Search

Purpose: Experimental results and numerical simulations are presented, concerning effects of microwave generation in coaxial transmission lines which are fed with unipolar, high voltage electric pulses. The work is aimed at clarifying the relative importance of several mechanisms that could be responsible for the appearance of microwave-frequency oscillations in the course of pulse propagation through the guiding structure. Design/methodology/approach: Dispersive and filtering properties of coaxial waveguides that involve three structural sections are discussed. These latter follow one another along the axis of symmetry. Two identical sections at the input and output are filled with an isotropic liquid dielectric, while the middle part may, in addition, be either partially or fully filled with a non-conductive gyrotropic material. The inserted core represents a set of ferrite rings showing a nonlinear response to the initial high voltage, pulsed excitation. Throughout the series of measurements, the diameters of the inner conductor and of the ferrite core were kept constant. The outer conductor’s diameter was varied to permit analysis of the effect of that size proper and of the degree to which the cross-section is fi lled with ferrite. The gyrotropic properties of the ferrimagnetic material were realized through application of a magnetic bias field from an external coil. The measurements were made for a variety of pulsed voltage magnitudes from the range of hundreds of kilovolts, and magnetic bias fields of tens kiloamperes per meter. Findings: As observed in our experiments, as well as in papers by other writers, a unipolar pulse coming from the radially uniform front-end section, further on gives rise to quasi-monochromatic voltage oscillations. These appear as soon as the pulse has advanced a sufficient distance into the radially nonuniform portion of the guide. The oscillations may consist of a small number of quasi-periods, which suggests a large spectral line width. However, by properly selecting geometric parameters of the wave guiding line and the characteristics of the initial pulsed waveform it proves possible to obtain output frequencies of about units of gigahertz and pulse powers at subgigawatt levels. Conclusions: The frequencies and amplitudes of the appearing oscillations, as well as their spectral widths, are governed by the complex of dispersive and non-linear properties of the guiding structure. The diameters of the inner and outer coaxial conductors in the line, diameter of the ferrimagnetic insert and its intrinsic linear dispersion determine the set of waveguide modes capable of propagating through the line. An oscillating part of the waveform may appear and get separated from the main body of the pulse if it has originated at a higher frequency than the cut-off value for a different mode than the initial TEM. Key words: unipolar pulse, coaxial transmission line, microwave frequency oscillations, dispersion laws, waveguide modes

Experimental data and results of numerical modeling are presented, concerning excitation of microwave oscillations by a wave of pulsed ‘dc’ current (eventually, a shock wave) traveling through a radially non-uniform coaxial guiding structure. Similar experiments with ‘standard’ structures that involve a nonlinear dielectric insert (ferrite) in the coax and another dielectric, characterized by a smaller dielectric constant, result in appearance of a short radiofrequency pulse, in the form of decaying sinusoidal voltage at the line’s output. The decay is shown to be associated with a lack of velocity synchronism between the principal ‘quasi-TEM’ wave mode in the system and the slow Emode excited by the electromagnetic shock. Numerical experiments within 3-D models have demonstrated possibilities for obtaining radio pulses of various lengths, involving oscillations of a stable frequency and nearly constant amplitude – provided that Cherenkov-type synchronism were satisfied, owing to slowing down of the faster ‘quasiTEM’ mode. To cut its speed down two methods can be suggested, (i) using a dielectric material with a high value of the dielectric permittivity, and (ii) introducing a periodic slow-wave structure whose period would be smaller than the wavelength of the oscillations considered.

The paper presents methodologies and results concerning one- and two-dimensional numerical modeling of radio frequency oscillations in a coaxial transmission line fed with a short pulse of electric current. The line is partially filled with a ferrite material, magnetized longitudinally close to saturation. The 2D model has permitted analyzing, for the first time in the art, the spatial structure and dynamics of the wave field within the radially non-uniform cross-section planes of the non-linear and dispersive guiding structure. This opens ways for optimizing size parameters of the line and the extent to which it is filled with the ferromagnetic material, thus increasing the line’s electric strength and intensity of the r.f. oscillations.

Experimental results and data of numerical simulations are presented, concerning generation of wideband radio frequency (RF) oscillations in a nonlinear transmission line (NLTL) which contains a pre-magnetized core of ferrite material. Emphasis is made on the means for extracting the RF signal from the line, in order to radiate it into free space. Antennas of two types that can be used for the purpose are considered, both featuring a coaxial design. This permits availing of the principal advantages of coaxial systems, such as operation in the mode of a traveling TEM wave; wide range of the transmitted frequencies, and a reasonably simple design. The antennas studied, specifically a disc-cone dipole (DCD) and an impulse reflector antenna (‘Half-IRA’, or HIRA type) differ significantly in effective width of the radiated spectrum and in spatial characteristics of the radiated field in far region.

Electric current pulses of nanosecond length and amplitude up to 300 kV traveling through a transmission line partially filled with a magnetically saturated ferrite demonstrate excitation of quasi-monochromatic oscillations at 0.7–2.5 GHz. The amplitude efficiency of this DC-to-RF conversion can reach 50%. The two-dimensional model used for numerical analysis of the non-linear electromagnetic effects in the structure shows good agreement of the calculated and measured results.

The operation of millimeter-wave relativistic magnetrons is considered for devices of the traditional and the inverted design. The high-voltage, pulsed devices employ explosive emission of electrons in electric fields about (1÷5)·107 V/m near the emitting surface. The electric parameters like these allow magnetron generation with either polarity of the DC driving pulse, both in the traditional and inverted configuration, thus enabling four combinations of geometry and voltage polarity. The operation modes where the source of electrons is near the slow-wave structure have been studied for the first time.

The dynamics of charged particle beams emitted from a cathode into a ‘smooth’ coaxial diode with magnetic insulation (MICD) is studied with the aid of 3-D simulation. The processes controlling space charge formation and its evolution in the MICD are modeled for the structure geometries typical of high-voltage millimeter wave magnetrons1 that are characterized by very high values of emission currents, hence high space charge densities. The beam particles that interact both with the space charge which they belong to, and with external dc. electric and magnetic fields give rise to an azimuthal electron flow in the anode-cathode gap and to axially oriented flows outside that. In experiments with millimeter wavelength relativistic magnetrons1 the axial currents were identified as a major factor to negatively affect magnetron efficiency.

The ultra-low-frequency (ULF) geomagnetic pulsations observed at two nearly conjugate mid-latitude sites are examined to study their spatial structure and polarization, and learn about the role of ionospheric conductivity in forming their ground signatures. The data of 1999–2002 from Antarctica and New England ( L of 2.4) are compared with the numerical results obtained in a simple plane model of ULF wave propagation through the ionosphere and atmosphere. The multi-layered model environment includes an anisotropic and parametrically time-dependent ionosphere, a uniform magnetosphere and a conducting Earth, all placed in a tilted geomagnetic field. The measured diurnal and seasonal variations in the orientation angle of the polarization ellipse are interpreted as effects of hydromagnetic wave propagation through the ionosphere and conversion to an electromagnetic field below. Essentially, the phase, amplitude and polarization of ULF waves observed at the ground are controlled by the wave's spatial structure in the magnetosphere and ionospheric transverse conductivities. The differences shown by the characteristics of simultaneous pulsations in conjugate areas arise mainly from different local ionospheric conditions, while the source waves of the pulsations are common to both sites.

Results of Doppler radar observations are discussed of HF scattering from heater induced inhomogeneities in an underdense ionospheric plasma. The quasiperiodic variations shown by the Doppler frequency shift are associated with the E×H plasma drift in the electric field of geomagnetic pulsations. Combined with a theoretical model of hydromagnetic wave propagation, with allowance for an arbitrary dip angle of the geomagnetic field and arbitrary angle of incidence, the Doppler shift measurements at several points along the geofield tube permit estimating height-integrated ionospheric conductivities

The paper describes several versions of the ‘half-reflector’ impulse radiating antenna, intended for operation with high voltage sources, such as to provide electric field intensities about 70 kV/m at distances r > 20 m away from the aperture. The TEM horn feed of the antenna consists of cylindrical conductors (metal pipes), which geometry ensures a better stability against electric breakdown. The design of the feed arms and the TEM horn as a whole is optimized, via numerical simulation of the system's performance, for higher values of the radiated E-field, lower level of reflections throughout the feed line, and safe operation at very high voltages.

An attempt has been made to apply fractal dimension analysis to HF radio signals scattered from natural and artificially stimulated ionospheric turbulence. Envelopes of the scattered signals in all cases showed a finite fractal dimension as estimated by the correlation integral technique. The signals scattered from artificial turbulence are characterized by a higher correlation dimension, while the statistical characteristics of the signal envelopes of both kind are practically identical. >

The scattering of radiowaves in the outer solar corona is discussed. Results are given of the decametric wave observations. In the theoretical analysis both regular refraction and the gradients in the electron density fluctuations are considered. The theory is in satisfactory agreement with the experimental data. From their comparison the ratio n=(l r /l t ) is deduced of the correlation scales in the radial (l r )and transverse directions. This value is not equal to the ratio of the observed distribution dimensions. The frequency dependence of the angular spectrum rms width is not λ2 at longer wavelengths. At small separations from the Sun, however, the rms angular size cannot serve as the only characteristic of the spectrum, the latter being non-Gaussian.