Variation Of Field Amplitude Research Articles

Evolution of the electron distribution function during gas ionization by a sub-nanosecond microwave pulse of hundreds MW power

The electron velocity distribution function in the plasma, formed by gas ionization by a microwave pulse of sub-nanosecond timescale width and hundreds of megawatts power, is studied by a theoretical model and by 3D numerical simulations which confirm quite well the model. It is shown that the distribution function is defined by the field amplitude variation during the entire pulse. After the pulse's passage through the gas, the remaining plasma distribution function follows a decreasing power-law function. Experiments performed in a waveguide filled with helium gas confirm that energetic (from several keV to several tens of keV) electrons remain in plasma long after the pulse has crossed the experimental volume. These electrons continue the gas ionization over extended times up to tens of nanoseconds.

ВИПАДКОВІ ГРАВІТАЦІЙНІ ХВИЛІ У ДВОШАРОВІЙ ГІДРОДИНАМІЧНІЙ СИСТЕМІ

The article is devoted to the study of the propagation of random gravitational waves in a three-dimensional hydrodynamic system half-space– half-space. An overview of studies on the analysis of the propagation of random waves in different systems is given. Mathematical statement of the problem contains second-order differential equations with respect to velocity potentials, kinematic and dynamic conditions on the contact surface. To study the problem, the field of deviations and the potentials of the wave velocities are presented in the form of expansions in Fourier-Stiltjes integrals. Stochastic amplitudes of the corresponding fields are expressed through the amplitude of the deviation field in the form of recurrent relations. Using the expansion in series in a small parameter for the stochastic field amplitude variations, the dynamic equation in integral form has been received. It should be noted that the use of a small parameter makes it possible to control the contribution of the nonlinearity of the corresponding terms. Subintegral functions of two- and three-wave interaction are obtained in symmetrized form. Based on the obtained equation, a linear dispersion relationship is derived. In the two-dimensional case, it degenerates into the dispersion relationship obtained by A. Naifehfor deterministic wave motions in a two-layer system. Using the equations for the amplitude of the deviation field and the ensemble averaging procedure, the equation for the spectrum of the first harmonics is obtained. The reliability of the obtained results is confirmed by a comparison with previous studies of the problem of propagation of random surface gravitational waves performed in the works of Masuda and others. The obtained results can be used in the study of the propagation of random internal waves in the oceans.

Relationship Between the Polar Electrojet Dynamics and the Amplitude of ELF/VLF Signal from the Ionospheric Source in the Modulated Ionospheric Heating Experiment

We consider the results of the experiment on the polar ionosphere heating by a high-power modulated HF wave conducted by the AARI at the EISCAT/heating facility in October 2016. The data on joint processing of the values of the equivalent polar electrojet current above the heated ionosphere region retrieved by the IMAGE network of ground-based stations and the amplitudes of a signal of the horizontal component of the magnetic field from an ionospheric source at the modulation frequency recorded in the PGI network of high-latitude stations are presented. The events with a strong positive correlation between the magnetic field amplitude variations and the polar electrojet current variations are considered. It is shown that if the direction of the current changes from eastward to northward, correlation between the current variations and magnetic field variations almost disappears at a modulation frequency of 3017 Hz. Temporal and spatial variations of the coefficient of linear regression of the amplitude of the horizontal magnetic-field component with respect to the electrojet current are analyzed. In the transition from day to night, which is accompanied by the electrojet current decrease, the regression slope increases, and it decreases with increasing distance between the heater and the observation point.

Analysis of Orbital Eccentricity and UMP in Large Salient Pole Synchronous Machines

The problem with unbalanced magnetic pull (UMP), often analyzed for induction motors, is also a huge issue in large hydro generators. This paper focuses on large vertical shaft synchronous machines describing the UMP as a function of eccentricity, irrespective of its origin on the rotor, stator, or both. The static and the dynamic eccentricity impact on the air-gap length is described through an original mathematical model in which the magnetic flux density spatial distribution is amplitude modulated and the UMP is calculated applying Maxwell's tensor. This process allows including the machine saturation when calculating the no-load rated voltage. Hence, with constant rotor excitation current, corresponding to the rated generator voltage at no-load, the magnetic field amplitude variation results in a linear function of the air-gap length variation. Seven case studies are presented comparing traditional methodology with the proposed one. Two of them are also calculated by applying the finite element method (FEM). The proposed methodology gets more realistic answers if FEM results are taken as a reference. Finally, UMP triggered by a dynamic eccentricity is shown, and the resulting force that followed the orbital eccentricity is depicted clearly.

Dielectric properties of self-assembled spherulite films of organic ferroelectric 2-methylbenzimidazole

Thin films of organic ferroelectric 2-methylbenzimidazole (MBI) were grown by evaporation method on different substrates. The films consist of self-assembled spherulite blocks formed by elongated crystallites of MBI. Temperature, frequency, and electric field amplitude variations of in-plane hysteresis dielectric loops were measured using interdigital system of electrоdes and compared with analogous dependences in MBI single crystals. Simulation of remnant polarization Prem dependence on amplitude of electric field Emax in films is carried out in frames of a model considering the dipole moment of block as a sum of dipole moments of crystallites, which form spherulite with account of different orientation of crystallites relative to applied electric field. Deviations from the model are assosiated with complicated domain structure and presence of depolarizing fields.

Experimental study of alternating magnetic field in laminated ferromagnetic core

At engineering simulations of magnetic fields in ferromagnetic cores standard Preisach and Jiles-Atherton models are commonly used. At that a material magnetic properties are considered similar in all points of a core. However, the view of experimental hysteresis loops can differ from calculated even for materials with well-known magnetic properties, e.g. transformer steel. Experimental registration of hysteresis loops in characteristic zones of laminated steel core have revealed substantial discrepancy as in the loops view as in the field strength distribution in comparison with calculation results. Measurements at the frequency and field amplitude variation in order to reveal their influence on the magnetization process are conducted. The results obtained point to possible significant influence of magnetostriction effect on functioning of the ferromagnetic core. This influence can appear at frequencies of the core mechanical resonance and lead to changes in the core inductance and quality factor. The effect is relatively weakly pronounced at laminated steel core, but in some cases needs to be considered, e.g. if big size core operates at network frequency.

Nonlocal properties of ponderomotive force in strongly nonuniform high intensity laser fields

In order to study the particle dynamics in strongly nonuniform and/or localized laser fields, we extend the theory of the ponderomotive force by taking into account the laser field structure up to the higher order with respect to � , the ratio between the particle excursion length and the scale length of the laser field amplitude variation. This extension is realized by keeping the rigorous phase space Lagrangian structure utilizing the noncanonical Lie perturbation up to higher orders. As a result, the equations of motion in a oscillation center coordinate is derived up to the third order of � , which include the second and third spatial derivatives of the laser field amplitude. This denotes that the ponderomotive force depends not only on the local field gradient, but also on the curvature and its derivative. The additional higher-order force originates from the nonlocal particle motion around the oscillation center, which could have an influence especially in the case of tight focusing in the level of laser wavelength.

Variable-period permanent magnet undulators

To change the wavelength of undulator radiation, variation of the undulator magnetic field amplitude is frequently applied. Another option is changing the undulator period. A corresponding scheme is described. Both the period and number of periods can be changed. For a set of undulator sections (as in the x-ray free-electron lasers ), the mechanical motion of the periods allows doing without phase shifters between the undulator sections. Magnetic field calculations for some undulator parameters of interest were performed. Numerous advantages of the new undulators (fixed gap, strong dependence of the undulator radiation wavelength on the period, relatively low field amplitude variation, and variable number of periods) look very attractive. The prospects for this new type of undulators are discussed.

AC susceptibility and intergranular critical current density study in pure and Ag doped (La 1− xY x) 2Ba 2CaCu 5O z superconductors

The temperature and AC field amplitude variations of AC susceptibility have been measured on pure and 5 wt% Ag doped (La 1− x Y x ) 2Ba 2CaCu 5O z superconductors. The AC susceptibility as a function of field have been analyzed using Kim's critical state model. The temperature dependence of intergranular critical current density and the effective volume fractions of the grains have been estimated. The Ag doped samples show relatively large critical current density due to the improved intergranular coupling. The exponent of temperature variation of critical current density suggests that the weak links form superconductor–normal metal–superconductor (SNS) type of junctions for all the samples.

Compressional Pc5 pulsations as sloshing in the plasma sheet

In the morningside plasma sheet close to the equatorial plane, satellites often observe magnetic field amplitude variations on timescales of several minutes (Pc5 range). Here we present an event study which shows that in some cases these variations can be a result of magnetospheric regions with different magnetic field amplitudes sloshing back and forth past the satellite. The sloshing is caused by toroidal Alfvén oscillations.

Dielectric losses behaviour under frequency and a.c. field amplitude variation in TGSe crystals

Dielectric losses in triglycine selenate crystals were studied in the frequency range from 20 Hz to 1 MHz for several a.c. field amplitudes. The peculiar shape of the temperature dependence of the loss factor D = tan δ reflects the character of the phase transition, which is very close to tricritical. In the range from 20 Hz to 10 kHz the temperature behaviour of tanδ is described according to the model presented previously for TGS crystals, adapted to the TGSe, taking into account the specific features of the phase transition. A low and a higher frequency relaxation band in D vs. frequency were found and a dip in D was noticed in the range of a few kHz for several temperatures near and below T C. The relaxation frequency attributed to domain wall motion increased rapidly, reached a maximum around 3 K below T C' and then dropped in relative value toward lower temperatures, as it has been observed in other ferroelectric transitions.

Graphical study of Laguerre-Gaussian beam modes

Following a brief survey of the theory of Laguerre-Gaussian beam modes, expressions are derived for the paraxial vector electric and magnetic fields for wave modes of arbitrary order. The variation of field amplitude in the plane transverse to the axis of propagation is illustrated by three-dimensional graphs, and constant-amplitude contours are obtained. Also, the electric and magnetic fields in longitudinal (xz and yz) planes are plotted for some low-order modes.

Reduction of RF penetration effects in high field imaging.

A method of correcting for the RF inhomogeneity in the body by dielectric loading of the coil-to-shield space in an RF resonator (coil and shield assembly) is described. The presence of the RF coil and RF shield have significant effects on RF homogeneity. Based on theoretical calculations, a method for adjusting the RF homogeneity by manipulating the axial propagation constant, kappa z, is proposed. This is accomplished by loading the coil-to-shield space with dielectric material of suitable relative permittivity so as to increase kappa z and decrease the radial propagation constant, kappa rho. In this manner, the radial wavelength (lambda rho = 2 pi/kappa rho) can be increased relative to the body dimensions, and the field amplitude variations in the axial plane minimized. Theoretical calculations indicate that a value of between 30 and 40 for the relative permittivity of the dielectric material in the coil-to-shield space would reduce the RF field inhomogeneity from +/- 15% to about +/- 3% over a central 30-cm-diameter region of a homogeneous 40-cm-diameter body at both 64 and 170 MHz. The theoretical model was verified in laboratory measurements of the [formula; see text] field generated in a test coil at 170 MHz which was scaled to correspond to a body at 64 MHz. However, the improved RF field homogeneity would be accompanied by increased RF power requirements and reduced coil sensitivity.

Graphical depiction of the electromagnetic fields of hermite—gaussian modes

The theory of Hermite-Gaussian beam waveguide modes is briefly reviewed, leading to expressions for the paraxial vector electric and magnetic fields for wave modes of arbitrary order. The variation of field amplitude in the transverse xy-plane is depicted by three-dimensional graphs, and constant-amplitude contour plots are derived. Also, the electric and magnetic field lines in longitudinal xz- and yz-planes are plotted for transverse modal orders ranging from (0,0) to (2,2).

Low frequency emission and magnetic field amplitude variation at field-aligned current region

The study of the results of the Intercosmos-18 experiment has revealed simultaneous fluctuations in the LF noise emission amplitude and in the intensity of one the magnetic field components in the region of invariant latitudes where field-aligned currents are usually observed. The observed zone of fluctuations is shown to move spatially as a geomagnetic storm develops. The correlation between the magnetic field fluctuation amplitude and the amplitude of LF noise in the field-aligned current region suggests a common source of the fluctuations. Presumably, they are related to the filament structure of the plasma. The occurrence of simultaneous small-scale variations of the emissions and of the magnetic field components is experimental evidence for LF magnetic emission generation by field-aligned current streams.