Allowable propagation of short pulse laser beam in a plasma channel and electromagnetic solitary waves

Nonparaxial and nonlinear propagation of a short intense laser beam in a parabolic plasma channel is analyzed by means of the variational method and nonlinear dynamics. The beam propagation properties are classified by five kinds of behaviors. In particularly, the electromagnetic solitary wave for finite pulse laser is found beside the other four propagation cases including beam periodically oscillating with defocussing and focusing amplitude, constant spot size, beam catastrophic focusing. It is also found that the laser pulse can be allowed to propagate in the plasma channel only when a certain relation for laser parameters and plasma channel parameters is satisfied. For the solitary wave, it may provide an effective way to obtain ultra-short laser pulse.     

相对论行波管慢波结构几何参数研究

 推导了无引导磁场下具有离子通道的波纹波导中的色散方程,并通过数值计算分析了该慢波结构的几何参数如波纹周期、波纹深度以及波导平均半径对相对论行波管的带宽及增益的影响,为相对论行波管的设计提供了一定的依据。     

强激光束在等离子体通道中传输的变分法研究

关键词：     

Laser Self-Trapping for the Plasma Fiber Accelerator

A short-pulsed intense laser is injected into an underdense plasma to sustain a self-trapped photon channel. With either high-enough intensity or strong-enough focusing the optical beam causes total electron evacuation on the beam axis. Under appropriate conditions this laser and plasma fiber system can provide a slow wave structure of the electromagnetic wave that is suitable for high-energy acceleration.     

超强激光等离子体中钻孔效应的二维粒子模拟

 当非均匀超短脉冲强激光照射均匀分布的过稠密等离子体时, 相对论效应导致电子质量增大,相对论电子等离子体频率减小,激光能更深地进入等离子体产生强烈吸收。由于入射激光的径向不均匀性,钻孔效应显著。通过二维粒子模拟结果清晰地观察到了等离子体通道的产生。     

Analysis of the instability in relativistic traveling wave tube

The relativistic traveling wave tube is an important high power microwave source. The corrugated cylindrical waveguide is usually used as slow wave structure of this device. Starting from wave equation and using boundary conditions, dispersion relation is derived for the corrugated waveguide, in which an intense relativistic electron beam propagates along the axis. Two cases which are shorter period and longer period are discussed in this paper respectively. The small signal gain of the relativistic traveling wave tube is analyzed and some conclusions are drawn. The analysis method presented in this paper can be extended to other types of slow wave structures of relativistic traveling wave tube.     

Using the Steepened Plasma Profile and Wave Breaking Threshold in Laser‐Plasma Interaction

In this work we evaluate the interaction of high intense laser beam with a steepened density profile. During laser interaction with underdense plasma by freely expanding plasma regime, modification of density profile is possible. In this paper we have investigated the ultra short laser pulse interaction with nonisothermal and collisionless plasma. We consider self–focusing as an effective nonlinear phenomenon that tends to increase when the laser power is more than critical rate. By leading the expanded plasma to a preferred location near to critical density, laser reflection is obtained, so the density profile will be locally steepened. The electromagnetic fields are evaluated in this new profile. We show the amplitude and period of electrical field oscillation are increased by reducing the steepened scale length. Also our numerical results identify that by reducing the steepened scale length, the electrical field is increased to wave breaking threshold limit. This high gradient electrical field causes the effective beam loading during the wave breaking phenomenon. The wave breaking can be the initial point for other acceleration regime as cavity or channel guiding regime. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Non‐linear interaction of a q‐Gaussian laser beam in a plasma channel created by the ignitor‐heater technique

This paper presents a theoretical investigation of the propagation characteristics of a q‐Gaussian laser beam propagating through a plasma channel created by the ignitor‐heater technique. The ignitor beam creates the plasma by tunnel‐ionization of air. The heater beam heats the plasma electrons and establishes a parabolic channel. The third beam (q‐Gaussian beam) is guided in the plasma channel under the combined effects of density non‐uniformity and non‐uniform ohmic heating of the plasma channel. Numerical solutions of the non‐linear Schrodinger wave equation (NSWE) for the fields of laser beams are obtained with the help of the moment theory approach. Particular emphasis is placed on the dynamical variations of the spot size of the laser beams and the longitudinal phase shift of the guided beam with the distance of propagation.     

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)     

Laser-hole boring into overdense plasmas measured with soft X-Ray laser probing

A laser self-focused channel formation into overdense plasmas was observed using a soft x-ray laser probe system with a grid image refractometry (GIR) technique. 1.053 &mgr;m laser light with a 100 ps pulse duration was focused onto a preformed plasma at an intensity of 2x10(17) W/cm (2). Cross sections of the channel were obtained which show a 30 &mgr;m diameter in overdense plasmas. The channel width in the overdense region was kept narrow as a result of self-focusing. Conically diverging density ridges were also observed along the channel, indicating a Mach cone created by a shock wave due to the supersonic propagation of the channel front.     

Intense laser beam propagating in a plasma channel with flat-bottom leaky density profile

The propagation characteristics of an intense laser beam in a preformed plasma channel with the flat-bottom leaky density profile are investigated in detail. The evolution equation of the laser spot size is derived by employing variational technique. Seven propagation modes of the laser spot size are identified and some numerical results are presented. By comparison, we find that the results in this Letter may be more realistic since the flat-bottom leaky plasma channel comes closer to the practical plasma channel.     

Elastic wave propagation in sinusoidally corrugated waveguides

The ultrasonic wave propagation in sinusoidally corrugated waveguides is studied in this paper. Periodically corrugated waveguides are gaining popularity in the field of vibration control and for designing structures with desired acoustic band gaps. Currently only numerical method (Boundary Element Method or Finite Element Method) based packages (e.g., PZFlex) are in principle capable of modeling ultrasonic fields in complex structures with rapid change of curvatures at the interfaces and boundaries but no analyses have been reported. However, the packages are very CPU intensive; it requires a huge amount of computation memory and time for its execution. In this paper a new semi-analytical technique called Distributed Point Source Method (DPSM) is used to model the ultrasonic field in sinusoidally corrugated waveguides immersed in water where the interface curvature changes rapidly. DPSM results are compared with analytical solutions. It is found that when a narrow ultrasonic beam hits the corrugation peaks at an angle, the wave propagates in the backward direction in waveguides with high corrugation depth. However, in waveguides with small corrugation the wave propagates in the forward direction. The forward and backward propagation phenomenon is found to be independent of the signal frequency and depends on the degree of corrugation.     

The anomalous penetration of intense circularly polarized electromagnetic beam through overdense magnetized plasma

The steady state nonlinear propagation of an intense, circularly polarized electromagnetic beam in an inhomogeneous magnetized plasma has been investigated in paraxial approximation. The laser induces a large oscillatory velocity on electrons, raising their mass and lowering the plasma frequency. Further, rising due to cyclotron resonance effect. The propagation of the electromagnetic waves in magnetized plasma in both the extraordinary and ordinary mode is analyzed. The nonlinearity in dielectric function is considered in presence of external magnetic field due to saturation effects for arbitrary large intensity, which leads to focusing/defocusing of the beam. The focusing effect along with magnetic field helps in the process of anomalous penetration of the beam by enhancing the depletion of the plasma from the axial region. The penetration increases with the incident beam power up to some critical value beyond which it rises abruptly when all electrons have been driven out of the axis. The cyclotron resonance effect awfully supports the laser beam to propagate inside the overdense plasma region. Numerical computations are performed for typical parameters of relativistic laser–plasma interaction applicable for underdense and overdense plasma.     

Design of the Upgraded TJ-II Quasi-optical Transmission Line

TJ-II plasma start-up and heating are made by electron cyclotron resonance waves at the second harmonic of the electron cyclotron frequency. Two quasi-optical transmission lines transmit the microwave power of the gyrotrons to the vacuum vessel. The first line launches the microwave power under fixed injection geometry, i.e. there is no possibility to change the launching angle and the wave polarization. The second line has a moveable mirror installed inside the TJ-II vessel. To get high absorption efficiency and a narrow energy deposition profile the internal mirror focuses the wave beam at plasma center.To get more flexibility in the experiments on heating and current drive the first transmission line needs to be upgraded. The design is presented in this paper. The new launching antenna includes an internal mirror to focus the beam and to change the injection angle. Both launchers are then symmetrical. A polarizer consisting of two corrugated mirrors is used to get any wave polarization. Two mirrors with an array of coupling holes and calorimetric measurements of the energy absorbed in the barrier window allow the estimation of the microwave power launched into the TJ-II.     

等离子体填充波纹波导中低频模式特性分析

采用等离子体流体模型和线性场理论，导出了在强引导磁场下，一无限薄环形等离子体加载 波纹波导中电磁波传播的色散关系.数值计算并分析了在不同的等离子体填充密度下，低频 等离子体模式的色散特性和行波管的小信号增益.研究发现，低频等离子体模式可与相对论 电子束发生同步互作用使高频信号放大.同时，在无限薄环形等离子体填充条件下，波纹波 导中的低频等离子体模式严格满足Floquet定理所要求的周期性，其上截止频率不再受到等 离子体频率的限制，当密度较大时，等离子体模式还可与TM模式发生耦合. 关键词： 等离子体 波纹波导 色散关系 增益     

Expanded-core channel waveguides: Single-mode analysis and application

A buried channel waveguide composed of several core layers (i.e., a expanded-core waveguide) is proved to exhibit single-mode wave characteristics and a mode field compatible with that of a standard single-mode fiber. A sufficient condition for achieving single-mode operation is derived using an effective-index method. The concept of expanded-core waveguides is then applied to the design of MMIC-based beam splitters. Numerical results show that the mode field would exhibit a strong evanescent wave for TM mode in case the core layers are not many enough, even the TE mode field is well confined in the expanded core region. This situation induces substantial polarization dependent loss and a large fiber coupling loss for TM wave. However, this situation can be improved by using more core layers for the expanded-core waveguide.     

Dispersion relation and growth rate for a corrugated channel free-electron laser with a helical wiggler pump

The effects of corrugated ion channels on electron trajectories and spatial growth rate for a free-electron laser with a one-dimensional helical wiggler have been investigated. Analysis of the steady-state electron trajectories is performed by solving the equations of motion. Our results show that the presence of a corrugated channel shifts the resonance frequency to smaller values of ion channel frequency. The sixth-order dispersion equation describing the coupling between the electrostatic beam mode and the electromagnetic mode has also been derived. The dispersion relation characteristic is analyzed in detail by numerical solution. Results show that the growth rate of instability in the presence of corrugated ion channels can be greatly enhanced relative to the case of an uniform ion channel.     

Production of a keV x-ray beam from synchrotron radiation in relativistic laser-plasma interaction

We demonstrate that a beam of x-ray radiation can be generated by simply focusing a single high-intensity laser pulse into a gas jet. A millimeter-scale laser-produced plasma creates, accelerates, and wiggles an ultrashort and relativistic electron bunch. As they propagate in the ion channel produced in the wake of the laser pulse, the accelerated electrons undergo betatron oscillations, generating a femtosecond pulse of synchrotron radiation, which has keV energy and lies within a narrow (50 mrad) cone angle.     

Large amplitude lower hybrid wave driven by laser and its effect on electron acceleration in a magnetic plasma channel

A laser propagating through a magnetic plasma channel can drive a lower hybrid wave having frequency less than the plasma frequency. The lower hybrid wave can be driven to large amplitude if the laser pulse duration is comparable to the lower hybrid wave period. We derived the expression for the maximum amplitude of lower hybrid mode. This large amplitude mode is responsible for additional electron acceleration by betatron resonance in laser acceleration schemes.     

Divergence of far-infrared laser beam and collimation for Galilean and Keplerian system designs

Local confinement of the electromagnetic fields in a waveguide is a boundary condition problem which is often discussed as an example of wave propagation. A wave can also propagate in the air in a region without boundaries. In this case, the beam of electromagnetic waves is merely diverging as no condition of total internal reflection exists in a uniform medium such as air. We develop equations predicting the proper separation distance between a pair of lenses to collimate a far-infrared laser beam for both a Galilean and Keplerian telescopic systems. For a typical CO₂ laser Gaussian beam at λ=10.6 μm, it is found that the separation distance is much more than the simple sum of the focal lengths for both lenses used in a given telescopic system. This substantial increase in the separation distance between the pair of lenses is mostly due to the divergence of the emitting radiation of the CO₂ laser in the air, which is large compared to that of other commercially available laser sources. Some experimental results are given for both the Galilean and Keplerian telescopes and the discrepancies are compared from the calculations. A simple methodology is also described to test the collimation of the invisible CO₂ laser beam after it is transmitted through the pair of collimated ZnSe lenses. Good estimations for the separation distance between a pair of collimation lenses and beam magnifications are very useful when focusing a beam of CO₂ laser to a very small spot in various designs, where high-power density is required. Computations for the separation distance and magnification are found to be very close to the experimental values reported with a discrepancy of less than 4%.