Sideband Waves Research Articles

The onset of parametric decay of lower hybrid waves during lower hybrid current drive experiments on Experimental Advanced Superconducting Tokamak

Abstract An ad hoc calculation is presented to interpret the growth of sideband waves during lower hybrid current drive (LHCD) experiments on Experimental Advanced Superconducting Tokamak (EAST). We adopt the solution of the non-linear dispersion relation for parametric instabilities to describe the growth rate of ion cyclotron quasi-modes (ICQMs) in the peripheral plasma, and use the WKB solution of the propagation of electrostatic waves in two dimensions to model the variation of the electric field in the scrape-off-layer (SOL). We then calculate the growth rates and amplification factors of the sideband waves associated with ICQM in the parallel and perpendicular coupling limits. The onset and substantial growth of the first down-shifted sideband of lower hybrid waves (LHWs) are associated with elevation of the plasma density in the outer SOL where the growth rate of ICQM in the parallel coupling limit increases sharply after the density is elevated. The peak frequencies associated with the first harmonic of ICQM obtained from calculating the amplification factors in the parallel coupling limit ( A para ) agree very well with the observed frequencies of the first down-shifted sideband of 2.45 GHz LHWs in EAST pulse 43 772 and 86 521. The consistency between the calculation and the observation indicates that the sideband waves are associated with parametric decay instability of ICQM excited in the outer SOL through parallel coupling. The amplitude of A para obtained from a single pass of LHWs through the SOL of EAST is, however, one order of magnitude smaller than unity, meaning that it remains elusive whether the substantial growth of sideband waves mainly occurs in front of the antenna or requires multiple passes of LHWs across the SOL region.

Higher order dispersions effect on high-order soliton interactions

The object of the research is deleting the interaction of the higher order soliton interaction by introducing the third and fourth order dispersions inside an optical fiber. The results are obtained by the simulation of the nonlinear Schrödinger equation, which models the propagation of solitons in the optical fiber using the method of Fast Fourier Transform. The interaction of two higher order solitons due to the attraction of their electric field can lead to losing the solitons' properties. Hence, this can prevent the use of solitons in high-bit-rate optical fiber communication systems because it increases the bit error rate, significantly limiting the potential of the communication system. To resolve this problem, we should diminish the bit rate error by avoiding the interaction of the co-propagative solitons when they are too close. It is well known that, during higher order soliton propagation in the presence of the third order dispersion, the irregular shape of the higher order soliton disappears, and a splitting towards its fundamental constituents occurs after a considerable propagation. As for the fourth order, dispersion gives rise to two dispersive wave sidebands on the red or blue side. Our results reveal that bringing two higher order solitons together in the presence of the fourth order dispersion, a series of interactions between the components generated after their fission is obtained. In the third-order distribution, besides the fourth-order diffusion, the rare form and the supercontinuum generated by the fission of the higher-order solitons disappear, and we get two fundamental solitons propagating in parallel with a temporal shift and some inconsiderable dispersive waves. The most important aspect is that both higher-order dispersions are able to suppress the interactions of higher-order solitons thanks to the time shift induced by the third-order distribution and the intermittent compression caused by the fourth-order scattering. These results can be obtained in practice inside the dispersion-engineered photonic crystal waveguide (PhC-wg), which allows for manipulating the high order dispersion.

Theoretical Study on Weibel instability in the existence of large amplitude Langmuir wave inside a Plasma

This paper investigates how an electrostatic Langmuir wave having large amplitude affects the Weibel instability (WI) in the existence of ions and an electron beam. Two Langmuir side band waves are produced by coupling of EM perturbation to the Langmuir wave. The Langmuir wave (LW) increases the growth rate beyond its linear value. Here, we noticed that the growth rate Γ(sec−1) scales linearly with the electron beam velocity vbe and 1/2 power of the electron beam density nbe. As we increase the density of ions inside the plasma, the growth rate stabilizes. Additionally, we find that the growth rate is very sensitive to the plasma frequency of ions. Therefore, our work finds an application in space, galactic cosmic rays and supernovas. Also, our work covers a range of application from the development of fusion power to understand the various astrophysical phenomena.

Electron Bernstein wave aided cosh-Gaussian laser beam absorption in plasma

In this paper, we investigate the electron Bernstein wave aided cosh-Gaussian laser beam absorption in a plasma with static magnetic field. The cosh-Gaussian laser beam has potential to couple with pre-existed electron Bernstein wave and the coupled wave has frequency ω1=ω0+ω and wave number k1=k0+k where ω0 and ω are the laser and electron Bernstein wave frequencies respectively and k0 and k are the wave numbers of laser and electron Bernstein wave respectively. The oscillatory velocity due to laser couples with the density perturbation due to electron Bernstein waves to produce nonlinear current density driving the electromagnetic sideband wave. An expression of absorption coefficient is analytically derived when the nonlinear current density is in phase with cosh-Gaussian laser field. The graphical discussion promises the more absorption in plasma by varying laser beam decentered parameter, beam width, laser frequency, electron cyclotron frequency, parameter b, and electron Bernstein wave frequency. The laser beam decentered parameter behaves as a very sensitive parameter. The absorption process is found to be very robust even for very small change in electron Bernstein wave frequency as compared with laser frequency. This tunable and efficient absorption theory might be applicable in heating of plasma.

Narrow-edged beamforming based on individual phase inversion in amplitude-modulated wave for parametric array loudspeaker

The parametric array loudspeaker (PAL) can realize sharp directivity using the straightness of ultrasound. Some recent studies have applied phase array approaches to PAL to control the directivity. The PAL is composed of a large aperture emitter consisting of multiple small ultrasonic transducers to exhibits sharper directivity and reproduce audible sound with sufficient sound pressure. In contrast, the directivity of the small-scale PAL may diffuse. In the present paper, we focus on forming a narrow acoustical beam for practical use of the PAL, so that it is able to control the area where the audible sound reproduced by the PAL can be heard. We propose a narrow-edged beamforming method based on individual phase inversion in the amplitude-modulated wave. The PAL is divided into interior part and exterior part, and the phases of the components in the signal fed to the exterior of the PAL are controlled individually. In the processing of the proposed method, the lower frequency component between the carrier and the sideband wave is phase-inverted in the signal fed to the exterior of the PAL. The proposed method uses phase cancellation of both ultrasound and demodulated audible sound to form a narrower beam. This method can also be combined with beam steering techniques to form a narrow steered beam. The experimental results demonstrate the effectiveness of the proposed method in forming a narrower beam than the common PAL.

Neutral beam driven ion cyclotron instability of lower hybrid wave in a tokamak plasma

The effect of dust grains on the parametric coupling of neutral beam driven ion–cyclotron wave instability with a lower hybrid pump wave is studied. A high amplitude lower hybrid pump, which is launched into a tokamak for heating purposes in the presence of neutral beam driven ion-cyclotron waves, can excite the parametric coupling involving two lower hybrid sidebands. In a tokamak, the lower hybrid waves result in parametric excitation of the ion-cyclotron mode and quasi-modes near the edge when the electron oscillatory velocity is greater than the sound velocity. This parametric coupling increases the growth rate of instability when the lower sideband wave is resonant. Moreover, the presence of dust grains in the tokamak plasma, their radius, and the number density significantly affect the growth rate of the instability, which in turn can affect the advanced stage operations of a tokamak. The growth rate of parametric instability scales with the amplitude of the pump wave. The growth rate is found to be linearly increased with the dust grain density, but it decreased with increasing size of dust grains, which means large sized dust grains stabilize the instability. The theoretical results explained in the present paper are very helpful in explaining the complexity in the plasma properties of a tokamak due to the dust–plasma interactions, which can diminish the performance of the International Thermonuclear Experimental Reactor due to potential safety issues.

Stimulated forward Raman scattering in density rippled magnetized plasma

In inertial confinement fusion, the development of stimulated Raman scattering is observed in the presence of an azimuthal magnetic field and a density rippled plasma. The Gaussian laser beam propagating through a density rippled plasma is amplified by forward Raman scattering in the presence of an azimuthal magnetic field, resulting in two radially localised electromagnetic sideband waves and a lower hybrid wave. It is observed that as the laser power is increased, the growth rate in the presence of density rippled plasma reduces the probability of plasma pre-heating in ICF.

Electrostatic electron plasma wave envelope with nonlinear Landau damping in nonthermal plasmas

The characteristics of nonlinear electrostatic electron plasma wave modulation are investigated in collisionless plasmas in the context of the Cairns and Kappa (generalized Lorentzian distribution with spectral index κ) nonthermal electron distribution, with particular attention to the nonlinear Landau damping process with resonance effect at the group velocity of the modulated wave. A modified nonlinear Schrödinger equation (mNLSE) with a nonlocal nonlinear Landau damping term governs the dynamics of the wave modulations. The results predict a far equilibrium region () where the Kappa distributed electrons lead to new dispersive corrections, an overall reduction of the carrier wave frequency () and the phase velocity and the existence of parameter regions where . On the contrary, the results in the near equilibrium region () are qualitatively similar to that of Cairns distributed electrons. The carrier waves always become damped by transferring energy to the side band waves due to nonlinear Landau damping and nonthermal particles enhance the energy transfer rate. The mNLSE is simulated extensively by a 2nd order split-step Fourier method with various initial pulses in different parameter regions.

Stimulation of Waves and Instabilities in Response to Irradiation of Electromagnetic Fields into Plasmas

Plasma is a unique phase of matter constituting positively or negatively charged atoms, excited atoms, neutral atoms, electrons, radicals, etcetera displaying a unique role in the nuclear fusion research besides studying electrical discharges in the domain of switching devices and biomedical applications lately. We discuss in this extensive proposition the fundamental plasma characteristics such as Debye length, plasma oscillations, plasma sheath and condition for sustainability and confinement of plasmas, besides examining the elementary waves in plasmas namely zero waves, electron plasma wave and ion plasma wave. The inherent electron plasma wave and ion plasma wave is associated with the driving of plasma currents which in turn depends upon the density perturbation and thermal velocities of the electrons and ions. The application of external electromagnetic radiation such as laser (pump wave) into the plasma modifies the dispersion relations of electron and ion plasma wave, respectively. The laser stimulates a plethora of waves in the plasma and undergoes remarkable physical phenomena such as self-focusing and filamentation of laser beams. The excitation of sideband waves of the laser beams into the plasma plays a key role in imparting ponderomotive force on the electron plasma waves leading to turbulence in the plasmas due to coupling of the waves. The oscillatory velocity of the electron due to pump wave, plasma density perturbation, ponderomotive force and current densities are associated with the excitation of instabilities in the plasma. Conclusively, such waves and instabilities in unmagnetized and magnetized plasma is comprehensively studied and concluded by proposing the investigation of unexplored twisted electromagnetic wave-plasma interaction.

Effect of dust grains on the parametric coupling of a lower hybrid wave driven ion cyclotron wave in a tokamak plasma

In this article, the effect of dust charge fluctuations on the parametric upconversion of a lower hybrid wave into an ion cyclotron wave and a side band wave in a two-ion species tokamak plasma is studied. When the oscillatory velocity of plasma electrons is a few percent of the sound velocity, the lower hybrid wave becomes unstable and decays into two modes: an ion cyclotron wave mode and a low frequency lower hybrid side band wave. Furthermore, a ponderomotive force by a lower hybrid pump and a side band wave is exerted on the existing electrons, which drives the ion cyclotron decay mode. The presence of negatively charged dust grains and their shape, size, radius, and density influence the instability. The growth rate of instability is calculated by considering typical existing D–T (Deuterium–Tritium) dusty plasma parameters, and it is observed that the growth rate increases with the relative density of dust grains, number density of dust grains, oscillatory velocity of electrons, and amplitude of pump waves. However, the normalized growth rate increases with the unstable wave frequency, and it also increases as we increase the ratio of deuterium to tritium density. Here, the growth rate decreases with the increase in the size of dust grains and electron cyclotron frequency. The theoretical results summarized in the present study are able to efficiently elaborate the complexity produced in plasma properties in a tokamak due to the dust–plasma interactions, which are briefly discussed here.

Generation of ion cyclotron instability by parametric coupling of gyrating ion beam with lower hybrid wave in a complex plasma

In a two-ion species tokamak plasma, with the help of gyrating ion beam was to investigate the impact of dust charge fluctuations on the parametric up-conversion of lower hybrid wave (LHW) into an upper side band wave and an ion cyclotron wave. On the existing electrons, through an upper side band wave and lower hybrid pump, a ponderomotive force has been exerted which in turn drives the ion cyclotron decay mode. The growth rate of instability is calculated by considering typical existing D-T (Deuterium-Tritium) dusty plasma parameters and observed that with an increment in amplitude of pump waves, wave number of ion cyclotron mode, number density of dust grains and relative density of dust grains, an enhanced growth rate is reported. However, a reduction in the growth rate has been detected with enhancing dust grain size and electron cyclotron frequency. The findings of this work can play an imperative role in magnetized plasma sources used in plasma material interaction / surface processing. The tokamak operation is severely affected by the various sizes and density of dust grains, which in turn, considerably affect the choice of material requisite in tokamak.

Controlled beat-wave Brillouin scattering in the ionosphere

Stimulated Brillouin scattering experiments in the ionospheric plasma using a single electromagnetic pump wave have previously been observed to generate an electromagnetic sideband wave, emitted by the plasma, together with an ion- acoustic wave. Here we report results of a controlled, pump and probe beat-wave driven Brillouin scattering experiment, in which an ion-acoustic wave generated by the beating of electromagnetic pump and probe waves, results in electromagnetic sideband waves that are recorded on the ground. The experiment used the EISCAT facility in northern Norway, which has several high power electromagnetic wave transmitters and receivers in the radio frequency range. An electromagnetic pump consisting of large amplitude radio waves with ordinary (O) or extraordinary (X) mode polarization was injected into the overhead ionosphere, along with a less powerful probe wave, and radio sideband emissions observed on the ground clearly show stimulated Brillouin emissions at frequencies agreeing with, and changing with, the pump and probe frequencies. The experiment was simulated using a numerical full-scale model which clearly supports the interpretation of the experimental results. Such controlled beat-wave experiments demonstrate a way of remotely investigating the ionospheric plasma parameters.

Audio Hotspot Attack: An Attack on Voice Assistance Systems Using Directional Sound Beams and its Feasibility

We propose a novel attack, called an “Audio Hotspot Attack,” which performs an inaudible malicious voice command attack, by targeting voice assistance systems, e.g., smart speakers or in-car navigation systems. The key idea of the approach is to leverage directional sound beams generated from parametric loudspeakers, which emit amplitude-modulated ultrasounds that will be self-demodulated in the air. Our work goes beyond the previous studies of inaudible voice command attack in the following three aspects: (1) the attack can succeed on a long distance (3.5 meters in a small room, and 12 meters in a long hallway), (2) it can control the spot of the audible area by using two directional sound beams, which consist of a carrier wave and a sideband wave, and (3) the proposed attack leverages a physical phenomenon i.e., non-linearity in the air, to attack voice assistance systems. To evaluate the feasibility of the attack, we performed extensive in-lab experiments and a user study involving 20 participants. The results demonstrated that the attack was feasible in a real-world setting. We discussed the extent of the threat, as well as the possible countermeasures against the attack.

Kinetic theory of effect of dust charge fluctuations on the parametric decay of lower hybrid wave instability by relativistic runaway electrons in tokamak

Dust charge fluctuation effect on the parametric decay of lower hybrid wave instability by relativistic runaway electrons is studied in a tokamak using kinetic treatment. Parametric upconversion of lower hybrid pump waves into relativistic runaway electron mode and upper sideband mode is described. A ponderomotive force is exerted on the runaway electrons by lower hybrid pump wave possessing large amplitude and upper sideband wave, which drives the runaway electron mode. The coupling of the oscillatory velocity of electrons with density perturbations produces nonlinear density perturbations on the upper sideband frequency. As a result, runaway generation is enhanced by a lower hybrid wave and the growth rate of the instability is measured as the square of the amplitude of the pump wave. Moreover, the presence of dust charge fluctuations and their number density in the tokamak have an appreciable effect on the growth rate of lower hybrid wave instabilities, which in turn affects the actual performance of the ITER due to the potential safety and operational issues.

Stimulated Raman Scattering of X-Mode Laser in a Plasma Channel

Stimulated Raman forward scattering (SRFS) of an intense X-mode laser pump in a preformed parabolic plasma density profile is investigated. The laser pump excites a plasma wave and one/two electromagnetic sideband waves. In Raman forward scattering, the growth rate of the parametric instability scales as two-third powers of the pump amplitude and increases linearly with cyclotron frequency.

A Review of Bolt Tightening Force Measurement and Loosening Detection.

Pre-stressed bolted joints are widely used in civil structures and industries. The tightening force of a bolt is crucial to the reliability of the joint connection. Loosening or over-tightening of a bolt may lead to connectors slipping or bolt strength failure, which are both harmful to the main structure. In most practical cases it is extremely difficult, even impossible, to install the bolts to ensure there is a precise tension force during the construction phase. Furthermore, it is inevitable that the bolts will loosen due to long-term usage under high stress. The identification of bolt tension is therefore of great significance for monitoring the health of existing structures. This paper reviews state-of-the-art research on bolt tightening force measurement and loosening detection, including fundamental theories, algorithms, experimental set-ups, and practical applications. In general, methods based on the acoustoelastic principle are capable of calculating the value of bolt axial stress if both the time of incident wave and reflected wave can be clearly recognized. The relevant commercial instrument has been developed and its algorithm will be briefly introduced. Methods based on contact dynamic phenomena such as wave energy attenuation, high-order harmonics, sidebands, and impedance, are able to correlate interface stiffness and the clamping force of bolted joints with respective dynamic indicators. Therefore, they are able to detect or quantify bolt tightness. The related technologies will be reviewed in detail. Potential challenges and research trends will also be discussed.

Numerical Investigation of Parametric Frequency Dependence in the Modeling of Octave-Spanning Kerr Frequency Combs

A generalized Lugiato-Lefever equation in frequency domain for the modeling of Kerr frequency combs, including full-order frequency dependence of all parameters, is derived and solved without sacrifice of computational time. The proposed model shows high accuracy and efficiency in simulations, agreeing well with previous reports. Based on the proposed model, the influence of parametric frequency dependence is systemically investigated by generating octave-spanning combs in a silicon-nitride micro-ring-resonator with an air slot. The device is dispersion-tailored for broadband comb generation, and its parameters are strongly dependent on frequency. When considering three kinds of parametric influence separately, we reach the following conclusions. The frequency dependent propagation constant determines whether the dispersive wave is generated, matching well with the phase-matching condition. Besides, the frequency dependence of effective area and nonlinear refractive index can both introduce spectral blue-shift. Combining full-order frequency dependence of the three parameters together, a dispersive wave sideband emerges and this phenomenon has never been reported. Our work reveals that full-order frequency dependence of all parameters can be considered with high computational efficiency, which enables precise prediction of dispersive wave frequency for f-2f self-referencing and opens up more reliable avenues for the understanding of Kerr frequency comb generation, especially octave-spanning cases.

Nonlinear frequency shift of acoustic waves in semiconductor plasmas

The evolution of nonlinear instability of an electron acoustic wave from the coupling of pair modes of slightly different frequencies in semiconductor plasmas is studied. The linear dispersive properties of pair modes, viz., pump wave and sideband wave, are analyzed on employing the quantum hydrodynamic model. The linear fields of pair modes couple through a second order convective term to unfold the nonlinear beat frequency of the electron acoustic mode. The nonlinear dispersion relation leads to the derivation of the growth rate of three-wave parametric instability. The quantum effects of semiconductor electrons are taken through the exchange-correlation potential, the Bohm potential, and statistical Fermi pressure. The growth rate is also analyzed graphically. This study seeks its applications in semiconductor devices.

Temporal characteristics of relativistic stimulated Brillouin scattering of a laser in plasmas

This article presents our study of the relativistic stimulated Brillouin scattering (SBS) process of a laser in a plasma in the presence of an electron–ion collision damping coefficient by following the energy and momentum conservation. The change in plasma density and the inclusion of the damping coefficient of the electron–ion collision modify the dispersion relations of the pump wave, ion acoustic wave, and sideband waves; consequently, the temporal characteristics of the interacting waves have been affected. The electron–ion collisions and relativistic mass effect are seen to be important to the temporal characteristics of the interacting waves. In this work, we primarily introduce the physical model of SBS under different plasma parameter conditions. The influence of collisional plasma parameters on the temporal evolution of SBS is analyzed. This work may be crucial to understand laser energy transfer in the hohlraum plasma in inertial confinement fusion.

A conservative scheme for simulation of free-surface turbulent and wave flows

A numerical scheme with good conservation properties is developed for the simulation of free-surface turbulent and viscous wave flows using a surface-fitted curvilinear grid. The Navier–Stokes equations are written in a strong conservative formulation with respect to the curvilinear coordinates, and are discretized by a pseudo-spectral method in the horizontal directions and a finite-difference method in the vertical direction. Large-eddy simulation (LES) is implemented with the conservative scheme to extend the simulation capability to turbulent flows with higher Reynolds numbers. Fully nonlinear kinematic and dynamic boundary conditions are implemented at the free surface. The numerical scheme is validated using a variety of wave and vortical flow test cases. The results show good agreement with previous theoretical and numerical predictions, whereas the present scheme achieves significant improvement in the conservation of mass and momentum over the non-conservative scheme developed by Yang & Shen [1]. Meanwhile, the present conservative scheme is found to be more stable than the non-conservative scheme for the simulation of sideband waves and broadband waves. The effect of viscous dissipation on the long-term nonlinear wave evolution is also captured by the present scheme. The ability of the present scheme for simulating long-term wave-current-turbulence interaction is demonstrated by the computation of Langmuir circulation, for which the non-conservative scheme produces significant errors in mass and momentum conservation and the simulation fails. Flow features of the Langmuir circulation, such as the counter-rotating vortices and converging-diverging zones, have been successfully captured with our numerical scheme. The turbulence statistics also agree with the characteristics of Langmuir circulation.