Electromagnetic Waveforms Research Articles

Measuring Intramolecular Charge Transfer via Coherent Generation of THz Radiation

We report the direct measurement of intramolecular electron transfer by detecting the electromagnetic (EM) waveform that is emitted during this process. It is detected in the time domain via free-space electro-optic sampling and then related to the dynamics of the charge-transfer event. Electromagnetic pulse generation from two systems, Betaine-30 in chloroform and DMANS in toluene, are studied to illustrate this technique. A finite-difference time-domain calculation with a time-dependent polarization is used to model the EM generation and propagation through the solution. This method is very general, since the movement of charge itself generates the EM waveform, and is sensitive to charge transfer occurring on a 0.1-10 ps time scale.

Measurement of electromagnetic fields near a monopole antenna excited by a pulse

To observe transient radiation fields, a technique for reconstructing electromagnetic (EM) waveforms using the complex antenna factor (CAF) have been developed. However, the CAF is originally defined assuming plane wave incidence, while the waveforms are measured in a vicinity of the radiating source. In order to examine the reconstruction technique using the CAF in the near-field measurements, the EM fields radiated from a monopole antenna excited by pulsed input voltage was reconstructed and compared with calculated results. For the geometry of the experiment, the waveforms of the reconstructed and calculated EM fields have shown good agreement. Therefore, the reconstruction technique using the CAF can be used for similar dimensions or at a greater distance when sufficient sensitivity is provided.

Escape of electron Langmuir waves from a magnetized plasma

A study is made of electromagnetic oscillations of a plasma in open field line geometry (open magnetic devices). The oscillations that propagate from the critical surface and are originally of the nature of the electron Langmuir waves are shown to continuously change their nature and to escape from the plasma into vacuum in the form of electromagnetic waves. This phenomenon may give rise to wave energy losses from a thermodynamically nonequilibrium (unstable) plasma, e.g., a plasma penetrated by charged particle beams.

Slow electromagnetic solitons in electron-ion plasmas

A set of nonlinear differential equations that describe moving relativistic solitons is investigated analytically and solved numerically. The influence of the ion motion on the soliton structure is investigated. It is demonstrated that, depending on the propagation velocity, relativistic solitary waves can occur in the form of bright solitons, dark solitons, or collisionless electromagnetic shock waves. In the limit of a low propagation velocity, a dark soliton can trap the ions and accelerate them. In the case of a bright soliton, the effects of ion dynamics limit the soliton amplitude. The constraint on the maximum amplitude is related to either the breaking of ion motion or the intersection of electron trajectories. The soliton breaking provides a new mechanism for ion and electron acceleration in the interaction of high-intensity laser pulses with plasmas.

3-D imaging of large scale buried structure by 1-D inversion of very early time electromagnetic (VETEM) data

A simple and efficient method for large scale three-dimensional (3D) subsurface imaging of inhomogeneous background is presented. One-dimensional (1D) multifrequency distorted Born iterative method (DBIM) is employed in the inversion. Simulation results utilizing synthetic scattering data are given. Calibration of the very early time electromagnetic (VETEM) experimental waveforms is detailed along with major problems encountered in practice and their solutions. This discussion is followed by the results of a large scale application of the method to the experimental data provided by the VETEM system of the U.S. Geological Survey. The method is shown to have a computational complexity that is promising for on-site inversion.

Generation of pulsar radio emission

AbstractThe generation of radio emission from plasma waves excited by two-stream instability in pulsar magnetospheres is considered. Induced scattering transforms the excited longitudinal waves into waves that escape freely in the form of transverse electromagnetic waves. It is shown that the spectrum and the luminosity of the generated radio emission are compatible with those observed.

Transition radiation of a charge in media with a nonuniform potential

The influence of a potential barrier on the transition radiation in the form of volume and surface electromagnetic waves emitted by a charged particle crossing an interface between media is investigated. It is shown that the volume-wave radiation field arises not only as a result of the jump in the dielectric constant at the boundary but also as a result of the velocity jump and the reflection of an electron induced by the presence of a nonuniform potential barrier. The angular distribution of the transition radiation intensity is obtained.

Standing Waves: Transmission Line Failure Caused by Reflected Energy

The creation of standing waves in systems vulnerable to harmonics is of interest in both mechanical and electrical contexts. There are cases of harmonics in electrical oscillating systems as well as mechanical ones. Common characteristics of these wave phenomena are transport energy, reflection, and resonance, to list a few. Failures of systems that transport this wave energy, either in the form of sound (longitudinal) waves or electromagnetic (transverse) waves have significant similarities. This paper examines the failure of an energy transport system for electromagnetic waves, a television transmission system, and interprets the investigation of the failure in terms common to both mechanical and electrical systems. Investigating failures that sometimes have an obscure cause-effect relationship, especially if they are rare occurrences, often leads to enhanced design parameters that will prevent future occurrences. The paper concludes that if the cause of the failure cannot be significantly reduced, it can be minimized by closer attention to methods that diminish the effects.

GEM‐3: A Monostatic Broadband Electromagnetic Induction Sensor

We have designed, fabricated, and field‐tested a new, unique, monostatic, broadband, electromagnetic sensor for subsurface geophysical investigation. The sensing unit consists of a pair of concentric, circular coils that transmit a continuous, broadband, digitally‐controlled, electromagnetic waveform. The two transmitter coils, with precisely computed dimensions and placement, create a zone of magnetic cavity (viz., an area with a vanishing primary magnetic flux) at the center of the two coils. A third receiving coil is placed within this magnetic cavity so that it senses only the weak, secondary field returned from the earth and buried targets. This monostatic configuration has many advantages including (1) compact sensor head, (2) a large transmitter moment, (3) high spatial resolution, (4) no spatial distortion of an anomaly common to bistatic sensors, (5) circular symmetry that greatly simplifies mathematical description, and, therefore, (6) simplified forward and inverse modeling processes. Three prototype GEM‐3 units have been built and tested at various environmental sites, including those containing unexploded ordnance and land mines.

Electromagnetic field radiation model for lightning strokes to tall structures

This paper describes the observation and analysis of electromagnetic field radiation from lightning strokes to tall structures. Electromagnetic field waveforms and current waveforms of lightning strokes to the CN Tower have been simultaneously measured since 1991. A new calculation model of electromagnetic field radiation is proposed. The proposed model consists of the lightning current propagation and distribution model and the electromagnetic field radiation model. Electromagnetic fields calculated by the proposed model, based on the observed lightning current at the CN Tower, agree well with the observed fields at 2 km north of the tower.

Microwave radiometry and applications

The radiometry in general is a method of detecting the radiation of matter. All material bodies and substances radiate energy in the form of electromagnetic waves according to Planck s Law. The frequency spectrum of such thermal radiation is determined, beyond the properties of a blackbody, by the emissivity of surfaces and by the temperature of a particular body. Also, its reflectivity and dispersion take part. Investigating the intensity of radiation and its spectral distribution, one may determine the temperature and characterize the radiating body as well as the ambient medium, all independently of distance. With the above possibilities, the radiometry represents a base of scientific method called remote sensing. Utilizing various models, temperature of distant bodies and images of observed scenes can be determined from the spatial distribution of radiation. In this method, two parameters are of paramount importance: An instrument usable to conduct radiometric observations thus consists of two basic elements: a detector or radiometer, which determines the temperature resolution, and an antenna which determines the angular or spatial resolution. For example, a photographic camera consists of an objective lens (antenna) and of a sensitive element (a film or a CCD). In remote sensing, different lenses and reflectors and different sensors are employed, both adjusted to a particular spectrum region in which certain important features of observed bodies and scenes are present: frequently, UV and IR bands are used. The microwave radiometry utilizes various types of antennas and detectors and provides some advantages in observing various scenes: the temperature resolution is recently being given in milikelvins, while the range extends from zero to millions of Kelvins. Microwaves also offer a chance to penetrate surfaces of non-metallic objects down to some wavelengths, by which it is advantageous in certain applications over e.g. IR waves. An extreme example of capabilities of the microwave radiometry is found in radio astronomy, where it determines temperatures and spectral features of bodies so remote that their distance from us is measured in millions of light years. Other apparatus serve in remote observation of Earth s resources: soils, water regions and atmosphere. Similar systems also have found applications in medical studies of human body, e.g. in cancer and inflammation diagnostics. The paper presents a background of the radiometric method, comments to equipment design and outlines some of the applications.

Design and experimental investigation of the asymptotic conical dipole antenna

The paper describes the application of the "equivalent charge distribution method" for designing the geometry of a special type of dipole antenna used for the faithful reproduction of electromagnetic pulse type waveforms. Using this method, the design equation of the surfaces of an asymptotic conical dipole antenna (ACD) has been derived. The surface of the asymptotic conical dipole antenna, obtained by this method, is compared with those computed by the method of Baum (1969). The response of an ACD antenna, fabricated using design data computed from the design equations derived in the paper, is measured and presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Reflection of wideband electromagnetic pulses at a plasma boundary

We propose a new method to determine reflection coefficients for nonsinusodial electromagnetic waveforms containing only a few oscillations. This method is suitable for studies of wideband pulses which cannot be treated by the traditional Fourier techniques. The incident waveform is described by means of Hermitian functions in order to illustrate how the reflection is influenced by the pulse shape and duration.

Low-power electromagnetic stimulation of osteotomized rabbit fibulae. A randomized, blinded study.

The purpose of this study was to determine whether low-power-consuming symmetrical-waveform electromagnetic stimuli could increase the stiffness of fracture sites in a rabbit fibular-osteotomy model. Both active and placebo devices were used in a blinded study protocol. Dose-response studies of pulse amplitude and pulse width were performed by continuous application (twenty-four hours a day) of repetitive (fifteen-hertz), bursted (five-millisecond-long) symmetrical, rectangular electromagnetic stimulus waveforms. The power consumed by these stimuli is approximately one-fifth that consumed by the pulsing electromagnetic field devices that are in current clinical use. Significant increase of callus bending stiffness was produced by pulse widths of five to seven microseconds and pulse amplitudes of fifty to 100 millivolts.

Radiation-triggered breakdown phenomena in high-energy e/sup +/e/sup -/ colliders

The frequency of breakdown phenomena in a HV system under ultrahigh vacuum, can be increased by orders of magnitude by the presence of radiation, either in the form of electromagnetic waves (laser and UV light, synchrotron light, X-rays, gamma rays) or charged particles and ions. Both types of radiation are abundant in high-energy e/sup +/e/sup -/ colliders, and studies of the effect of a particular radiation source in this difficult environment are subject to many limitations. Thus, the principal strategy is to reduce the radiation flux incident on critical devices by absorbing the electromagnetic radiation or by trapping the charged particles. However, in the case of the 'pretzel' separation project for CERN's large electron positron (LEP) collider, this strategy failed. As a result many experiments have been carried out in order to achieve a better understanding of radiation-triggered breakdown phenomena. Observations made in some other e/sup +/e/sup -/ colliders are also reported.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

An efficient formulation to determine the scattering characteristics of a conducting body with thin magnetic coatings

Two new and efficient surface integral equations, derived from corresponding volume integral equations, are developed to calculate the scattering of electromagnetic (EM) waveform from an arbitrarily shaped conducting body coated with thin lossy magnetic film. Their numerical solutions by the method of moments (MM) for two-dimensional structures with full or partial coatings are presented. It is shown that the radar cross-section of a conducting body can be significantly reduced by coating it with a lossy magnetic film. To verify the validity and accuracy of the proposed formulation, another method based on the expansion of cylindrical harmonic functions with real arguments is also developed to calculate the scattering of a plane EM wave from an electrically large coated circular cylinder. The same problem was also solved by the proposed formulation, and excellent agreement between the two approaches was achieved. In addition, numerical results of the scattering from a rectangular coated cylinder is shown to be consistent with that obtained by a modified finite-difference-time-domain (FDTD) method.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Transient electric and magnetic fields associated with establishing a finite electrostatic dipole, revisited

The authors reexamine the analysis of M.J. Master and M.A. Uman (1983), who derived and plotted the electromagnetic field waveforms generated by a square wave of current propagating with constant speed along a finite linear path, resulting in the formation of a finite electrostatic dipole. It was concluded that such a square pulse of current had charge only at its front and rear. The authors argue that errors were made in that analysis, correct these errors, and extend the overall analysis: (1) to show that the decomposition of the electromagnetic fields into electrostatic, induction, and radiation components on the basis of distance dependence is not general; (2) to demonstrate the use of the continuity equation in finding the charge distribution along a current path when the current distribution is known; and (3) to show how a seemingly simple analytical technique, the treatment of an abrupt step of current as a linearly rising current in the limit that the risetime goes to zero, can lead to erroneous results if there is not a good physical and mathematical understanding of all the variables involved.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Multistroke cloud‐to‐ground strike to the NASA F‐106B airplane

An analysis of electromagnetic waveform records and video images of a multistroke cloud‐to‐ground (CG) strike to the NASA F‐106B instrumented airplane is presented. The CG flash started as a lightning strike triggered by the airplane and later produced multiple return strokes to the ground (the ground strike network registered six return strokes). Although there are some uncertainties in data interpretation resulting from lack of independent measurements with other than the airborne instruments, recoil streamers and eight sequences of dart leaders followed by return strokes (two more than were indicated by the ground network) were identified in the airborne data. At least three of the subsequent return strokes were attached to the airplane. The analysis provides evidence that formation of recoil streamers and dart leaders is accompanied by a surge in continuous current. This feature is similar to that observed in the bidirectional leader development during lightning initiation on the airplane.

Time evolution of the fields associated with a cosmic string

The time development of the fields associated with a straight cosmic string in the localU(1) model is studied numerically in an expanding universe. The effective potential is time dependent due to the cooling of the universe. After the spontaneous breaking of theU(1) symmetry the fields start to oscillate as the system tends towards the static state of the stable Nielsen-Olesen string. It is shown that energy flows out of the comoving cylinder in the form of electromagnetic waves. Due to Hubble damping and energy outflow the oscillation amplitudes decrease but static fields are not obtained without additional damping terms originating, e.g., from particle production.

The asymptotic field of an accelerating point charge

We consider the question: Is there an explicit limit in which the radiative part of the electromagnetic field of an accelerating charge viewed in Minkowskian space-time takes the form of plane electromagnetic waves? This paper provides an affirmative answer by means of an explicit transformation and limit except for some special motions of the charge which are clearly defined in a Lorentz-invariant manner. Among the excluded motions are runaway motions in which the charge accelerates to the speed of light.