Effect of external actions on the characteristics of vortex sound

The dependence of the characteristics of vortex sound caused by an air flow around a rigid cylinder on various factors, including the turbulence of the incident flow, the inhomogeneity of the cylinder surface, and the sound radiation of an external source, is experimentally investigated. Measurements have made it possible to specify the mechanism of vortex sound radiation under the action of external factors, to relate the radiation intensity and the drag to the type of air flow around the body, and to propose possible ways of reducing the vortex sound radiation intensity.     

Numerical study of acoustic radiation dynamics of a Rankine vortex

The paper continues investigations of a fundamental problem important for understanding the nature of vortex flows: sound radiation by a perturbed Rankine vortex. Results of numerical simulation are presented for both quadrupole and higher-mode radiations. The structure and dynamics of tonal acoustic radiation generated by a vortex are considered in detail. The transition of a Rankine vortex to a perturbed state under small external perturbation is considered. Calculations are performed using the EBR scheme implemented by NOISEtte software.     

Cherenkov radiation of a Josephson vortex moving in a sandwich embedded in a dielectric medium

A motion of a Josephson vortex in a long sandwich embedded in a dielectric medium is described. If the velocity of the vortex is greater than the velocity of light in the dielectric, terahertz-band Cherenkov radiation is generated and emitted from the lateral surface of the sandwich. The radiation loss power is determined. In the case when radiation loss is compensated for by the energy gain due to transport current, a relation between the current and the velocity of the vortex is obtained.     

Cherenkov radiation emitted by a vortex into an anisotropic dielectric

The fields produced by a Josephson vortex moving in a sandwich placed into an anisotropic dielectric are investigated. When the vortex velocity exceeds the velocity of light in the direction of the normal to the sandwich surface, Cherenkov emission of electromagnetic waves propagating from the sandwich to the bulk of the dielectric takes place. The Poynting vector of outgoing waves is determined. It is shown that the radiation directivity considerably depends on the degree of anisotropy in the permittivity. The radiation loss power of the vortex is determined, and the relation between the transport current and the vortex velocity is established.     

Explosive plasma-vortex optical radiation sources

The results of experimental study of explosive radiation sources based on pulsed injection of a cumulative plasma jet into atmospheric air are considered. The injection process is accompanied with intense vortex formation as well as the formation of a large-scale toroidal plasma vortex. High-power electromagnetic radiation in the optical range is generated due to shock-wave processes during deceleration of a plasma jet in air and plasma-chemical processes in the vortex. The temporal structure of a radiation pulse being generated contains components from the micro- and millisecond range. For a 20-g mass of the explosive charge, a peak radiation power of 300 kW/sr and an energy yield of 400–600 J/sr integrated over the emission spectrum are attained. The efficiency of conversion of the chemical energy of the explosive into radiation is 5.0–7.5%.     

Vortex viscosity in magnetic superconductors due to radiation of spin waves

In type-II superconductors that contain a lattice of magnetic moments, vortices polarize the magnetic system inducing additional contributions to the vortex mass, vortex viscosity, and vortex-vortex interaction. Extra magnetic viscosity is caused by radiation of spin waves by a moving vortex. Like in the case of Cherenkov radiation, this effect has a characteristic threshold behavior and the resulting vortex viscosity may be comparable to the well-known Bardeen-Stephen contribution. The threshold behavior leads to an anomaly in the current-voltage characteristics, and a drop in dissipation for a current interval that is determined by the magnetic excitation spectrum.     

Cherenkov radiation from an Abrikosov-Josephson vortex

The Cherenkov radiation of generalized Swihart waves is investigated in connection with the slow motion of an Abrikosov-Josephson vortex, which corresponds to a 2 π kink in the phase difference of Cooper pairs on opposite sides of a tunnel junction. The radiative friction force acting on such a vortex is determined. An evaluation is made of the steady-state vortex velocity when the accelerating influence of an electric current through the Josephson junction is compensated by radiative slowing of the vortex due to Cherenkov radiation from the Abrikosov-Josephson vortex. Fiz. Tverd. Tela (St. Petersburg) 39, 444–448 (March 1997)     

可压缩涡流场中空泡运动规律及声辐射特性研究

基于可压缩流体力学基本理论, 通过边界积分方程, 采用不同表面压力模型, 求解空泡在计及可压缩性的涡流场中的运动规律; 通过表面离散及坐标变换, 采用Kirchhoff动边界积分方程, 将空泡表面视为运动变形边界, 作为直接噪声源, 获得涡流场中空泡运动产生的时域声压分布; 分析了涡流场参数对空泡运动规律及声辐射特性的影响. 研究结果表明: 计及流场可压缩性, 空泡的脉动幅度会随时间减弱, 辐射声压幅值随之减小; 空泡在涡流场中会发生延展、 颈缩、 撕裂, 并在撕裂后子空泡中形成射流; 当流场中的压力减小时, 空泡运动过程中的最大半径与撕裂前的最大长度逐渐增加, 且当流场中压力较小时, 空泡撕裂时形成的子空泡增多; 空泡辐射声压的指向性较弱, 撕裂会使辐射声压产生突变, 形成极大峰值; 随着涡通量的增大或空泡数的减小, 空泡脉动周期及其诱导的辐射声压波动周期随之延长, 辐射声压峰值逐渐滞后并减小. 本文结果旨在为涡流场中空泡运动规律及声辐射特性的相关研究提供参考. 关键词： 可压缩 涡流场 空泡 声辐射     

Macroscopic far-field observation of the sub-wavelength near-field dipole vortex

The energy flow lines of the radiation emitted by a rotating electric dipole moment have a vortex structure near the source. The spatial extend of this vortex is well below an optical wavelength. This near-field vortex has a macroscopic effect which could be observed in the far field.     

Low-frequency transition radiation from a short laser pulse at the plasma boundary

The transition radiation that appears when a short laser pulse crosses the vacuum-plasma boundary and that is attributable to an vortex electric current produced at the plasma boundary by averaged ponderomotive forces has been considered for the first time. The spectral, angular, and spatiotemporal characteristics of the transition radiation in a vacuum are analyzed. It is shown that under typical conditions of present-day laser plasma experiments, the frequency of the transition radiation lies in the terahertz range and its power can reach several megawatts.     

Electromagnetic radiation from vortex flow in Type-II superconductors

We show that a moving vortex lattice, as it comes to a crystal edge, radiates into a free space the harmonics of the washboard frequency, omega(0)=2pi v/a, up to a superconducting gap, Delta/variant Planck's over 2pi. Here v is the velocity of the vortex lattice and a is the intervortex spacing. We compute radiation power and show that this effect can be used for the generation of terahertz radiation and for characterization of moving vortex lattices.     

Enhancement of nonlinear properties of superconducting films in low-frequency magnetic field

The effect of enhancement of nonlinear properties of superconducting films is found in low-frequency magnetic field. Harmonic mixing of the electromagnetic radiation is detected. These results provide an argument in support of a new approach to design the active superconducting elements. A low-frequency oscillating magnetic field directed perpendicularly to the film surface forms an unstable vortex structure in the film. The frequency multiplication is intensified in the vortex unstable state. A transition of the vortex structure into the ground state synchronized by an external resonant circuit causes the generation of the electromagnetic radiation. Main advantage of these oscillators over Josephson junctions is high power emitted into a broadband system.     

Generation of electromagnetic radiation in the motion of vortices in magnetically coupled superconducting films

We examine theoretically the generation of electromagnetic radiation in the relative motion of vortex lattices in magnetically coupled films in the dc transformer geometry. We establish the conditions under which the force of mutual pinning of the vortex lattices varies according to a harmonic law as a function of the relative displacement of the vortices in the films within a given range of magnetic field inductions. In this case the equation describing the viscous flow of vortex lattices in magnetically coupled films is the same as the equation of the resistively shunted Josephson junction model. We show that magnetically coupled superconductors exhibit the properties of a Josephson element without any restrictions on the geometrical size of such a system imposed by the coherence length ξ. The frequency f of the electromagnetic radiation generated by the relative motion of vortex lattices in magnetically coupled superconductors depends on the spatial period of the vortex lattices and the velocity of relative vortex motion, which means that the frequency of the radiation can be tuned by applying a magnetic field or a current. Zh. éksp. Teor. Fiz. 113, 1319–1338 (April 1998)     

Control of surface plasmon excitation via the scattering of light by a nanoparticle

We analyze the scattering of vortex pairs (the particular case of 2D dark solitons) by a single quantum vortex in a Bose–Einstein condensate with repulsive interaction between atoms. For this purpose, an asymptotic theory describing the dynamics of such 2D soliton-like formations in an arbitrary smoothly nonuniform flow of a ultracold Bose gas is developed. Disregarding the radiation loss associated with acoustic wave emission, we demonstrate that vortex–antivortex pairs can be put in correspondence with quasiparticles, and their behavior can be described by canonical Hamilton equations. For these equations, we determine the integrals of motion that can be used to classify various regimes of scattering of vortex pairs by a single quantum vortex. Theoretical constructions are confirmed by numerical calculations performed directly in terms of the Gross–Pitaevskii equation. We propose a method for estimating the radiation loss in a collision of a soliton-like formation with a phase singularity. It is shown by direct numerical simulation that under certain conditions, the interaction of vortex pairs with a core of a single quantum vortex is accompanied by quite intense acoustic wave emission; as a result, the conditions for applicability of the asymptotic theory developed here are violated. In particular, it is visually demonstrated by a specific example how radiation losses lead to a transformation of a vortex–antivortex pair into a vortex-free 2D dark soliton (i.e., to the annihilation of phase singularities).     

Stimulated emission of the coherent electromagnetic radiation by superconducting film

Coherent microwave radiation has been directly detected in the frequency range of 0.15–200 MHz from superconducting thin films excited by an oscillating magnetic field directed perpendicular to the film plane. Due to pinning, a metastable state of the vortex system can be set up in the superconducting films. Vortex-jump synchronization reached via their electromagnetic interaction with external resonant circuit causes the generation of the electromagnetic radiation.     

Observation of an optical vortex beam from a helical undulator in the XUV region

The observation of an optical vortex beam at 60 nm wavelength, produced as the second‐harmonic radiation from a helical undulator, is reported. The helical wavefront of the optical vortex beam was verified by measuring the interference pattern between the vortex beam from a helical undulator and a normal beam from another undulator. Although the interference patterns were slightly blurred owing to the relatively large electron beam emittance, it was possible to observe the interference features thanks to the helical wavefront of the vortex beam. The experimental results were well reproduced by simulation.     

Sound scattering by a potential vortex

The scattering of a monopole wavefield by the flow of a potential vortex is considered by using a scheme due to Lighthill. The problem is two-dimensional, the monopole being a line source adjacent to a parallel line vortex. The cases of large and small separation of vortex and source are examined in detail. The far field density is asymptotically estimated in both limits. A quadrupole type radiation field is calculated in each case.     

Controlled vortex-sound interactions in atomic Bose-Einstein condensates

The low temperature dynamics of a vortex in a trapped quasi-two-dimensional Bose-Einstein condensate are studied quantitatively. Precession of an off-centered vortex in a dimple trap, embedded in a weaker harmonic trap, leads to the emission of sound in a dipolar radiation pattern. Sound emission and reabsorption can be controlled by varying the depth of the dimple. In a shallow dimple, the power emitted is proportional to the vortex acceleration-squared over the precession frequency, whereas for a deep dimple, periodic sound reabsorption stabilizes the vortex against radiation-induced decay.     

回声谐波放大型自由电子激光相干涡旋X射线产生方案

外种子型自由电子激光具有全相干、频谱稳定、极高亮度的优点,可以实现在超小空间和超快时间尺度下对物质结构的研究。具有特殊横向相位模式的光特别是具有螺旋相位的带轨道角动量的涡旋光已经在众多科学领域有了应用,基于自由电子激光原理产生的辐射横向模式基本上为简单的高斯模式,为产生具有横向螺旋相位的相干涡旋X射线,对基于回声谐波放大型(EEHG)自由电子激光产生涡旋光方案进行了深入研究,并且根据上海软X射线自由电子激光装置(SXFEL)的参数,进行了相关方案设计和模拟研究。三维模拟结果表明,外种子型EEHG自由电子激光可以产生峰值功率可达到GW量级的相干涡旋软X射线。     

Acoustic waves emitted by a vortex ring passing near a circular cylinder

Acoustic waves emitted by a vortex ring moving near a circular cylinder have been studied experimentally and theoretically. The vortex rings used in the experiments had a translational speed ν₀ in the range 26 ⪅ ν₀ ⪅ 58 m/s and a radius of about 4·7 mm comparable in size with the cylinder radius. The acoustic pressure signals were detected by four microphones in the far field, and analyzed by digital methods. The observed pressure p obeys the scaling law p ∞ ν₀³L⁻⁴, where L is the impact distance of the vortex path to the cylinder. The observed sound wave is of dipole radiation type, and the direction of the dipole axis rotates as the vortex position changes relative to the cylinder. The direction of the dipole axis is related to that of the normal to the plane of the vortex ring. The instantaneous resultant force exerted on the cylinder by the vortex motion has also been examined, and the magnitude and the direction determined experimentally as a function of time. The theory of vortex sound predicts that the wave profile is proportional to the second time derivative of the volume flux (of a hypothetical potential flow around the cylinder) through the vortex ring. The observed scaling law and dipole directivity of the pressure are in good agreement with the theoretical predictions. The pressure profiles are calculated by using the observed vortex motion. These profiles also agree well with the observed ones, confirming the validity of the theory.