Nonlinear gain of a dipole antenna immersed in a collisionless plasma

This paper presents an investigation of the effect of collisionless plasma on the equatorial gain of a high power dipole antenna. The plasma experiences a ponderomotive force in the non-uniform radiation field and the electron density gets redistributed; the plasma gets depleted in the centre and the near equatorial zone of the antenna. This nonuniformity in the electron density (hence in the dielectric constant) tends to converge the electromagnetic power to the equatorial zone. For typical parameters a 20% (i.e., ∼1 dB) enhancement in the gain of the antenna has been predicted within the limits of the perturbation theory. Work partially supported by NSF (USA)     

Second Harmonic Generation of Self Focused Cosh‐Gaussian Laser Beam in Collisionless Plasma

This paper presents an investigation of self‐focusing of a Cosh‐Gaussian (ChG) laser beam and its effect on second harmonic generation in collisionless plasma. In the presence of ChG laser beam the carriers get redistributed from high field region to low field region on account of ponderomotive force as a result of which a transverse density gradient is produced in the plasma which in turn generates an electron‐plasma wave at pump frequency. Generated plasma wave interacts with the incident laser beam and hence generates its second harmonics. Moment theory has been used to derive differential equation governing the evolution of spot size of ChG laser beam propagating through collisionless plasma. The differential equation so obtained has been solved numerically. The effect of decentered parameter, intensity of ChG laser beam and density of plasma on self‐focusing of the laser beam and second harmonic yield has been investigated. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dust-Lower-Hybrid Surface Waves in Classical and Degenerate Plasmas

The dispersion relation for general dust low frequency electrostatic surface waves propagating on an interface between a magnetized dusty plasma region and a vacuum is derived by using specular reflection boundary conditions both in classical and quantum regimes. The frequency limit ω«ω_ci«ω_ce is considered and the dispersion relation for the Dust-Lower-Hybrid Surface Waves (DLHSW's) is derived for both classical and quantum plasma half-space and analyzed numerically. It is shown that the wave behavior changes as the quantum nature of the problem is considered.     

Microwave ponderomotive action on the inhomogeneous collisionless and collisional plasmas

The nonlinear interaction of a high-power microwave (MW) with an unmagnetized inhomogeneous plasma is investigated in collisionless and collisional regimes. The electron density distribution and the nonlinear wave equation in an inhomogeneous plasma are obtained by taking into account the ponderomotive force due to the high-power MW. It is shown that the electron density distribution becomes very steepened in the presence of the ponderomotive force. In the collisional regime, the expression for electron temperature is also found by considering ohmic heating. It is indicated that the amplitude of oscillations of the electron temperature and dielectric permittivity increases and the wavelength of these oscillations decreases with increasing energy flux, hence modulation occurs.     

Self-focusing of Gaussian laser beam in collisionless plasma and its effect on stimulated Raman scattering process

This paper presents an investigation of self-focusing of Gaussian laser beam in collisionless plasma and its effect on stimulated Raman scattering process. The pump beam interacts with a pre-excited electron plasma wave thereby generating a back-scattered wave. On account of Gaussian intensity distribution of laser beam, the time independent component of the ponderomotive force along a direction perpendicular to the beam propagation becomes finite, which modifies the background plasma density profile in a direction transverse to pump beam axis. This modification in density affects the incident laser beam, electron plasma wave and back-scattered beam. We have set up the non-linear differential equations for the beam width parameters of the main beam, electron plasma wave, back-scattered wave and SRS-reflectivity by taking full non-linear part of the dielectric constant of collisionless plasma with the help of moment theory approach. It is observed from the analysis that focusing of waves greatly enhances the SRS reflectivity.     

On the amplification mechanism of the ion-channel laser

The amplification mechanism of the ion-channel laser (ICL) in the low-gain regime is studied. In this concept, a relativistic electron beam is injected into a plasma whose density is comparable to or lower than the beam's density. The head of the electron beam pushes out the plasma electrons, leaving an ion channel. The ion-focusing force causes the electrons to oscillate (betatron oscillations) about the axis and plays a role similar to the magnetic field in a cyclotron autoresonance maser (CARM). Radiation can be produced with wave frequencies from microwaves to X-rays depending on the beam energy and plasma density: ω~2γ^3/2ω_pe, where γ is the Lorentz factor of the beam and ω_pe is the plasma frequency. Transverse (relativistic) bunching and axial (conventional) bunching are the amplification mechanisms in ICLs; only the latter effect operates in free-electron lasers. The competition of these two bunching mechanisms depends on beam velocity ν_0z; their dependences on ν_0z cancel for the cyclotron autoresonance masers. A linear theory is developed to study the physical mechanisms, and a PIC (particle-in-cell) simulation code is used to verify the theory. The mechanism is examined as a possible explanation for experimentally observed millimeter radiation from relativistic electron beams interacting with plasmas     

Effects of relativistic and ponderomotive nonlinearities on the interaction of high-power laser beam with an inhomogeneous warm plasma

The influence of relativistic-ponderomotive nonlinearities and the plasma inhomogeneity on the nonlinear interaction between a high-power laser beam and a warm underdense plasma are studied. It is clear that the relativistic ponderomotive force and the electron temperature modify the electron density distribution and consequently change the dielectric permittivity of the plasma. Therefore, by presenting the modified electron density and the nonlinear dielectric permittivity of the warm plasma, the electromagnetic wave equation for the propagation of intense laser beam through the plasma is derived. This nonlinear equation is numerically solved and the distributions of electromagnetic fields in the plasma, the variations of electron density, and plasma refractive index are investigated for two different background electron density profiles. The results show that the amplitude of the electric field and electron density oscillations gradually increase and decrease, during propagation in the inhomogeneous warm plasma with linear and exponential density profiles, respectively, and the distribution of electron density becomes extremely sharp in the presence of intense laser beam. It is also indicated that the electron temperature and initial electron density have an impact on the propagation of the laser beam in the plasma and change the plasma refractive index and the oscillations' amplitude and frequency. The obtained results indicate the importance of a proper choice of laser and plasma parameters on the electromagnetic field distributions, density steepening, and plasma refractive index variations in the interaction of an intense laser beam with an inhomogeneous warm plasma.     

On the effect of long-wavelength electron plasma waves onlarge-angle stimulated Raman scattering of short laser pulse in plasmas

Spectral features of a large-angle stimulated Raman scattering (LA SRS) of a short electromagnetic pulse in an underdense plasma, which are caused by the presence in a plasma of a given linear long-wavelength electron plasma wave (LW EPW), are investigated. It is shown that the LW EPW, whose phase velocity coincides with a group velocity of a pulse and a density perturbation normalized to a background electron density δn_LW/n₀ exceeds the ratio of the electron plasma frequency to the laser frequency ω_pe/ω₀ suppresses the well-known Stokes branch of the weakly coupled LA SRS. Under the same condition, the anti-Stokes band appears in the spectrum of the scattered radiation. Variation of a scattering angle and an electron temperature do not significantly modify qualitative features of the effect. In the case of strongly coupled LA SRS, the maximum of the increment is decreased by nearly one-half for δn_LW/n₀~(a₀ω_pe /ω₀)^2/3≫ω_pe /ω₀, where a₀ is an amplitude of an electron quiver velocity in the laser field normalized to a speed of light c, and it decreases further with an increase in plasma density perturbation in LW EPW     

General Formulation for the Ponderomotive Force in a Warm Two Fluid Magnetized Plasma

A general expression for the ponderomotive force in a warm, inhomogeneous, nonstationary, magnetized and collisionless two fluid plasma in the presence of electromagnetic field is obtained. We start with two fluid equations viz. continuity, momentum equations and Maxwell's equation. Expanding the physical quantities in terms of a high frequency part and a slowly varying part and averaging the quantities over one period of the high frequency, we have obtained the expression for the ponderomotive force. The results are compared with the results of earlier workers under different approximations.     

Variable density formation by intense microwave beams near the critical density

Modification of electron density of an inhomogeneous, unmagnetized plasma by the relativistic ponderomotive force of intense microwave beams near the critical density is studied. Using the Maxwell and fluid equations and taking into account the relativistic mass, relativistic ponderomotive force, linear inhomogeneity for electron temperature, and tangential inhomogeneity for electron density, the non‐linear electron density, non‐linear dielectric permittivity, and non‐linear wave equations are derived. Results show that positive and negative values of σ₁ and σ₂ (degree of inhomogeneity of the background electron density and electron temperature, respectively) parameters can affect the electric and magnetic field profiles. In addition, profiles of the non‐linear electron density indicate that by decreasing the σ₁ parameter, the amplitude of the peaks increases near the critical density. For positive values of the σ₂ parameter, by increasing this parameter the amplitude of the peaks increases, while for negative values of the σ₂ parameter, by decreasing this parameter the amplitude of the peaks increases.     

New method for polarization measurements of magnetic fields indense plasmas

Measurements of magnetic fields based on observations of the Zeeman splitting ω_B of spectral lines is a virtually impossible task in dense plasmas of powerful Z-pinches where the Stark splitting in the ion microfields ω_F is much greater than ω_B. In this situation, much better diagnostics of magnetic fields can be achieved through polarization difference contours obtained by subtracting profiles of the same spectral line observed in two orthogonal linear polarizations. In this way the obscuring role of the Stark effect is significantly diminished. In the present paper it is shown that the most sensitive and accurate measurements of the magnitude and the direction of the magnetic field in a dense plasma can be conducted employing central Stark components of hydrogen or hydrogen-like spectral lines. The polarization contour of a central Stark component turned out to be much more sensitive to the magnetic field than the polarization contour of a lateral component of the same line, namely by a factor of (ω_F/ω_B) ³≫1. This constitutes a drastic enhancement of the previously suggested method that had used the polarization contour of a lateral Stark component because in dense plasmas a typical value of the above factor is (ω_F/ω_B)³⩾10³ . The new method can also be used for laser fusion plasmas and for some astrophysical objects such as magnetic white dwarfs     

Quantum effects on magnetization due to ponderomotive force in cold quantum plasmas

The quantum effects on the magnetization due to the ponderomotive force are investigated in cold quantum plasmas. It is shown that the ponderomotive force of the electromagnetic wave induces the magnetization and cyclotron motion in quantum plasmas. We also show that the magnetic field would not be induced without the quantum effects in plasmas. It is also found that the quantum effect enhances the cyclotron frequency due to the ponderomotive force related to the time variation of the field intensity. In addition, it is shown that the magnetization diminishes with an increase of the frequency of the electromagnetic field.     

Compression of laser radiation in plasmas using electromagnetic cascading

Compressing high-power laser beams in plasmas via generation of a coherent cascade of electromagnetic sidebands is described. The technique requires two copropagating beams detuned by a near-resonant frequency Omega approximately < omega(p). The ponderomotive force of the laser beat wave drives an electron plasma wave which modifies the refractive index of plasma so as to produce a periodic phase modulation of the laser field with the beat period tau(b) = 2pi/Omega. A train of chirped laser beat notes (each of duration tau(b)) is thus created. The group velocity dispersion of radiation in plasma can then compress each beat note to a few-laser-cycle duration. As a result, a train of sharp electromagnetic spikes separated in time by tau(b) is formed. Depending on the plasma and laser parameters, chirping and compression can be implemented either concurrently in the same plasma or sequentially in different plasmas.     

有质动力和静电分离场对激光等离子体流体力学状态的影响

利用建立在欧拉坐标系上的一维电子_离子双流双温流体力学程序, 模拟了超短脉冲强激光 (1×1015W/cm2, 150fs)与线性密度梯度等离子体相互作用的流体力学过程. 模拟结果显示，入射激光与临界密度面的反射光叠加，在临界密度以下区域形成局域驻波， 产生的强有质动力在低密区驱动电子形成周期性密度结构——Bragg光栅，激光的反射被增 强. 临界密度处有质动力将等离子体分成向内和向外运动的两部分. 由于离子所受的有质动 力和热压强的梯度力远小于电子，体系产生了强静电分离场，离子的运动主要由该静电分离 场决定. 对双流双温模型和单流双温模型的模拟结果进行了比较. 当有质动力和热压强梯度 力较大时，两种模型对等离子体流体力学状态的描述有明显差异，单流双温模型无法描述此 时的流体力学状态. 关键词： 有质动力 密度调制 双流双温流体力学模型 单流模型     

Second harmonic generation of high power Cosh-Gaussian beam in cold collisionless plasma

The purpose of this study is to explore the second harmonic generation (SHG) of a high power Cosh-Gaussian beam in cold collisionless plasma. The ponderomotive force causes carrier redistribution from high field to low field region in presence of a Cosh-Gaussian beam thereby producing density gradients in the transverse direction. The density gradients so produced the results in electron plasma wave (EPW) generation at the frequency of the input beam. The EPW interacts with the input beam resulting in the production of 2nd harmonics. WKB and paraxial approximations are employed for obtaining the 2nd order differential equation describing the behavior of the beam's spot size against normalized distance. The impact of well-established laser-plasma parameters on the behavior of the beam's spot size and SHG yield are also analyzed. The focusing behavior of the beam and SHG yield is enhanced with an increase in the density of plasma, the radius of the beam and the decentred parameter, and with a decrease in the intensity of the beam. The results of the current problem are really helpful for complete information of laser-plasma interaction physics.     

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.     

Characteristics of single and dual radio-frequency (RF) plasma sheaths

The characteristics of radio-frequency (RF) plasma sheaths have been topics of much scientific study for decades, and have also been of great importance in the manufacture of integrated circuits and fabricating microelectromechanical systems (MEMS), as well as in the study of physical phenomena in dusty plasmas. The sheaths behave special properties under various situations where they can be treated as collisionless or collisional, single-or dual-RF, one-or two-dimensional (1D or 2D) sheaths, etc. This paper reviews our recent progress on the dynamics of RF plasma sheaths using a fluid method that includes the fluid equations and Poission’s equation coupled with an equivalent circuit model and a hybrid method in which the fluid model is combined with the Monte-Carlo (MC) method. The structures of RF sheaths behave differently in various situations and plasma parameters such as the ion density, electron temperature, as well as the external parameters such as the applied frequency, power, gas pressure, magnetic field, are crucial for determining the characteristics of plasma sheaths.     

Theoretical estimates of radial electric fields and plasma rotation induced in H-1NF by RF waves

The purpose of this study is to estimate, for planned experiments at the H-1NF heliac, nonlinear variations of the stationary radial electric field, and poloidal and toroidal plasma rotation, that result from ponderomotive forces exerted by large amplitude RF waves in the H-1NF. Similarly as in the previous studies of nonlinear transport effects induced by RF waves, the nonlinear ponderomotive force effects on the radial electric field, toroidal and poloidal plasma rotation become important for dissipated powers of the order of 1 MWm^–3 for RF waves with frequency of about 10 MHz. At these high RF powers, the nonlinear ponderomotive force effects might therefore result in important changes in plasma confinement and RF wave coupling in H-1NF.This work has been partially supported by the Australian National University, by the DIST Department of the Australian Government, by the Czech Grant Agency grants No. 202/96/1355 and 1350, and by the Queen Elisabeth II grant administered by G.G. Borg. The author is grateful to G.G. Borg and R.L. Dewar for many stimulating discussions.