Short Electromagnetic Pulses Research Articles

Importance of the electron plasma parameter for excitation of chorus and formation of magnetic field irregularity in the region of their excitation

A threshold condition has been established for excitation of VLF electromagnetic radiation with a chorus structure of the dynamic spectrum in the daytime magnetosphere using the BPA (Beam Pulse Amplifier) mechanism for amplifying short noise electromagnetic pulses. The kappa distribution was used as a model function of the electron velocity distribution in the magnetosphere. Calculations performed for this distribution have shown that the threshold for excitation of chorus largely depends on the electron plasma parameter equal to the ratio of gas-kinetic pressure of electrons to magnetic pressure. This pattern is not contradicted by the dependence of the probability of excitation of chorus on the degree of magnetic field irregularity, which we derived from observations made by the Van Allen Probe spacecraft. It is sharp fluctuations in the magnetic field strength near its local minima outside the plasmasphere where the radiation under study can be excited. If there is an irregularity, the probability of detecting chorus is >70 %; and if there is no or very low irregularity, the probability of the absence of any emissions is ~80 %. The results indicate a common reason for the excitation of chorus and the magnetic field irregularity — a small but finite value of the plasma parameter.

Существенность величины плазменного параметра электронов для возбуждения хоров и формирования нерегулярности магнитного поля в области их возбуждения

A threshold condition has been established for excitation of VLF electromagnetic radiation with a chorus structure of the dynamic spectrum in the daytime magnetosphere using the BPA (Beam Pulse Amplifier) mechanism for amplifying short noise electromagnetic pulses. The kappa distribution was used as a model function of the electron velocity distribution in the magnetosphere. Calculations performed for this distribution have shown that the threshold for excitation of chorus largely depends on the electron plasma parameter equal to the ratio of gas-kinetic pressure of electrons to magnetic pressure. This pattern is not contradicted by the dependence of the probability of excitation of chorus on the degree of magnetic field irregularity, which we derived from observations made by the Van Allen Probe spacecraft. It is sharp fluctuations in the magnetic field strength near its local minima outside the plasmasphere where the radiation under study can be excited. If there is an irregularity, the probability of detecting chorus is >70 %; and if there is no or very low irregularity, the probability of the absence of any emissions is ~80 %. The results indicate a common reason for the excitation of chorus and the magnetic field irregularity — a small but finite value of the plasma parameter.

“Middle way” in the description of ultrashort electromagnetic interactions: between Fermi’s golden rule and the numerical solution of the Schrödinger equation

A simple efficient approach describing the interaction of various quantum targets with short electromagnetic (EM) pulses firstly proposed in [1] is reviewed. This approach is based on the expression for the photoprocess probability in terms of the cross section and the Fourier transform of the EM pulse. Our method makes it possible to take into account the internal dynamics of the target and the influence of the pulse parameters in a simple way. The proposed approach is used both for the probability for the entire pulse duration, and for its dependence on the current time. In addition, the developed method is applied to generalize the simulation of the population kinetics of a quantum system to take into account the dependence of the photoprocess rate on time. It is also shown that our expression for the probability of excitation of a micro-target corresponds to the formula describing the energy loss of a non-monochromatic field in a dispersive medium and can be generalized on other EM processes.

The interaction of electron with the infinitesimally short electromagnetic pulse

After the classical approach to acceleration of a charged particle by the δ-form impulsive force, we consider the corresponding quantum theory based on the Volkov solution of the Dirac equation. We determine the modified Compton formula for frequency of photons generated by the scattering of the δ-form laser pulse on the electron in a rest. The article follows the physical ideas involved in the author text with the title Electron in an Ultrashort Laser Pulse (Pardy, 2003).

Determination of solutions for chorus excitation mechanism characteristic equation by means of Van Allen probe data analysis

In this paper, we focus on studying the quantitative characteristics of the excitation mechanism of chorus emissions closely related to the problem of relativistic electron flux formation in the radiation belts. The study is based on the processing of information accumulated by the Van Allen Probe spacecraft through the analysis of high-resolution data. We have chosen two typical examples of chorus emissions with spectral forms predominantly in the upper-frequency band (above half the electron cyclotron frequency) in the region of the local minimum of the magnetic field outside the plasmapause in the middle magnetosphere. We have developed and implemented a calculation algorithm that enables us to represent the results of wave field measurements in a high-resolution data channel in the form of a rectangular event matrix, each row of which corresponds to a cycle of the wave process. In the event matrix, we select the rows corresponding to fragments of chorus emissions that best characterize the natural source of short electromagnetic pulses. This method made it possible to determine the complex eigenvalues of the characteristic equation of the chorus emissions source. The proposed method for processing experimental information makes it possible to determine the main characteristics of the linear mechanism of chorus excitation. The results obtained in this way from observational data are in good agreement with relevant theory of chorus emissions excitation by amplifying noise electromagnetic pulses in a duct with a depleted density of a cold plasma.

Light-induced dynamic microcavities created in a resonant medium by collision of non-harmonic rectangular 1-fs light pulses

Recently, interest in extremely short unipolar electromagnetic pulses with non-harmonic shape (e.g. rectangular or triangular) is rising. Such pulses do not contain a carrier frequency and have an ultra-wide spectrum. Compared to conventional harmonic multicycle pulses, they are faster and more efficient in controlling the properties of quantum systems, which allows, in the future, for faster data transmission and processing. With coherent interaction of extremely short light pulses with a medium, formation and ultrafast control of population difference gratings and polarization structures are possible when the pulses both overlap and do not overlap in the medium. In this work, based on the numerical solution of Maxwell–Bloch equations, we demonstrate possible creation of a dynamic “microcavity” as a burst of population difference, arising from the collision of rectangular unipolar 1-femtosecond pulses of self-induced transparency in a two-level resonant medium. Potential to control the microcavities parameters (changing the depth, turning it on, erasing it, etc.) is demonstrated.

High-current space-charge-limited pulses using ultrashort laser pulses.

The maximum current that can be delivered by a 1-D vacuum diode is limited due to the space-charge reduction of the accelerating field. For a steady-state operation, this is given by the Child-Langmuir space-charge limit. However, for pulsed diode sources, the instantaneous current can be much higher than this limit, as long as the pulse length is much less than the transit time across the diode gap. This enables the generation of high current pulses with pulse durations of the order of tens of picoseconds using photocathodes driven by ultrashort laser pulses. The generation of such short and powerful electromagnetic pulses is important for numerous applications such as ultrawideband radar or as photocathode sources for accelerators. In this work, the limiting current pulse is investigated in the case of a very short, square-top initial pulse and it is found that these pulses propagate in a self-similar manner, remaining as a square-top charge cloud in space throughout their acceleration and propagation. The resultant current and pulse duration turn out to be dependent on only three parameters, which are the applied voltage, the vacuum transit time, and the fraction of the saturation charge density in the initial charge cloud. The resultant scaling of the resultant peak current and pulse duration are calculated numerically as a function of starting sheet-charge density, allowing the calculation of the resultant pulses for any ultrashort, pulse-driven vacuum photodiode design.

A New Approach to Formation and Directed Radiation of Powerful Short Radio Pulses

This article details a new method for formation and directional radiation of short high-power electromagnetic (EM) pulses, which is based on a wave analog of the Smith–Purcell effect. The presented device cycles through two stages. At the stage of energy accumulation, a slow wave of an open guiding structure interacts with a periodic chain of storage resonators and excites high-Q free field oscillations in each resonator of this chain. The phase of oscillations inside the storage resonators differs by a constant value, and the amplitude increases with time. At the radiation stage, the chain of storage resonators is opened to radiation (as it usually happens in active compressors of EM pulses), and short powerful pulses burst out into free space in strictly defined directions. The proposed design is considered in detail for the simplest model of a device. The results obtained during its rigorous electrodynamic analysis look rather promising; the continuation of this work can contribute to solving a number of urgent problems of physics and technology of millimeter and submillimeter waves.

Coherent x-ray radiation induced by high-current breakdown on a ferrite surface

For the first time observed that at the initial stage of a high-current discharge, a low-divergence short (<2 ns) electromagnetic pulse is formed over a ferrite surface. The 50% part of the energy spectrum of this pulse lies in the region of sufficiently hard x-ray radiation (hν > 1 keV) with an energy of 0.6 mJ and an average power of 0.3 MW. The radiation propagates parallel to the surface in the anode direction with the angle divergence <2°. The high directionality of the radiation in absence of the aperture-limiting devices for the radiation beam and the quadratic dependence of the spatial radiation energy flux density on the length of working part of the ferrite prism points to the coherent nature of the observed radiation. A possible generation mechanism of the radiation is proposed. It is based on the short-lived magnetization of the ferrite plots by a high-power electromagnetic pulse and the subsequent coherent interference of unit waves irradiated by these plots.

Conduction state transition induced by solitons in a graphene junction at room temperature

The aim of this research was the study of the transition between high and low electrical resistivity states of two overlapped graphene layers when subjected to short electromagnetic pulses (soliton waves). These transitions have already been previously observed by the authors in experiments carried out with different conductors, separated by a tiny insulating layer. The choice of a highly ordered material, such as graphene, was justified by the attempt to achieve greater stability and reproducibility of these transitions. What has been observed is an instantaneous reversible transition of the graphene overlapped layers to/from a state of insulator with resistance in the order of Mohms from/to a state of resistance of few ohms or, in some cases, of zero ohms. The transition from a high resistance state to a lower one requires EM pulses of different polarity than the transition from a low resistivity state to a higher one. Some intermediate relatively stable states have also been observed.

Entrainment, stopping, and transmission of microwave solitons of self-induced transparency in counter-propagating magnetized electron beam.

Based on numerical simulations of a boundary problem, we study various scenarios of microwave soliton formation in the process of cyclotron resonance interaction of a short electromagnetic pulse with a counter-propagating initially rectilinear electron beam taking into account the relativistic dependence of the cyclotron frequency on the electrons' energy. When a certain threshold in the pulse energy is exceeded, the incident pulse can propagate without damping in the absorbing beam, similar to the effect of self-induced transparency in optics. However, mutual motion of the wave and electrons can lead to some novel effects. For relatively small energy of the incident pulse, the microwave soliton is entrained by the electron beam opposite to the direction of the wave's group velocity. With an increase in the pulse energy, soliton stopping occurs. This regime is characterized by the close-to-zero pulse velocity and can be interpreted as a variety of the "light stopping." High-energy microwave solitons propagate in the direction of the unperturbed group velocity. Their amplitude may exceed the amplitude of the incident pulse, i.e., nonlinear self-compression takes place. A further increase in the incident energy leads to the formation of additional high-order solitons whose behavior is similar to that of the first-order ones. The characteristics of each soliton (its amplitude and duration) correspond to analytical two-parametric soliton solutions that are to be found from consideration of the unbounded problem.

On the area of extremely short electromagnetic pulses in medium for problems of a finite integration region

Based on the one-dimensional Maxwell equations of electrodynamics of continuous medium, the balance ratios of electric and magnetic areas for problems solved in a finite region in space and time are obtained. Keywords: extremely short pulses, electrical pulse area.

New precise results on dynamics of weak-pulse excitation of mirrorless slab of two-level atoms

We consider by precise calculations the result of passing a very short coherent electromagnetic pulse through a gaseous slab of resonant (“two-level”) atoms bounded by vacuum on both sides. By “very short” we mean that at time t=0 (just after the passage) the system is left in an initial condition in which (except in a negligibly small region next to the exit face) the atoms have not yet felt reflection from the exit boundary; we assume only weak excitation from the ground state. A large rôle is played in our calculations by what we call ENR (exact numerical results) which are obtained by summing over a very great number of eigenmodes in the modal expansion of our formulas. Among other things, we show that, for thicknesses exceeding 5 resonant wavelengths λ, the forward flux of radiation is almost precisely inverse to the thickness, contrary to Beer’s law. We also introduce a generalized definition of frequency shift by regarding the forward flux spectrum as a mass distribution and finding the shift in its center of mass. With this definition we are able to exhibit oscillations of extremely small amplitude but extremely well defined period in the shift as a function of thickness, starting around 5λ and losing no quality even at 80λ. We have given a new discussion of the landmark 1969 paper of Burnham and Chiao in which the slowly varying envelope approximation (SVEA) was used to solve the atomic excitation problem exactly subject to neglect of the backward wave. We have elucidated the distinction between the thick-slab approximation (decay of excitation faster than return from back face), used there, and our medium-thickness problem (return from back face faster than local decay). We have shown how the method of Burnham and Chiao can be adapted to the medium-thickness problem, yielding an SVEA result almost identical to the one they presented. Other new results are summarized in the Introduction.

Amplification of a slipping quasi-monochromatic wave pulse by an electron flow with a wide velocity spread

Cherenkov interaction between a wave pulse and a flow of electrons possessing a very wide (on the scale of the characteristic band of the resonant electron–wave interaction) velocity spread is considered. We show that if the wave pulse is short enough, and its group velocity is close to the phase velocity, then the effect of the slippage of the resonant electrons with respect to the wave pulse leads to the transformation of an inert electronic medium into an active one (absorbing or amplifying the wave pulse, depending on the slippage sign). This can be a mechanism of formation of short powerful electromagnetic pulses as a result of amplification of short-pulse weak noises by electron flows which, due to natural reasons, have a large velocity spread, namely, electron flows in the magnetosphere of planets, in the plasma envelope of brown dwarfs and neutron stars, as well as in electron masers with weak electron–wave interaction (including ultra-relativistic electron beams used in free-electron lasers).

Coherent GHz lattice and magnetization excitations in thin epitaxial Ag/Fe/Cr/Fe films

We excited an epitaxial magnetic Ag/Fe/Cr/Fe multilayer nonthermally and nonoptically with very short (1 ps) electromagnetic pulses. We detected the synchronous phononic-magnetic response by time-resolved magneto-optical Kerr effect measurements. The Ag/Fe/Cr/Fe multilayer was patterned into a coplanar waveguide transmission line, and the electromagnetic pulses were generated by pulsed-laser illumination of an integrated GaAs photoconductive switch (PCS). The detected magnetic excitations comprise up to four narrow-band high-order modes with the highest frequency reaching 30 GHz. The mode frequencies are independent of both temperature in the range from 16 to 300 K and the applied external magnetic field up to 120 mT. Our analysis shows that the origin of the rigidity of these high-frequency modes is the strong coupling of the magnetic subsystem with the lattice of the Ag/Fe/Cr/Fe multilayer. The exciting electromagnetic pulse generated by the PCS induces, via magnetoelastic coupling, long-lived (ns) standing GHz acoustic waves normal to the Ag/Fe/Cr/Fe film plane. These lattice oscillations in turn couple back and drive the magnetization oscillations via the magnetoelastic coupling. The temperature and field dependence of the damping of the oscillations can be described by inelastic phonon-phonon and phonon-magnon scattering. Our study opens up a possibility of using coherent lattice and magnetization dynamics in ferromagnetic films for spintronic devices at GHz clock rates.

Antenna Array with TEM-Horn for Radiation of High-Power Ultra Short Electromagnetic Pulses

An antenna array with short shielded transverse electromagnetic horns (S-TEM-horns) for emitting high-power radiation of ultra-short electromagnetic pulses (USEMP) has been created and researched. The antenna unit consists of an ultra-wideband antenna array with four S-TEM horns, with each connected to a two-wire HF transmission line, and these four lines are connected to an antenna feeder. This feeder is connected to a semiconductor generator with the following parameters: a 50 Ohm connector, 10–100 kV high-voltage monopolar pulses, a rise time of about 0.1 ns, FWHM = 0.2–1 ns, and pulse repetition rates of 1–100 kHz. The antenna array was designed and optimized to achieve a high efficiency of about 100% for the antenna aperture by using a 2 × 2 array with S-TEM-horns, with shielding rectangular plates for the return current. The transient responses were studied by simulation using the electromagnetic 3D code “KARAT” at the time domain and experimentally with the use of our stripline sensor for measurement of the impulse electrical field with a 0.03 ns rise time and a 7 ns duration at the traveling wave. The radiators were emitting USEMP waves with a hyperband frequency spectrum of 0.1–6 GHz. The radiation with an amplitude of 5–30 kV/m of the E-field strength at a distance of up to 20 m was successfully applied to test the electronics for immunity to electromagnetic interference.

Dynamic chaos in the kicked “photon-qubit atom”

We study dynamics of a qubit coupled to a single photon mode in a high-quality resonator. For a weak coupling constant the qubit and photon form a long-lived bound state which may be called as a “photon-qubit atom”. We consider the stability and chaotic dynamics of the photon-qubit atom under the action of short electromagnetic pulses. The photon oscillator considered in the classical approximation. We plot the Poincare surface to indicate the appearance of the chaotic behaviour. It is shown that depending on the qubit and field parameters the system may be demonstrating the chaotic behaviour.

Localized optical vortex solitons in pair plasmas

Abstract The dynamics of short intense electromagnetic pulses propagating in a relativistic pair plasma is governed by a nonlinear Schrödinger equation with a new type of focusing-defocusing saturable nonlinearity. In this context, we provide an existence theory for ring-profiled optical vortex solitons. We prove the existence of both saddle point and minimum type solutions. Via a constrained minimization approach, we prove the existence of solutions where the photon number may be prescribed, and we get the nonexistence of small-photon-number solutions. We also use the constrained minimization to compute the soliton’s profile as a function of the photon number and other relevant parameters.

Magnetic pulse welding of copper to steel tubes–Experimental investigation and process modelling

Abstract Magnetic pulse welding allows joining of metallic tubes by a short electromagnetic pulse and the resulting high strain rate impact and plastic deformation. Since melting of materials is avoided, magnetic pulse welding is an efficient method for joining of dissimilar materials. However, the process occurs very fast with little opportunity for real-time monitoring and control, which is also difficult due to the presence of a high amplitude electromagnetic field. A novel attempt is presented here to examine the underlying phenomena for magnetic pulse welding of copper flyer tubes to steel target tubes by computational process modeling and a focused experimental investigation. A coupled electromagnetic and mechanical finite-element model was created to compute the electromagnetic field and pressure distribution, and the progressive nature of flyer tube impact velocity on the target tube. The consequent progress of the angle of impact and contact length are examined for a range of standoff distances between the flyer and target tubes, the target tube wall thickness. The computed results are validated extensively with corresponding experimentally measured results. Overall, the coupled theoretical and experimental investigation provided a useful quantitative insight of magnetic pulse welding of copper and steel tubes, which is expected to help in the advancement of practical application of the process.

Peculiarities of extremely short optical pulses propagation in carbon nanotube medium with nonlinear absorption

In this paper, we study the propagation of extremely short electromagnetic pulses in a medium with zig–zag carbon nanotubes taking into account pumping and nonlinear absorption introduced phenomenologically. Based on Maxwell’s equations, we obtain an effective equation for the vector potential of the electromagnetic field, which takes into account the dissipation of the pulse field under the conditions of the piezoelectric effect associated with the vibrations of heavy nuclei of the medium, pumping by an external electromagnetic wave, and nonlinear absorption of carbon nanotubes. We demonstrate the stability of the electromagnetic pulse shape on a time scale that is significantly longer than the pulse duration but not exceeding the relaxation time.