Ponderomotive Force Research Articles

Second-harmonic generation of hollow Gaussian laser beams in inhomogeneous plasmas in the presence of wiggler magnetic field

Second-harmonic generation of hollow Gaussian laser beams (HGLBs) in inhomogeneous plasmas is investigated in the presence of an external wiggler magnetic field. The wiggler magnetic field and electric field of the laser pulse exert a ponderomotive force on electrons deriving electron velocity. Electron velocity, wiggler magnetic field and magnetic field of the laser pulse beat to exert a nonlinear ponderomotive force on electrons at the second-harmonic frequency. The nonlinear ponderomotive force can derive a nonlinear current density. Using linear and nonlinear current densities and wave equation in cylindrical coordinates, two equations obtaining different components of the electric field of second-harmonic wave (SHW) are derived. Results indicate that by increasing the external wiggler magnetic field, order of HGLB, rippled electron density, rippled wave number and initial intensity of the laser pulse, the amplitude of the excited SHW increases. Finally, it is found that by decreasing the initial width of the laser beam, the amplitude of SHW increases, attains a maximum and then decreases.

Fluctuations of the plasma potential in atmospheric pressure micro-plasma jets

Fluctuations in plasma floating potential in an atmospheric pressure micro-plasma jet are investigated experimentally. The transport of charged particles toward the ambient air is often seen to be affected by plasma fluctuations. The dependence of fluctuations on operating parameters such as applied voltage, gas flow rate, and gas mixture ratio (He:Ar) is investigated so as to be able to control the fluctuations in the intended application. A single pin probe measures the fluctuations in the jet in a region where the plasma emerges out of the capillary. The fluctuations are characterized by fast Fourier transform and time-frequency analysis (TFA). It is found that with an increase in applied voltage at a fixed flow rate (1 l/min), the fluctuations increase and reach a peak value at ∼11 kV and thereafter decrease at higher voltages due to a high value of discharge current (∼4.77 mA) at 11 kV arising from the intense ionization taking place in the jet. When the gas flow rate is increased at a fixed voltage (14 kV), the fluctuations get enhanced for flow rates beyond 2 l/min due to transition to turbulent flow occurring at a buoyancy induced Reynolds number of ∼474. In the case of gas mixing, an increase in the concentration of Ar and a reduction in He at a fixed applied power (80.6 W) and flow rate (1 l/min) give rise to a higher level of fluctuations, which is considered to be due to lower thermal conductivity and ionization potential of Ar, leading to gas heating, and ponderomotive force, giving rise to filamentation. The TFA shows the time evolution of fluctuation frequencies, which can be tuned by varying the experimental parameters. Understanding the influence of experimental parameters is crucial in controlling the fluctuations in the micro-plasma jet.

Electron Bernstein wave aided beat wave of Hermite-cosh-Gaussian laser beam absorption in a collisional nanocluster plasma

In this paper, we investigate the electron Bernstein wave aided beat wave of Hermite-cosh-Gaussian laser beam absorption in a collisional nanocluster plasma. The interactions of electric field profile of Hermite-cosh-Gaussian laser beams with electron clouds of nanoclusters cause the beat wave. The nonlinear ponderomotive force is generated through the beat wave of two Hermite-cosh-Gaussian laser beams. The time average power absorption per electron per unit area per unit time by electron Bernstein wave aided electric field of laser beat wave is derived. The graphical discussions were promised to achieve extreme absorption via spatial shape of Hermite-cosh-Gaussian laser beam and resonance condition of beat wave to surface plasmon frequency. The electron Bernstein waves parameters such as thermal velocity, cyclotron frequency and perpendicular component of wavenumber is also leading with beat wave for control and enhancement of absorption process in plasma embedded with nanocluster. The absorption is controlled by tuning the laser beam width, mode index, collisional frequency, clustered radius, and density.

Field quantization in quantum plasmas: Photon and plasmon charge and mass

AbstractWe study electromagnetic field quantization in isotropic quantum plasmas, thus connecting two distinct problems of quantum plasma physics. We include plasmons and transverse photon states. We show that field quantization in a plasma is equivalent to that of massive boson fields and discuss the qualitative differences with respect to standard quantization in a vacuum. We consider nonlinear effects and show that photon and plasmon charges can be associated with the ponderomotive force. We also discuss the conservation properties of these affective charges during nonlinear mixing processes. The existence of photon and plasmon charge and mass could be relevant to laboratory experiments and to astrophysics.

Dynamics of resonant self‐focusing on second harmonic generation of Gaussian laser beam in a rippled density plasma

AbstractResonant second harmonic generation by a Gaussian laser beam in a rippled density plasma is studied using the moment theory approach. The nonlinearity arises through the relativistic mass effect and ponderomotive forces. The laser beam creates a plasma channel and gives rise to electron density perturbation at the laser frequency. The density perturbation beats with electron quiver velocity to produce second harmonics. The ripple provides phase matching and makes the process a resonant one. The second harmonic power efficiency is increased effectively with density ripple. Self‐focusing causes enhancement in the efficiency of harmonic generation.

Effect of pulse enhancement on beat wave THz generation in a ripple density magnetized plasma

Generation of terahertz pulse by the method of nonlinear mixing of two lasers in a plasma which is magnetized and have ripple density is studied permitting the effect of pulse enhancement. Due to nonlinear ponderomotive force, coupling of electron flow with density ripples occurs which leads to production of nonlinear current. Terahertz radiation are resonantly driven by this nonlinear current at beat frequency. Since here magnetic field is transverse to propagation of laser so we have transverse component of current density, while density ripples are responsible for phase matching. Thus, enhancement of pulse occurs due to the mismatch in group velocity of laser pulses and terahertz radiation, which further results in saturation of amplitude of terahertz waves.

The feature of the wake-plasma of a space vehicle induced by modulational instability

The characteristics of modulation instability in the wake plasma of space vehicle are obtained, based on the nonlinear coupling equations of the wake plasma and space vehicle in non-steady-state and non-static limit. It is found that the modulation instability is strongly related to the disturbance angle. The growth rate of transverse modulation instability is always much greater than that of longitudinal ones. Then the characteristic scale of the localized fields and density cavitons in the wake plasma of space vehicle are mainly determined by the transverse modulation instability. The growth rate of modulation instability increasing with the ponderomotive force of pump wave field, but the characteristic scale decreasing with it. Furthermore, the growth rate of modulation instability has maximum value on the central axis of space vehicle in the wake plasma, and the value increasing with the speed of space vehicle. The present results can provide a more detailed theoretical reference for the detection of space vehicle in ionospheric and stealth aircraft in plasma environments.

Magnetically tuned hybrid resonance emission of terahertz waves from the interaction of intense laser beams with warm collisional inhomogeneous plasma

Enhancing the terahertz emission at hybrid resonant frequencies is observed from a warm collisional inhomogeneous plasma irradiated by intense laser beams in the presence of an external transverse magnetic field. To reach this result, we utilize the dielectric permittivity tensor of the plasma affected by the strength and the direction of the magnetic field and analyze the nonlinear current density induced by the ponderomotive force due to the beating of two Gaussian laser beams. The calculations reveal resonance emission at several frequencies, which depend on the thermal speed of the electrons, the wavenumber, the electron plasma frequency, the collision frequency, and the cyclotron frequency. However, the amplitude of THz waves at these frequencies mainly depends on the electron temperature as well as the external magnetic field strength.

A Method for Calculating Ponderomotive Forces and Mutual Inductances in Coaxial Axisymmetric Magnetic Systems

A Method for Calculating Ponderomotive Forces and Mutual Inductances in Coaxial Axisymmetric Magnetic Systems

Control of the Properties of Welded Joints in Wet Underwater Welding

The aquatic environment will negatively affect the processes that occur during the formation of a welded joint and leads to a deterioration in its properties. The welding conditions do not allow the use of traditional methods to improve the quality of the weld metal - preheating or subsequent heat treatment. One of the possible effective ways to solve this problem when welding with flux-cored wire is the use of external electromagnetic exposure. Generation of axial alternating control magnetic fields in the welding zone allows controlling the hydrodynamics of the molten weld pool. Because of the interaction of an external electromagnetic field with an electric field of the arc, ponderomotive forces appear in the weld pool, which leads to mixing of the liquid metal. Changing the direction of electromagnetic induction with a given frequency and interval, you can periodically change the direction of flow in the weld pool. Intensive reverse mixing of the melt occurs, during which the hydrogen accumulated in front of the crystallization front as a result of concentration compaction is removed from it, the melt temperature in the central and peripheral areas of the pool is averaged, which slightly reduces the crystallization rate of the weld pool, thereby increasing its degassing time. As a result, it was possible to significantly reduce the porosity of the weld metal (more than 8 times) and reduce the hydrogen content (more than 2.5 times). Metallographic studies showed a significant refinement of the structure both weld metal and the heat-affected zone (2 and 1.3 times, respectively), its homogenization and a decrease in the degree of defectiveness. Thus, the investigations showed that the external electromagnetic effect on the liquid metal pool in the process of wet welding underwater is a useful tool that allows you to control the processes that occur during the crystallization stage of the weld pool and significantly increase the ductility of the weld metal (more than 1.5 times).

High-Density Dynamics of Laser Wakefield Acceleration from Gas Plasmas to Nanotubes

The electron dynamics of laser wakefield acceleration (LWFA) is examined in the high-density regime using particle-in-cell simulations. These simulations model the electron source as a target of carbon nanotubes. Carbon nanotubes readily allow access to near-critical densities and may have other advantageous properties for potential medical applications of electron acceleration. In the near-critical density regime, electrons are accelerated by the ponderomotive force followed by the electron sheath formation, resulting in a flow of bulk electrons. This behavior represents a qualitatively distinct regime from that of low-density LWFA. A quantitative entropy index for differentiating these regimes is proposed. The dependence of accelerated electron energy on laser amplitude is also examined. For the majority of this study, the laser propagates along the axis of the target of carbon nanotubes in a 1D geometry. After the fundamental high-density physics is established, an alternative, 2D scheme of laser acceleration of electrons using carbon nanotubes is considered.

Non-Magnetic Solid Body in Ferrofluid Containers: Wall Effects

Experimental and numerical investigations of a magnetic ponderomotive force acting on non-magnetic body immersed in ferrofluid were carried out. The study was performed on a nonmagnetic sphere in a cylindrical container filled with the magnetic fluid and magnetized by a uniform magnetic field. The symmetry of the experimental setup allowed the simplification of the problem: 2D axisymmetric numerical simulation corresponded to the 1D experimental measurements. The magnetic ponderomotive force turned out to be non-monotonic: it has two extrema and one or three zero values depending on the geometrical parameters of the container and magnitude of the applied field intensity. It was shown that wall effects are crucial for this problem, because the ponderomotive force in the vicinity of the container’s bottom (or top) may change its sign (direction). This behaviour can be described only in the framework of the inductive approach, which takes into account all demagnetizing fields generated by the ferrofluid container. On the contrary, the simplified non-inductive approach is unable to explain the magnetic force behaviour, especially the wall effects.

The Whistler Traveling Wave Parametric Amplifier Driven by an Ion-Ring Beam Distribution from a Neutral Gas Injection in Space Plasmas

A new parametric amplifier model describes the observations of intensified whistler waves produced by a dedicated burn of the BT-4 engine on the Cygnus spacecraft during the NG-13 mission. Ground very low frequency (VLF) radio emissions at 25.2 kHz from a Navy NML transmitter in North Dakota were amplified by 20-30 dB during the Cygnus burn at 480-km altitude and recorded at 1060 km by the e-POP/RRI plasma wave receiver on the SWARM-E satellite. The amplification process starts with charge exchange between the exhaust molecules and the ambient O+ in the ionosphere to produce water vapor ions that spiral around the earth's magnetic field lines. This ion-velocity-ring distribution generates broadband, oblique-lower-hybrid (OLH) waves, which act as a pump for the parametric amplifier. The nonlinear ponderomotive force on the electrons causes the high-amplitude OLH pump to mix with the input whistler signal, yielding an OLH idler wave. Resonant mixing of the pump and idler electric fields promotes temporal growth in the amplitude of the whistler waves as they propagate through the exhaust cloud. The key features to rocket exhaust-driven amplification (REDA) process are broadband gain, bi-directionality along the magnetic field, pump depletion, phasing, and feedback. Pump depletion limits the intensity of the output whistler waves by wave energy conservation. The total wave energy is the electrostatic energy of the pump and idler lower hybrid waves plus the energy extracted by the propagating whistler signal. The whistler traveling wave parametric amplifier provides an efficient mechanism for active amplification of signals in space.

Constructions of new abundant traveling wave solutions for system of the ion sound and Langmuir waves by the variational direct method

In this paper, a novel and simple approach called the variational direct method (VDM) is applied to construct the exact traveling wave solutions of the system of ion sound and Langmuir waves (SEISLWs), which is under the influence of the ponderomotive force caused by a nonlinear force experienced by a charged particle in an inhomogeneous oscillating electromagnetic field due to high-frequency field. These exact solutions include the bright soliton, bright-like soliton, kinky-bright soliton, bright-dark soliton and periodic wave solutions, which are all presented through the numerical results in the form of 3-D plots. Obtained results show that the VDM is simple, straightforward and powerful, which is expected to open new perspectives for the traveling wave theory.

Hermite-cosh-Gaussian laser-induced third harmonic generation in plasma

Present work focuses on Hermite-cosh-Gaussian (HchG) laser-induced third harmonic generation (THG) in plasma on account of self-focusing. HchG laser beam gets self-focused due to ponderomotive force while propagating in plasma. Interaction of intense laser pulse results ponderomotive force on electrons and induce density perturbation that produces density oscillations at second harmonic frequency. Induced density perturbation beats with quiver velocity of electrons at laser frequency to produce a third harmonic nonlinear current. Application of wiggler magnetic field satisfies the phase matching condition which results resonant THG. Significant enhancement in the efficiency of THG is observed by using HchG laser profile for mode index m = 2. Small variation in decentered parameter also affects the THG efficiency which shows the sensitivity of decentered parameter.

Disruption mitigation using ion cyclotron wave in ADITYA tokamak

Controlling and mitigating plasma disruptions are a matter of serious concern for tokamak operation since they can cause serious damage to the machine. Dedicated experiments on disruption mitigation have been carried out in ADITYA (R = 75 cm, a = 25 cm), an Ohmically heated circular limiter tokamak. A rapid growth of magneto hydrodynamic (MHD) modes is found to be the major cause of disruptions in ADITYA tokamak. Stimulated disruptions induced by hydrogen gas puffing are successfully mitigated through stabilization of these MHD modes by applying a biased-electrode placed inside the last-closed flux surface prior to the gas injection. However, as biased electrodes cannot be placed inside the plasma in bigger tokamaks, the application of ion-cyclotron waves (ICWs) prior to disruption has been successfully attempted to mitigate disruptions through stabilization of MHD modes in ADITYA tokamak. The amplitude of MHD modes (m/n = 3/1, 2/1) is significantly reduced upon the application of ICW prior to disruption, and the stimulated disruptions are successfully mitigated by preventing the growth and overlapping of these MHD modes. These experimental results demonstrate that MHD driven disruptions may be mitigated due to stabilization of m = 2 modes by ponderomotive force of the IC waves.

Nonlinear interaction of elliptical q-Gaussian laser beams with plasmas with axial density ramp: effect of ponderomotive force

Theoretical investigation on optical self action effects of intense q-Gaussian laser beams interacting with collisionless plasmas with axial density ramp has been presented. Emphasis are put on investigating the dynamics of beam width and axial phase of the laser beam. Effect of the ellipticity of the cross section of the laser beam also has been incorporated. Using variational theory based on Lagrangian formulation nonlinear partial differential equation (P.D.E) governing the evolution of beam amplitude has been reduced to a set of coupled ordinary differential equations for the beam widths of the laser beam along the transverse directions. The evolution equation for the axial phase of the laser beam has been obtained by the Fourier transform of the amplitude structure of the laser beam from coordinate space to \((k_x, k_y)\) space. The differential equations so obtained have been solved numerically to envision the effect of laser-plasma parameters on the propagation dynamics of the laser beam.

Terahertz radiation generation by beating of two chirped laser pulses in a warm collisional magnetized plasma

Analytical equations of terahertz (THz) radiation generation based on beating of two laser beams in a warm collisional magnetized plasma with a ripple density profile are developed. In this regard, the effects of frequency chirp on the field amplitude of the terahertz radiation as well as the temperature and collision parameters are investigated. The ponderomotive force is generated in the frequency chirp of beams. Resonant excitation depends on tuning of the plasma beat frequency, magnetic field frequency, thermal velocity, collisional frequency, and effect of the frequency chirp with the plasma density. For optimum parameters of frequency and temperature the maximum THz amplitude is obtained.

Effect of orbital angular momentum of light on self-action effects of Laguerre Gaussian laser beams in collisionless plasmas

Nonlinear optical self-action effects like self-focusing, self-trapping of intense laser beams propagating through underdense plasmas have been investigated theoretically. The optical nonlinearity of the plasma has been modelled by ponderomotive force originating through intensity gradient over the cross section of the laser beam. In order to investigate the effect of orbital angular momentum of the photons on propagation characteristics of the laser beam, the irradiance over the cross section of the beam has been considered to be Laguerre Gaussian (LG). Effect of self-focusing of the laser beam on the evolution of its axial phase also has been investigated. Formulation is based on finding semi-analytical solution for the wave equation for the field of laser beam with the help of moment theory approach.

Harmonic generation by an interaction of laser with an array of anharmonic carbon nanotubes

A scheme of second and third harmonic generation from laser irradiated anharmonic carbon nanotubes (CNTs) embedded in silica is proposed. An intense ultra short laser pulse interacting with the array of CNTs displaces its electrons. When this displacement becomes comparable to the radius of electrons of CNTs, the restoring force of electrons varies nonlinearly with the displacement. As a result, plasmon resonance gets broadened and the large resonance absorption of the laser by the electrons of CNTs occurs. The power conversion efficiency of harmonic generation due to the anharmonicity of CNTs is much higher than that due to the ponderomotive force nonlinearity. The effect of amplitude modulated parameter on the amplitude of second and third harmonic is studied. The high amplitude of generated harmonics can be realized with the amplitude modulated laser as compared to ordinary unmodulated laser.