Pulsed Electromagnetic Wave Research Articles

Ultrafast electro-optic field sensors

We report our recent study of ultrafast electro-optic field sensors for the coherent measurement of freely propagating subpicosecond pulsed electromagnetic waves (THz beams). The sensitivity and bandwidth of these electro-optic sensors are comparable with the conventional ultrafast photoconductive dipole antennas. The simplicity of the detection geometry and capability of optical parallel processing make these sensors suitable for real-time 2D subpicosecond far-infrared imaging.

Response of a dielectric sphere for electromagnetic pulsed wave—cases of back‐ and forward‐scattering

AbstractIt is important to analyze the scattering phenomena of a dielectric sphere in conjunction with the improvement of performance of weather radar to observe rain clouds. This paper considers the transient scattering phenomena when a plane electromagnetic pulse wave is incident on a dielectric sphere. Specifically, the responses of the back‐ and forward‐scattering observed at distant points are presented. The response waveforms are compared when the permittivity, loss, and incident waveform are varied. Analyzing the typical response waveforms, the propagation paths were estimated from the characteristics of the response time and waveform so that the scattering phenomena were studied. The fast inversion of Laplace transform (FILT) method was used for the calculation of the transient responses.

Frequency self-upshifting of focused electromagnetic pulse producing gas ionization

Nonlinear evolution of both the ultrashort focused electromagnetic wave pulse and the breakdown plasma produced by the pulse itself due to tunnel gas ionization is investigated by computer simulation on the basis of the 2-D wave equation. High self-blueshifting effect of the pulse spectrum ( Δω gw >40% ) and formation of short leading peaks of the field (half-wave ionizing “leaders”) with resulting increase in plasma density much over the known refraction limit are found.

Propagation of low-frequency electromagnetic-wave pulses in ionospheric plasma

The problem of one-dimensional propagation of low-frequency electromagnetic waves in homogeneous ionospheric plasma at an angle to the external magnetic field is examined. An oscillating pulse function is obtained for such waves. The shape of the envelope is determined by the Pedersen conductivity. The oscillation period increases with distance and the angle between the propagation direction and the external magnetic field. The period is determined by the Hall and Pedersen conductivities and varies from tenths of a second to seconds.

Ultrashort light pulse generation under coherent interaction of a field with a medium

We consider the dynamics of propagation and compression of ultrashort electromagnetic-wave pulses in a multicomponent two-level field, one component of which emits or absorbs light in a regime of coherent interaction of the radiation with the substance. We consider also the question of the dynamics of the field in planar superconducting structures (such as, e.g., layered high-temperature superconductors of the YBa2Cu3O7=x type). With Josephson tunneling of the electrons between the layers it becomes possible here to describe the field-induced current in a model using Bloch equations for particles with near-zero frequency of the transition between levels and with infinite relaxation times, i.e., to reduce the problem to that of the dynamics of a field coherently interacting with a two-level medium.

Theory of free-electron-laser heating and current drive in magnetized plasmas

The introduction of a powerful new microwave source, the free-electron laser, provides new opportunities for novel heating and current-drive schemes to be used in toroidal fusion devices. This high-power, pulsed source has a number of technical advantages for these applications, and its use is predicted to lead to improved current-drive efficiencies and opacities in reactor-grade fusion plasmas in specific cases. The Microwave Tokamak Experiment at the Lawrence Livermore National Laboratory will provide a test for some of these new heating and current-drive schemes. Although the motivation for much of this research has derived from the application of a free-electron laser to the heating of a tokamak plasma at a frequency near the electron cyclotron frequency, the underlying physics, i.e., the highly nonlinear interaction of an intense, pulsed, coherent electromagnetic wave with an electron in a magnetized plasma including relativistic effects, is of general interest. Other relevant applications include ionospheric modification by radio-frequency waves, high-energy electron accelerators, and the propagation of intense, pulsed electromagnetic waves in space and astrophysical plasmas. This review reports recent theoretical progress in the analysis and computer simulation of the absorption and current drive produced by intense pulses, and of the possible complications that may arise, e.g., parametric instabilities, nonlinear self-focusing, trapped-particle sideband instability, and instabilities of the heated plasma.

Reflection of pulsed electromagnetic waves from a randomly stratified half-space

We consider the reflection of temporally pulsed electromagnetic waves from a weakly dispersive and dissipative, randomly stratified half-space. The incident pulse width is chosen to be short relative to deterministic background variations in the constitutive parameters but broad relative to the rapid, fine-scale random fluctuations in these parameter values. We study both plane-wave excitation (normal and oblique incidences) and excitation by a point current source. We are interested in the coherent reflected field and the mutual coherence function of the reflected field at the surface. Explicit results are given in certain special cases. We also compare the theory with numerical simulations.

Propagation of transients in dispersive dielectric media

The propagation of transient electromagnetic fields in dispersive dielectric media is studied. The dielectric medium is assumed to be linear, isotropic, and homogeneous, and is described by the Debye model. Incident fields are assumed to be transverse electromagnetic plane wave pulses. The dielectric body can assume the form of infinite half space or an infinite circular cylinder, either of which may be homogeneous or stratified. The electric fields induced in the dielectric are calculated from time-domain Maxwell's equations using the finite-difference time-domain method. >

Trapping and entrainment of electromagnetic wave pulses into supercritical regions of inhomogeneous plasma by ion-acoustic waves

A mechanism of trapping and entrainment of electromagnetic (EM) field pulses into supercritical regions of an inhomogeneous plasma by ion-acoustic waves is considered.

Mirror Surface EDM by Electric Field Partially Induced

Mirror surface EDM has been recently presented to meet the demands for smooth plastic moulds and blanking dies. But it is difficult to polish a large work's surface by spark-erosion sinking. In this paper, the ideal electric pulse parameters of polishing, including discharge duration te and discharge current ie, are determined by technological experiments. The performance of pulse electromagnetic waves in small spark gap is searched theoretically. On the basis of the research, mirror surface EDM by electric field partially induced is first presented to realise the polishing of the workpieces with large working area, such as several thousand sq. millimeters.

Singularities in the transverse fields of electromagnetic waves. I. Theory

Electromagnetic waves generally contain three kinds of singularities called C lines, S surfaces and disclinations. These singularities are features of the transverse electric and transverse magnetic fields of the waves and all three kinds usually occur in any given wavefield. We show that in the case of nominally uniformly polarized waves, the simple line zeros predicted for interference fringes by scalar wave theory in fact have an underlying polarization structure consisting of two C lines and an S surface. In consequence, virtually all monochromatic electromagnetic waves contain polarization states ranging from right-hand circular, through linear to left-hand circular polarization. Singularities of the electric and magnetic fields are not generally coincident in space; in fact they can be separated by arbitrarily large distances. The separation of the electric and magnetic S surfaces means that there are regions where the transverse electric and transverse magnetic vectors counterrotate. C lines are probably the most significant of the singularities, since they are not only structural features of polarization, but also organize the time structure of electromagnetic waves. They play a crucial role in determining the topology of disclinations in paraxial wavefields. In pulsed electromagnetic waves all three singularities move through space. Their behaviour, including interactions between pairs of C lines, S surfaces or disclinations, which are likely to be frequent events in pulsed waves, is discussed.

SEM approach to transient scattering by a lossy, radially inhomogeneous dielectric circular cylinder

The singularity expansion method is applied to the computation of fields associated with the transient scattering of a pulsed electromagnetic plane wave of finite duration by an isotropic, lossy, radially inhomogeneous dielectric circular cylinder. An angular Fourier transformation reduces the problem to solving a series of one-dimensional transient scattering problems, each of which is similar to the problem of scattering by a dielectric slab. Each of the resulting time-dependent Fourier coefficients is then expressed as a Laplace inversion integral over the Bromwich contour. This integral is evaluated by supplementing the contour of integration by circular arcs at infinity, and applying Cauchy's theorem. For each angular order, this leads to contributions from a denumerable set of poles and from a branch cut along the negative half of the real-frequency axis. Procedures are devised to determine these singularities and their contributions numerically. For a representative configuration, results are presented and discussed. A physical interpretation of these results is given.

Electromagnetic pulses in a strongly magnetized electron-positron plasma.

The conditions for the existence of large-amplitude localized electromagnetic wave pulses in an electron-positron plasma penetrated by a very strong ambient magnetic field are obtained. It is shown that such pulses can exist in pulsar polar magnetospheres.

Pulsed electromagnetic radiation from a line source in the presence of a semi-infinite screen in the plane interface of two different media

The two-dimensional diffraction of a pulsed electromagnetic wave by a semi-infinite screen located in the interface of two different media is investigated theoretically. The incident electromagnetic field is taken to be generated by a line source. With the aid of the Wiener-Hopf technique and the Cagniard-De Hoop technique closed-form expressions for the field components anywhere in the configuration are obtained. Numerical results are presented in the case of E-polarization.

SEM approach to the transient scattering by an inhomogeneous, lossy dielectric slab; Part 2: The inhomogeneous case

In two companion papers, the singularity expansion method is applied to the computation of the transient scattering of a pulsed electromagnetic wave of finite duration by an isotropic, inhomogeneous, lossy dielectric slab. The time-domain electric field is expressed as a Laplace inversion integral over the Bromwich contour. Conditions for the validity of a representation in terms of natural-mode quantities only are derived, i.e. for a vanishing integral along the closing contour in the left halfplane, the latter being the contribution from the essential singularity at infinity. In the first of these two papers, this procedure was analyzed for the case of a homogeneous slab. Here, this analysis is generalized for the inhomogeneous case. For that case, an alternative method exists, where the nonvanishing contribution from the essential singularity at infinity is suppressed by taking into account at each instant only the part of the incident pulse that has emerged into the slab. This method also applies when the closing conditions are violated. The location of the poles and the corresponding natural-mode field distributions and coupling coefficients are determined numerically with the aid of a Runge-Kutta integration method and Muller's root-finding procedure. In the subsequent summation of the residual contributions, acceleration of the convergence is applied. Numerical results are presented and discussed.

SEM approach to the transient scattering by an inhomogeneous, lossy dielectric slab; Part 1: The homogeneous case

In two companion papers, the singularity expansion method is applied to the computation of the transient scattering of a pulsed electromagnetic wave of finite duration by an isotropic, inhomogeneous, lossy dielectric slab. The time-domain electric field is expressed as a Laplace inversion integral over the Bromwich contour. Conditions for the validity of a representation in terms of natural-mode quantities only are derived, i.e. for a vanishing integral along the closing contour in the left halfplane, the latter being the contribution from the essential singularity at infinity. In the present paper, this procedure is first carried out for a homogeneous slab. For that case, an alternative method exists where the computation of the integral along the closing contour is reduced to a modification of the residual contributions from the poles and the numerical evaluation of a principal-value integral along the branch cut of the complex index of refraction. This method applies when the closing conditions are violated. The location of the poles is computed numerically by determining the zero's of the analytically known slab-denominator with the aid of Muller's root-finding procedure. In the subsequent summation of the residual contributions, acceleration of the convergence is carried out to allow for discontinuities in the incident-pulse shape. Some numerical results are presented and discussed.

Strong electromagnetic waves in a magnetized relativistic electron-positron plasma

It is shown that in a strongly magnetized relativistic electron-positron plasma, strongly localized large amplitude circularly polarized electromagnetic wave pulses exist. The localization is due to relativistic mass variation as well as ponderomotive force effects. Three types of pulses are found analytically: the sharply spiked pulse in a strongly magnetized cold plasma, the smooth pulse in a week magnetized warm plasma, and the moderately spiked pulse for a weakly magnetized cold plasma. The physical mechanisms giving rise to these pulses are distinct for each case. Possible implications of our investigation to pulsar radiation are discussed.

Particle acceleration by an electromagnetic wave pulse propagating along a static magnetic field

Relativistic particle motion in an electromagnetic wave pulse propagating in vacuum along a static magnetic field is studied. A condition for synchronous particle acceleration which relates to the pulse width is given. As the particle energy increases, the particle motion becomes highly directional, and the associated radiation power is found to be proportional to the first power, rather than to the fourth power, of the particle energy.

Initial-boundary-value problem for diffusion of magnetic fields into conductors with external electromagnetic transients

The initial-boundary-value problem for the diffusion of an initially homogeneous magnetic field into a slab of conductivity σ<∞ and width Δx=2a is solved, under consideration of the electromagnetic wave pulses generated at the surfaces of the conductor by its interaction with the external magnetic field, which propagate into the surrounding vacuum. The analytical solutions show that (i) the external electromagnetic transients are necessary in order to correctly satisfy the boundary conditions for the tangential electric and magnetic field components, and (ii) the spatial and temporal development of the electromagnetic field and electric current in the conductor is quantitatively determined by a new dimensionless parameter group ℛ=μ0σac[c=(μ0ε0)−1/2]. This ‘‘magnetic Reynolds number of the vacuum’’ determines the coupling between the transient fields in the conductor σ>0 and the ambient space (σ=0).

Excitation of nonlinear impurity electron-phonon oscillations. Raman scattering

Abstract The impurity is shown to transit into the state of non-linear electron-phonon oscillation upon the passage of a resonance electromagnetic wave pulse. In this case some lines appear in the Raman spectrum at frequencies characteristic for the oscillations.