Abstract
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
Highlights
The effects of direct conversion of electric pulses of short duration into radio frequency oscillations, observed in transmission lines with nonlinear properties, have been studied quite intensely for more than a decade [1,2,3,4, 7, 8]
A critically essential feature of this technology is the employment of such transmission lines whose topology allows propagation of unipolar voltage pulses
The wave guiding structure used in our experiments involved two coaxial lines with linear response functions, and the line NL containing a ferrite core, placed in between
Summary
The effects of direct conversion of electric pulses of short duration into radio frequency oscillations, observed in transmission lines with nonlinear properties, have been studied quite intensely for more than a decade [1,2,3,4, 7, 8]. A critically essential feature of this technology is the employment of such transmission lines whose topology allows propagation of unipolar (roughly speaking, d.c.) voltage pulses. Falling into this category are planar strip lines and coaxial cables (along with lumped parameter, discrete cell circuits that will not be discussed here) [3, 6]. This paper is yet another attempt at clarifying the physics that underlies appearance of quasi-periodic field strength variations in the course of propagation of unipolar pulses through a transmission line of nonuniform cross-section, showing a nonlinear response to an external excitation
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