MOLECULAR DYNAMICS SIMULATION OF PHASE TRANSFORMATIONS KINETICS IN LIQUIDS AND SOLIDS

N.P. Lazarev

Full Text :   (2000 kB, Rus.) 

Abstract
Application of the molecular dynamics method in physics of condensed matter is reviewed. The main ideas and algorithms as well as performance and limitations of this approach are discussed. On the example of Ag-Cu glass-forming alloy we simulate phase diagram and find melting point T_m, glass transition temperature T_g and the Kauzmann temperature T_K. Structure properties are analyzed by means of radial distribution function and local order parameter. An anomalous diffusion is observed in an intermediate time region. The analysis of time evolution of van Hove correlation function indicates the existence of both jump displacements and short-range cooperative atomic rearrangements. Temperature dependences of the shear viscosity and the diffusion coefficient show the breakdown of the Stokes-Einstein relation well above the glass transition temperature T_g. During creep simulation, the activation volume is evaluated as 4 atomic volumes in liquid state. However, near T_g it increases several times. Temperature- and stress-reversible martensitic transformations in Ni-Al shape-memory alloy are demonstrated. During complete temperature cycle a wide hysteresis in enthalpy, volume, and shape of the simulated crystals is observed. The temperature T₀ of the phase transformation is found from the calculated free energy evolution. A heterogeneous nucleation of the new phase in a finite-size system with free external surface is observed. The effect of surface reconstruction on martensitic transformation is revealed. The role of composition ordering is studied in a Ni₅Al₃alloy. In order to find the location of the martensitic transformations in the temperature-composition-stress diagram, the influence of shear stresses is investigated. The effects of grain boundaries are simulated.

KEY WORDS: glass transition, anomalous diffusion, viscosity, order parameter, martensitic transformations.

PHYSICAL ELECTRON-MAGNETIC MODEL OF CATHODE SPOT VACUUM ARC

А.А. Andreev

Full Text :   (560 kB, Rus.) 

Abstract
The physical model of cathode spot has been considered, in which the main factors are field emission of electrons in its surroundings and interaction with its own magnetic field. The movement of electrons in the neighborhood of a cathode spot cell is considered. The cell current is a pulse with a rising-up part and a descending part of about 5·10^-9s time duration each. This current generates the magnetic field around the cell. During these rising-up and descending current stages, electric fields with an amplitude up to 3.5·10⁷ V/cm are induced in the neighborhood of the cell. Thus, around the cell, near the cathode surface, the crossed fields E ´ B are building up which amplitudes become lower with the increase of a distance from the cell center, approximately as 1/R. Under the action of electric fields the currents become to flow along the whole perimeter of a cell current channel. These currents are in some aspect similar to halo-currents arising in the plasma of tokamak during a pulsed ohmic current. When the cell current grows, the "halo" currents are directed oppositely to the cell current, i.e. electrons are moving to the cathode and ions are accelerated to the anode. When the cell current decreases, the induced electric field causes an autoelectronic emission from the cathode surface. The emitted electrons move along cycloids in the direction to the cell center making the gas desorption and building up the near-surface plasma (aureole) around the cell. Within the framework of this model it is possible to explain many observed experimental facts: appearance of a weak glow around the cathode spot, division of cathode spots, the spot motion in the tangential magnetic field, X-rays, etc.

KEY WORDS: cathode spot, sell, retrograde motion, electrons, ions, vacuum arc, magnetic field, plasma.

RELATION CONTRIBUTION OF DIFFRACTED BREMSSTRAHLUNG AND PARAMETRIC X-RAY RADIATION IN PERFECT CRYSTALS

D.A. Baklanov, A.N. Baldin, I.E. Vnukov, D.A. Nechaenko, R.A. Shatokhin

Full Text :   (630 kB, Rus.) 

Abstract
A simple model for calculating of diffracted bremsstrahlung (DB) from perfect crystals thickness of by far greater than extinction length is suggested and realized. The model was experimentally confirmed in measurements of bremsstrahlung yield in forward direction. It is shown that in the dependence of experimental condition the DB and parametric X-ray radiation (PXR) contributions may be comparable. The DB contribution is essential for Θ≈Θ _D /2 only, where Θ and Θ_D are the plane orientation angle and the detector position one, where the DB and PXR photons energy dependencies are practically the same. Therefore for detector energy resolution Dw >150 eV measured dependence of radiation peak energy from the crystal orientation angle will be coincide with prediction of the PXR theory as was observed experimentally.

KEY WORDS: parametric X-ray radiation, electron, crystal, radiation yield, diffracted bremsstrahlung, diffraction.

ANALYSING COMPOSITION OF THE RADIOACTIVE SPECIES WITH HELP OF EVOLUTIONARY ALGORITHM

V.Yu. Korda, A.S. Molev, L.P. Korda

Full Text :   (2000 kB, Rus.) 

Abstract
The evolutionary algorithm with the additional turn of the probability distribution of the normally distributed multi-dimensional random variable is developed. The proposed algorithm appears to be capable of investigating the spaces of optimization problems with complicated topology. The algorithm is implemented for the problem of determining the composition of the radioactive species. The software created on the ground of the novel evolutionary algorithm can be used in improving the existed and developing the new systems for the diagnostics, control and prediction of state of the radioactive waste.

KEY WORDS: radioactive waste, radioactive decay, evolutionary algorithm.

MODES OF LONGITUDINAL COMBINED DISCHARGE IN LOW PRESSURE NITROGEN

V.A. Lisovskiy, N.D. Kharchenko

Full Text :   (530 kB, Rus.) 

Abstract
The modes of low pressure combined (RF + DC) gas discharge were studied in this work. Longitudinal combined discharge (RF and DC voltages were applied to the same electrodes) can exist in three modes: 1) unself-sustained RF discharge, disturbed with DC electrical field, 2) combined discharge, 3) unself-sustained DC discharge, disturbed with RF electrical field. Conditions of existance of these modes were determined. The parameter range in which combined discharge burning in the first mode can be extinguished by increasing of DC voltage is limited in the low-pressure side with the extinction curve and the curve of the transition of combined discharge from the first mode to the second one. Relation between thicknesses of the "cathode" and "anode" layers was analytically obtained which agrees to the experimental results.

KEY WORDS: RF capacitive gas discharge, DC discharge, longitudinal combined discharge, low pressure, nitrogen.

INFLUENCE OF SKIN-EFFECT ON A PLASMA IMPEDANCE IN INDUCTIVELY COUPLED RF DISCHARGE

D. Rafalskyi, K. Polozhiy

Full Text :   (450 kB, Rus.) 

Abstract
Phenomenological model of a plasma impedance in an inductively coupled rf discharge are presented. The model takes into account a skin-effect in plasma for both cases of low and high density plasmas. General electrical characteristics of an inductively coupled rf discharge are obtained. Comparison to existing models has been done. It is shown that a consideration of the plasma as conductive loop with thickness of skin-depth order reduces a obtained plasma impedance by factor 2. The results can be use for further development of the existing models of the plasma impedance in an inductively coupled rf discharge.

KEY WORDS: rf discharge, icp, skin-depth, plasma impedance, transformer model.

FEATURES OF ELECTRON BEAM FORMATION IN RF GUN WITH PLASMA FERROELECTRIC CATHODE

I.V. Khodak, V.A. Kushnir

Full Text :   (290 kB, Rus.) 

Abstract
Electron beams with a charge over 102 nC in a pulse of ~10^-8 s duration can be produced in RF guns. It is possible upon the condition that an electron emitter has emission current density over 10² A/cm². One of electron sources that are able to provide sufficient emission current density is a plasma ferroelectric cathode. Such cathodes are widely used in electron beam sources with pulsed accelerating voltage. Features of shaping of electron beams with pulse charge ~102 nC in RF guns with such cathodes are bounded with features of plasma interaction with RF field of ~10⁷ V/m strength. The subject of the research in the present paper is the beam parameters obtained through the effect of plasma cathode configuration on its emitting surface formation. The most of the research is carried out experimentally. The estimation and analysis of the experimental results has been carried out using computer simulation. There are also analyzed the results of experimental study of plasma behavior on the surface of the plasma ferroelectric cathode of special design for application in RF gun.

KEY WORDS: RF gun, ferroelectric, plasma cathode, space charge, pulse current, particle dynamics.

PARTICULARITIES OF THE BAND STRUCTURE AND DEPENDECE OF MOLYBDENUM TEMPERATURE RESISTIVITY

T.A. Ignatyeva

Full Text :   (320 kB, Rus.) 

Abstract
The temperature dependences of the resistivity of Мо and Мо-Rе 0,5at % are investigated at low temperatures. For Мо on the background of smoothly varying non-linear dependence of the ¶r(T)/¶T at Т < 10К with a positive growth of the ¶r/¶T there are jumps, which one do not vanish at doping 0,5at % Re. In alloy they are existed on a background of not temperature-independent resistivity, which one is increased comparatively with Мо on two order. The temperature dependence r_i(T) of Mo is well described by a polynomial's of the 4-th extent at 2K< Т < 40K. It is supposed, qualitatively, feature in dependence Мо, both nonlinearity, and jumps it is possible to explain by there are in an electronic spectrum of critical energy of an electron lense., which can reduce to an features in density of electron states and in electron-phonon interplay (phonon entrainment, processes of overshoot for a step). Besides of, the other features in dependence r_i(T) there are exhibited also, bound with features of dynamics of an electron motion of smal groups.

KEY WORDS: resistivity, molybdenum, the Fermi surfaces, electron - phonon interplay, processes of overshoot for a step change.

IMPROVEMENT OF MECHANICAL CHARACTERISTICS OF MAGNESIUM ALLOYS FOR THE MEDICAL PURPOSES

I.I. Papirov, V.S. Shokurov, A.I. Pikalov, V.S. Sivtsov, A.I. Mazin, V.A. Shkuropatenko

Full Text :   (410 kB, Rus.) 

Abstract
Preforms of a ultra fine-grained magnesium alloy with the improved mechanical characteristics (UTS - 298 MPa, elongation up to rupture - 25 %) were received by methods of the intensive programmed deformation. The programmed deformation was carried out under two schemes: alternation of an extrusion and an upsetting at gradually lowering temperature and extrusion through equal-channel angular press-form. The estimation of high-temperature deformation mechanisms of the received ultra fine-grained material was made on the base of sensitivity analysis of flow stress vs. strain-rate (m) in the field of temperatures 280-380°С.

KEY WORDS: magnesium alloy, intensive deformation processing, ultra fine-grained structure.

SELF-FIELD COMPENSATION OF HIGH-CURRENT REB TRANSPORTED BY GRADIENT DRIFT IN AZIMUTHAL MAGNETIC FIELD

V.V. Chornyi, G.V. Tsepilov, A.I. Frolov, V.N. Dubina, V.S. Solovyov, A.V. Chornyi

Full Text :   (200 kB, Eng.) 

Abstract
The research on the basic physical factors influencing the compensation of self-fields of a high-current electron beam transported in azimuthal magnetic field using gradient drift was conducted. The magnetic field was produced by the current carrying conductor placed on the beam axis. The beam self-field compensation is provided by the plasma produced by the electron beam when it is injected into some rarified gas. It was shown, that under conditions of the experiments carried out by the authors, the transversal magnetic field considerably reduced the electron beam compensation degree. On the whole, the obtained results confirm the general concepts of the physical processes occurring in the beam-produced plasma when the beam is injected into some rarified gas. So, when solving some technical tasks associated with effective transport of a high-current relativistic electron beam (REB), the parameters of the electron beam and those of the transport channel are to be matched carefully.

KEY WORDS: high-current relativistic electron beam, beam-self-field compensation, gradient drift, transport, azimuthal magnetic field.

EFFECT OF THE TRANSVERSAL MAGNETIC FIELD ON FLUCTUATION PARACONDUCTIVITY OF YBa₂Cu_3-zAl_zO_7-d SINGLE CRYSTALS ALLOYED BY ALUMINIUM WITH THE GIVEN TOPOLOGY OF TWIN

R.V. Vovk

Full Text :   (520 kB, Ukr.) 

Abstract
In this work, effect of the magnetic field up to 12.7 kE on temperature dependences of conductivity of YBaCuO single crystals alloyed by an aluminium with the system of the unidirectional twin boundaries are investigated. On the basis of the resistivity measured temperature dependences of excess paraconductivity and field dependence off-plane coherent length x_с(0,Н) were determined. Temperature dependences of excess paraconductivity discussed in terms of the Hikami - Larkin theory of the fluctuation conductivity for layered superconductivity systems. The reasons of suppression of three-dimensional superconductivity fluctuations and field dependence off-plane coherent length x_с(0,Н) in the weak magnetic fields during the orientation of vector of the magnetic field along с axis are discussed.

KEY WORDS: fluctuation conductivity, YBa₂Cu₃O_7-d single crystals, twin boundaries, crossover, alloy, coherence length.