the shape of the membrane in the vicinity of the object are computed numerically, and analytical expressions are given for the boundaries separating the different wrapping regimes in the parameter space.     

Switching of output coupling in a grating coupler by diffraction transition to the distributed Bragg reflector regime

In a proposed switching grating coupler (SGC), the grating period Lambda is chosen so that the SGC converts from a first-order grating coupler to a third-order distributed Bragg reflector by means of a small change in the guided mode index. The principle for switching the radiated wave power and the effective aperture of the SGC were experimentally confirmed by use of the thermo-optic effect of a polymer waveguide. The extinction of the peak intensity of the wave radiated by the SGC and collected by an external lens was measured to be 5 dB with a temperature change of 10 degrees.     

Studying the effect of the temperature factor on the transition from a laminar to a turbulent regime of flow in a boundary layer

Based on the generalization of experimental data, we propose a formula for the calculation of the effect of the temperature factor and the transition from a laminar flow regime to one that is turbulent in the boundary layer of a plate; this formula has been confirmed by experiments performed in the following range of parameter variation: 0.5 2.6; 0.2 M 3.6; 0.1 9%.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 16, No. 2, pp.218–224, February, 1969.     

Supervised algorithms for particle classification by a transition radiation detector

Support vector machines (SVMs) provide an interesting computational paradigm for the classification of data from high-energy physics and particle astrophysics experiments. In this study, the classification power of SVMs is compared with those from standard supervised algorithms, i.e. likelihood ratio and artificial neural networks (ANN), using test beam data from the transition radiation detector prototype of the PAMELA satellite-borne magnetic spectrometer. Concerning signal/background discrimination, SVM and ANN show the best performance. Moreover, our analysis shows that the use of SVM allows an accurate estimate of the discrimination efficiency of unseen data points: indeed, since almost the same efficiency is obtained with or without the cross-validation technique, the performance of SVM appears to be stable. On the other hand, the ANN shows a tendency to overfit the data, while this tendency is not observed using SVM. For these reasons, SVM could be used in particle astrophysics experiments where, due to the harsh experimental conditions, efficient and robust classification algorithms are needed.     

Propagation of shear waves generated by a modulated finite amplitude radiation force in a viscoelastic medium

An effective way to generate localized narrow-band low-frequency shear waves within tissue noninvasively, is by the modulated radiation force, resulting from the interference of two confocal quasi-CW ultrasound beams of slightly different frequencies. By using approximate viscoelastic Green's functions, investigations of the properties of the propagated shear-field component at the fundamental modulation frequency were previously reported by our group. However, high-amplitude source excitations may be needed to increase the signal-to-noise-ratio for shear-wave detection in tissue. This paper reports a study of the generation and propagation of dynamic radiation force components at harmonics of the modulation frequency for conditions that generally correspond to diagnostic safety standards. We describe the propagation characteristics of the resulting harmonic shear waves and discuss how they depend on the parameters of nonlinearity, focusing gain, and absorption. For conditions of high viscosity (believed to be characteristic of soft tissue) and higher modulation frequencies, the approximate shear wave Green's function is inappropriate. A more exact viscoelastic Green's function is derived in k-space, and using this, it is shown that the lowpass and dispersive effects, associated with a Voigt model of tissue, are more accurately represented. Finally, it is shown how the viscoelastic properties of the propagating medium can be estimated, based on several spectral components of the shear wave spectrum.