Short Electromagnetic Pulses Research Articles

UWB time‐domain antennas beam scanning

AbstractUltra‐wideband (UWB) antenna is indispensable in impulse radar that transmits and receives short electromagnetic pulse signals and preserves the waveform well. Phased array antenna systems cannot meet the demand of the system. Optical true‐time delay techniques open the possibility of ultra‐wide bandwidth antenna systems, while at the same time meeting the stringent weight and size requirements. Continuously tunable true‐time delay techniques can provide continuous beam scanning. With advanced optoelectronic technique, fast wavelength tuning speed is possible. To detect and track moving‐target [Bi and Ren, Prog Electomagn Res 87 (2008), 15–41)], UWB time‐domain antenna array beam scanning is studied in this article. First, the theory of time‐domain antenna array scanning is analyzed, some useful conclusions are achieved. Then, an experiment system of UWB time‐domain antenna array scanning is built, which includes an antenna array, fiber‐optic true‐time delay device, and other equipments. Finally, the theoretical and experimental results combined to are made to validate the theory of UWB time‐domain antenna array beam scanning. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 1110–1112, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25102

Detection of laser induced dielectric breakdown in water using a laser doppler vibrometer

Abstract This study is focused on exploring the feasibility of an all-optic surface scanning method in determining the size and position of a submerged, laser generated, optoacoustic (OA) source. The optoacoustic effect in this case was generated when the absorption of a short electromagnetic pulse in matter caused a dielectric breakdown, a plasma emission flash and a subsequent acoustic wave. In the experiment, a laser pulse with λ = 1064 nm and 12 ns pulse length was aimed at a volume of deionized water. When the laser beam was focused by a f = 16 mm lens, a single dielectric breakdown spot occurred. When a f = 40 mm was used several breakdowns in a row were induced. The breakdowns were photographed using a double shutter camera. The acoustic wave generated by the dielectric breakdowns were detected at a point on the water surface using a laser Doppler vibrometer (LDV). First, the LDV signal was used to calculate the speed of sound with an accuracy of 10 m/s. Secondly, the location and length of the dielectric breakdown was calculated with an accuracy of 1 mm. The calculated position matched the breakdown location recorded by a camera. The results show that it is possible to use LDV surface measurements from a single spot to determine both the position and length of the OA source as well as the speed of sound in the medium. Furthermore, the LDV measurements also show a secondary peak that originates from the OA source. To unravel the origin and properties of this interesting feature, further investigations are necessary

Propagation of short pulses in irregular electromagnetic waveguides with a variable topology of the cross section

A method for real-time electrodynamic calculation of the dynamics of propagation and transformation of short ultrawideband electromagnetic pulses (UWB EMP) in irregular waveguides with discontinuous junctions is presented. The results of numerical simulation of a pulse passing through a junction of a coaxial waveguide having doubly connected cross sections and a waveguide horn with singly connected cross sections and through a discontinuity of the inner conductor in an irregular coaxial waveguide are presented.

Ground Penetrating Radar (GPR): an application for evaluating the state of maintenance of the building coating

Ground Penetrating Radar (GPR) is a non-destructive methodology. For the localisation of buried structures, it uses short time duration electromagnetic (EM) pulses lasting from about 1 ns to about 30 ns. Therefore, GPR is characterised by a wide frequency band ranging from 10 MHz to some GHz, and is useful in the localisation of EM discontinuities in the subsurface with high resolution. This paper describes an application of GPR to evaluate the state of maintenance of some travertine panels attached to the inside walls of a building housing the Bank of Naples in Campobasso and in danger of falling because of the numerous voids present between the wall and the panels. The aim of the survey was to assess whether the GPR technique could be used to detect the voids behind the travertine panels. The study was made to assist the design of the restoration works of the travertine covering. Because of the very narrow thickness of both the travertine plates and the voids, special care was needed in the acquisition and processing steps. The measurements were performed on two panels: one purposely put on to the laboratory wall with a known position of the voids; the other one selected from among the panels to be restored. Although pushed to the limit of the resolution achievable by the available antenna, the study has given quite good results.

The time-dependent Dirac equation for atoms exposed to short intense electromagnetic pulses

The time dependent Dirac equation for hydrogen-like systems exposed to intense laser pulses, is solved numerically and compared to the corresponding prediction from the non-relativistic Schrödinger equation. It is found that virtual electron-positron pairs are instrumental for a correct description of effects beyond the dipole approximation. Relativistic effects in the ionization dynamics of highly charged systems are studied

Accounting for relaxation processes during the pulse in surface NMR data

Under favorable conditions, the surface nuclear-magnetic-resonance (NMR) technique can provide direct quantitative estimates of subsurface water variations and indirect information on pore size and hydraulic conductivity. The technique is based on various exponential relaxation processes that become measurable after the intrinsic spin magnetic moments of groundwater protons have been rotated out of equilibrium by a pulse of alternating electromagnetic (EM) field generated at the surface. An implicit assumption in previous surface NMR studies is that relaxation processes need to be considered only after the EM pulse has been extinguished. Although this approximation is valid for short EM pulses, neglecting relaxation during the pulse (RDP) can result in significant errors for the generally long pulses used in surface NMR investigations. Because the influence of RDP cannot be isolated and quantified using field-scale approaches, this study is based on sample-scale NMR experiments and numerical simulations (Bloch equations) that mimic field practices and conditions. The results demonstrate that standard surface NMR methods that ignore the effects of RDP may yield significantly erroneous estimates of water volume (RDP-related errors of 25% are possible) and the key relaxation parameter (RDP-related errors of 50% are possible) that supplies information on pore size and hydraulic conductivity. Fortunately, the study also demonstrates that relatively simple interpretational approaches can reduce the RDP-related errors to less than 2%.

Pulse Propagation in Gigahertz Transverse Electromagnetic Cells

This paper deals with short electromagnetic pulse propagation in a large gigahertz transverse electromagnetic (GTEM) cell. The aim of the investigation is to analyze to what extent a voltage pulse with subnanosecond risetime, applied to the coaxial feeding port, is distorted when transformed into a field pulse in the testing volume. The analysis focuses on how the expected excitation of higher order modes results in a pulse distortion within GTEM cells. Therefore, an entire 3-D simulation model of the cell has been created and measurements have been conducted. Considering the GTEM cell as a linear time-invariant system, it is possible to calculate the impulse response in the time domain of each electromagnetic field component in the distinct observation points in the testing volume. The obtained simulation and measurement results are used to verify the proposed characterization of the GTEM cell. This method leads to a prediction of the pulse propagation of arbitrary transient excitation signals concerning both the pulse shape and the derived field magnitude.

Excitation of multiple wakefields by short laser pulses in quantum plasmas

We present a theoretical investigation of the excitation of multiple electrostatic wakefields by the ponderomotive force of a short electromagnetic pulse propagating through a dense plasma. It is found that the inclusion of the quantum statistical pressure and quantum electron tunneling effects can qualitatively change the classical behavior of the wakefield. In addition to the well-known plasma oscillation wakefield, with a wavelength of the order of the electron skin depth ( λ e = c / ω p e , which in a dense plasma is of the order of several nanometers, where c is the speed of light in vacuum and ω p e is the electron plasma frequency), wakefields in dense plasmas with a shorter wavelength (in comparison with λ e ) are also excited. The wakefields can trap electrons and accelerate them to extremely high energies over nanoscales.

Time reversal in thermoacoustic tomography—an error estimate

In thermoacoustic tomography an object is irradiated by a short electromagnetic pulse and the absorbed energy causes a thermoelastic expansion. This expansion leads to a pressure wave propagating through the object. The goal of thermoacoustic tomography is the recovery of the initial pressure inside the object from measurements of the pressure wave made on a surface surrounding the object. The time reversal method can be used for approximating the initial pressure when the sound speed inside the object is variable (non-trapping as well as trapping). This paper presents error estimates for the time reversal method in the cases of variable, non-trapping sound speeds. Numerical examples for non-trapping as well as for trapping sound speeds are provided.

Waveform Parameter Estimation and Dispersive Material Characterization

The re∞ection of short duration electromagnetic pulses from dielectric media is of interest in diverse technological applications, e.g., geophysics, material science and biomedical engineering. In this paper, the time domain pulse re∞ection from a dispersive lossy dielectric half space is investigated. The properties of a half space are described in frequency domain by the Debye and Cole-Cole models, respectively. The two models are commonly used to capture the relaxation-based dispersive properties. First, transient re∞ected pulses are analyzed and waveform parameters are estimated. Then, based on the estimation, the relationships between the waveform parameters of re∞ected pulses and the properties of dispersive material as well as incident angles are discussed. Meanwhile, the results obtained with the Debye model are compared to those obtained with the Cole-Cole model. The application of these results to material characterization and diagnosis is explored. There is excellent agreement between our results and those in the literature, which validates the correctness and efiectiveness of this work. Our technique is based on the numerical inversion of the Laplace transform, leads to good accuracy, and has a simple algorithm, short calculation time, small required memory size, readily controlled error and wide range of applicability. The knowledge of material properties is required in various technological flelds, such as geophysics, material science and biomedical engineering. The characterization of bulk materials would be the most direct way to acquire this knowledge and greatly helpful to understand the underlying physics at the microscopic level, which is much more complicated in comparison with the existing formula- tions of the bulk efiects. A typical approach to bulk material characterization is to examine re∞ected electromagnetic pulses from the interface between free space and the investigated material. Many kinds of materials show the relaxation-based dispersive properties that are commonly captured by the Debye (1) and Cole-Cole (2) models. The transient analysis of pulses re∞ected from a dispersive interface can be conducted in frequency domain flrst. Then the results in time domain may be ob- tained by carrying out a numerical Fourier transform of the frequency domain response. However, this is very time consuming since a wide frequency range needs to be considered. Furthermore, it is preferable to solve the problems directly in time domain under certain circumstances. The flnite difierence time domain (FDTD) technique can be applied to this problem, but computation costs can be high under some conditions. Rothwell (3) worked out the time domain re∞ection coe-cients of a Debye half space for both horizontal and vertical polarizations that involve exponential and modifled Bessel functions and require convolution operations to evaluate. To our knowledge, the time domain re∞ection coe-cient of a Cole-Cole half space for any polarization has been not avail- able so far. It is the purpose of this paper to develop a new technique for transient analysis of pulse re∞ection from Debye and Cole-Cole media, and apply this technique to waveform parameter estimation and material characterization. This technique is based on the numerical inversion of the Laplace transform and has several signiflcant advantages.

Amplification of ultrashort electromagnetic pulses propagating along quasi-continuous electron beams

Specific features of amplification of short electromagnetic pulses propagating along steady-state nonequilibrium electron flows with a group velocity differing from the translational velocity of particles are analyzed. It is shown that an amplitude level substantially higher than the saturation level in amplification of quasi-continuous signals can be attained by permanent injection of electrons without initial modulation to one of the pulse fronts. The effective duration of the pulse being amplified is reduced simultaneously. The Cherenkov and undulator interaction mechanisms are considered. Analysis is carried out using a simple 1D model based on the averaged description of the electron-wave interaction, as well as direct numerical simulation based on the KARAT code taking into account the parameters of planned experiments on observation of this effect.

Nonlinear electrodynamics in layered superconductors

We analyze theoretically the effect of a weak nonlinearity on the propagation of Josephson plasma waves in layered superconductors. The nonlinearity originates from the Josephson relation between the current density across superconducting layers and gauge-invariant phase difference of the order parameter. We show that strong nonlinear effects can be observed for electromagnetic waves with frequency slightly above or slightly below the plasma frequency. We study the nonlinear plasma resonance accompanied by the hysteretic dependence of the wave amplitude on the frequency. This hysteresis transforms the continuous terahertz radiation into a series of short electromagnetic high-amplitude pulses. We also consider the propagation of a nonlinear terahertz beam localized in the direction across the superconducting layers. This phenomenon is an analog of the self-focusing effect in nonlinear optics. The nonlinear phenomena in layered superconductors considered here can be potentially useful for the design of a new generation of terahertz devices.

Nonstationary 2D models of the electron-wave interaction

A 2D nonstationary model of induced emission of ribbon-shaped electron beams and clusters in free space is developed in the quasi-optical approximation. On the basis of this model, the problem of enhancement of a short electromagnetic pulse propagating along a quasi-stationary ribbon electron flow, the theory of a BWT-type oscillator with radiation channeling by an electron beam, and the process of collective acceleration of a short electron cluster in the field of an intense cocurrent wave are considered.

Nanosecond electromagnetic sounding of a fluid-saturated layered formation

Mathematical modeling was applied to study attenuation of a nanosecond pulse that propagated in an oil-bearing formation and was reflected from the oil-gas or oil-water interfaces. The problem was formulated and solved for propagation of a short electromagnetic Gaussian pulse in a layered hydrocarbon reservoir in the case of excitation by a long electric line. Complex permittivity of oil-bearing rocks was calculated using a refractive mixing dielectric model for oil, saline water, methane, quartz, and bentonite in each layer. We obtained and analyzed space-time diagrams of wave propagation and reflection and estimated effective attenuation for both cases.

Time-domain reconstruction using sensitivity coefficients for limited view ultrawide band tomography

This article addresses time-domain ultrawide band (UWB) electromagnetic tomography for reconstructing the unknown spatial characteristic of an object from observations of the arrivals of short electromagnetic (EM) pulses. Here, the determination of the first peak arrival of the EM traces constitutes the forward problem, and the inverse problem aims to reconstruct the EM property distribution of the media. In this article, the finite-difference time-domain method implementing a perfectly matched layer is used to solve the forward problem from which the system sensitivity maps are determined. Image reconstruction is based on the combination of a linearized update and regularized Landweber minimization algorithm. Experimental data from a laboratory UWB system using targets of different contrasts, sizes, and shapes in an aqueous media are presented. The results show that this technique can accurately detect and locate unknown targets in spite of the presence of significant levels of noise in the data.

Propagation of a videopulse through a thin layer of two-level dipolar atoms

The excitation of a thin layer of two-level permanent dipole moment atoms by ultimately short (less than the field oscillation period) electromagnetic pulses (videopulse) is observed. The numerical analysis of the matter equations free of the rotating wave approximation and relaxation reveals a strong influence of the local field and the Stark effect on temporal behaviour of transmitted field. Specifically, it is demonstrated that a dense film irradiated by a videopulse emits a short response with a delay much longer even than the characteristic cooperative time of the atom ensemble. It is supposed that the local field in the thin layer of permanent dipole atoms is able to re-pump the atomic subsystem. A close analogy with nonlinear pendulum motion is discussed.

Dynamics of light propagation in spatiotemporal dielectric structures

Propagation, transmission and reflection properties of linearly polarized plane waves and arbitrarily short electromagnetic pulses in one-dimensional dispersionless dielectric media possessing an arbitrary space-time dependence of the refractive index are studied by using a two-component, highly symmetric version of Maxwell's equations. The use of any slow varying amplitude approximation is avoided. Transfer matrices of sharp nonstationary interfaces are calculated explicitly, together with the amplitudes of all secondary waves produced in the scattering. Time-varying multilayer structures and spatiotemporal lenses in various configurations are investigated analytically and numerically in a unified approach. Several effects are reported, such as pulse compression, broadening and spectral manipulation of pulses by a spatiotemporal lens, and the closure of the forbidden frequency gaps with the subsequent opening of wave number band gaps in a generalized Bragg reflector.

A high-power periodic nanosecond pulse source of coherent 8-cm electromagnetic radiation

The generation of short electromagnetic pulses excited in an extended slow-wave system (SWS) of a relativistic backward wave tube (BWT) operating in the so-called superradiance regime with a carrier frequency of 3.7 GHz has been simulated and experimentally studied. At a decreased magnetic field (about 0.2 T) in the SWS, the BWT generated 2.5-ns microwave pulses with a power of up to 800 MW. At a pulse repetition rate of 100 Hz, the working life of the system was limited by the lifetime of an explosive emission cathode (106 pulses). The possibility of phase synchronization of the high-frequency field of the relativistic microwave oscillator with respect to the voltage pulse front is demonstrated for the first time.

Revivals, collapses, and magnetic-pulse generation in quantum rings

Using a microscopic theory based on the density-matrix formalism we investigate quantum revivals and collapses of the charge polarization and charge current dynamics in mesoscopic rings driven by short asymmetric electromagnetic pulses. The collapsed state is utilized for subpicosecond switching of the current and associated magnetization, enabling thus the generation of pulsed magnetic fields with a tunable time structure and shape asymmetry which provides an additional tool to study ultrafast spin dynamics and ratchet-based effects.

New class of extremely short electromagnetic solitons

A physical realization is presented for the integrable modified sine-Gordon equation. In this case, this equation describes the propagation of an extremely short vector electromagnetic pulse in the system of asymmetric quantum objects that have permanent dipole moments in the self-energy states. Using soliton solutions for ordinary and extraordinary components, new regimes of the dynamics of the pulse and medium that are specific only to asymmetric media have been found.