Growth of a laser ripple in a magnetoplasma and its effect on plasma wave excitation

Summary This paper presents an investigation of the growth of a radially symmetrical ripple, superimposed on a Gaussian laser beam in a collisionless magnetoplasma. The effect of the magnetic field and the intensity of the laser on the growth of the ripple is presented in some detail. The effect of the presence of the ripple on the excitation of an electron plasma wave is also investigated. Coupling of a weak plasma wave with the main laser beam is through the modified background density. The combined effect of increased intensity of the laser beam and magnetic field is observed to suppress the growth of the ripple as well as the excitation of the plasma wave. The authors of this paper have agreed to not receive the proofs for correction.     

An x-ray biplanar photodiode and the x-ray emission from magnetically confined laser produced plasma

Development of a single and multichannel biplanar vacuum photodiode for x-ray detection is reported, which has been used to study the x-ray emission from laser produced plasma expanding across an externally applied magnetic field. Two to three times enhancement in x-ray emission has been observed which was found correlated with decrease in size of the x-ray emitting plasma plume (expansion velocity of plasma). Experimental observations were found in close agreement with the analytical model based on an increase in plasma density as a result of plasma confinement in magnetic field. Temporal evolution of x-ray emission indicates that recombination radiation seems to be playing an important role in x-ray enhancement.     

激光偏振状态对磁化等离子体中电磁孤波的影响

强激光与等离子体之间相互作用,能够产生各种参量不稳定性过程和非线性效应。利用Karpman方法推导出横场包络所满足的非线性控制性方程,在一维情况下,获得孤波解。对孤波解进行分析,发现波包孤子的半宽反比于振幅;分析磁化等离子体中各参量对孤波半宽的影响。结果表明,在右旋圆偏振激光情况下,随着电子数密度的增大,孤波的半宽逐渐减小,而当磁场强度增大时,孤波的半宽逐渐增大;在左旋圆偏振激光情况下,随着电子数密度的增大,孤波的半宽逐渐增大,而当磁场强度增大时,孤波的半宽逐渐减小。     

Self-Focusing of Quadruple Gaussian Laser Beam in an Inhomogenous Magnetized Plasma with Ponderomotive Non-Linearity: Effect of Linear Absorption

The propagation of quadruple Gaussian laser beam in a plasma characterized by axial inhomogeneity and nonlinearity due to ponderomotive force in the paraxial ray approximation is investigated. An appropriate expression for the nonlinear dielectric constant has been developed in the presence of external magnetic field, with linear absorption and due to saturation effects for arbitrary large intensity. The effects of different types of plasma axial inhomogeneities on self-focusing of laser beam have been studied with the typical laser and plasma parameters. Self-focusing of quadruple Gaussian laser beam in the presence of externally applied magnetic field and saturating parameter is found significantly improved in the case of extraordinary mode. Our results reveal that initially converging beam shows oscillatory convergence whereas initially diverging beam shows oscillatory divergence. The beam is more focussed at lower intensity in both cases viz. extraordinary and ordinary mode.     

Self-Focusing of Quadruple Gaussian Laser Beam in an Inhomogenous Magnetized Plasma with Ponderomotive Non-Linearity:Effect of Linear Absorption

The propagation of quadruple Gaussian laser beam in a plasma characterized by axial inhomogeneity and nonlinearity due to ponderomotive force in the paraxial ray approximation is investigated.An appropriate expression for the nonlinear dielectric constant has been developed in the presence of external magnetic field,with linear absorption and due to saturation effects for arbitrary large intensity.The effects of different types of plasma axial inhomogeneities on self-focusing of laser beam have been studied with the typical laser and plasma parameters.Self-focusing of quadruple Gaussian laser beam in the presence of externally applied magnetic field and saturating parameter is found significantly improved in the case of extraordinary mode.Our results reveal that initially converging beam shows oscillatory convergence whereas initially diverging beam shows oscillatory divergence.The beam is more focussed at lower intensity in both cases viz.extraordinary and ordinary mode.     

Effect of self-generated magnetic field on the two-plasmon decay instability in a laser-produced plasma

Summary In this paper we have made a theoretical investigation on the two-plasmon decay instability of laser radiation in the presence of the selfgenerated magnetic field at the quarter-critical density region in a laserproduced plasma. The Vlasov equation in terms of guiding centre coordinates has been employed to obtain the nonlinear response of electrons in the plasma. The threshold power density of the incident laser radiation for the two-plasmon decay instability is always exceeded in currently employed power densities in laser-target experiments and above the threshold the growth rate of the instability is quite large. It is also noticed that the selfgenerated magnetic field enhances the threshold to a large extent, thus drastically reducing the growth rate of the instability. To speed up publication, the author of this paper has agreed to not receive the proofs for correction     

Helical instability of a semiconductor plasma in silicon wafers at 77 K

This paper reports an analysis of the experimental results on how the threshold electric field intensity required to excited helical instability of a semiconductor plasma in wafers of p-silicon depends on the magnitude of magnetic induction. Also cited are data on the dependence of the threshold frequency on electric field intensity. The variations of the amplitude of the alternating current, caused by the development of helical instability, with electric field intensity and magnetic induction well above the instability excitation threshold are examined.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 54–60, August, 1991.     

Parametric excitation of hybrid mode in magnetised piezoelectric semiconductors

Using the hydrodynamic model of homogeneous plasma, the parametric decay of a laser beam into an acoustic wave and another electromagnetic wave has been studied in heavily dopedn-type piezoelectric semiconductors in the presence of a transverse magnetostatic field. This decay process results in the parametric excitation of the hybrid mode. The threshold electric field necessary for the onset of instability equals to zero. The magnetostatic field couples the acoustic and the electromagnetic waves and in its absence the instability disappears. The growth rate increases with the square of the magnetic field.     

Laser-Plasma Interaction in Presence of an Obliquely External Magnetic Field: Application to Laser Fusion without Radioactivity

In this paper, the nonlinear interaction of ultra-high power laser beam with fusion plasma at relativistic regime in the presence of obliquely external magnetic field has been studied. Imposing an external magnetic field on plasma can modify the density profile of the plasma so that the thermal conductivity of electrons reduces which is considered to be the decrease of the threshold energy for ignition. To achieve the fusion of Hydrogen-Boron (HB) fuel, the block acceleration model of plasma is employed. Energy production by HB isotopes can be of interest, since its reaction does not generate radioactive tritium. By using the inhibit factor in the block model acceleration of plasma and Maxwell's as well as the momentum transfer equations, the electron density distribution and dielectric permittivity of the plasma medium are obtained. Numerical results indicate that with increasing the intensity of the external magnetic field, the oscillation of the laser magnetic field decreases, while the dielectric permittivity increases. Moreover, the amplitude of the electron density becomes highly peaked and the plasma electrons are strongly bunched with increasing the intensity of external magnetic field. Therefore, the magnetized plasma can act as a positive focusing lens to enhance the fusion process. Besides, we find that with increasing θ-angle (from oblique external magnetic field) between 0 and 90°, the dielectric permittivity increases, while for θ between 90° and 180°, the dielectric permittivity decreases with increasing θ.     

Instabilities in laser-produced carbon plasma expanding in a nonuniform magnetic field

The laser-produced carbon plasma expanding in an ambient atmosphere in the presence of an inhomogeneous magnetic field has been studied by emission spectroscopy and fast photography. A double-peak structure is observed in the temporal profile of CII and CIII transition. A sudden increase in delay observed in the second peak when the plasma expands in the concave region of a magnetic field is attributed to Rayleigh–Taylor instability in a magnetic field. An estimate of the growth rate of the instability inferred using intensity and velocity profile of the expanding plasma is reported. Received: 26 August 1999 / Revised version: 3 January 2000 / Published online: 20 September 2000     

Stabilization effect of Weibel modes due to inverse bremsstrahlung absorption in laser fusion plasma using Krook collisions model

In this work, the Weibel instability due to inverse bremsstrahlung absorption in laser fusion plasma has been investigated. The stabilization effect due to the coupling of the self-generated magnetic field by Weibel instability with the laser wave field is explicitly showed. The main result obtained in this work is that the inclusion of self-generated magnetic field due to Weibel instability to the inverse bremsstrahlung absorption causes a stabilizing effect of excited Weibel modes. We found a decrease in the spectral range of Weibel unstable modes. This decrease is accompanied by a reduction of two orders in the growth rate of instability or even stabilization of these modes. It has been shown that the previous analyses of the Weibel instability due to inverse bremsstrahlung have overestimated the values of the generated magnetic fields. Therefore, the generation of magnetic fields by the Weibel instability due to inverse bremsstrahlung should not affect the experiences of an inertial confinement fusion.     

激光等离子体不稳定性的入射光强度效应

用3维粒子模拟程序研究了相对论强激光和高密度等离子体相互作用引起的电磁不稳定。数值模拟表明,在线偏振强激光作用下,等离子体表面出现了电磁不稳定性。形成的不稳定结构随时间发展和激光功率密度的增加进一步深入到等离子体内部,最终使等离子体表面处激发饱和自生磁场。这种由电子速度各向异性而产生的自生磁场对激光有质动力推开电子时所形成的电子热流产生抑制作用,并将直接影响电子加速效率。     

Surface acceleration of fast electrons with relativistic self-focusing in preformed plasma

We report an observation of surface acceleration of fast electrons in intense laser-plasma interactions. When a preformed plasma is presented in front of a solid target with a higher laser intensity, the emission direction of fast electrons is changed to the target surface direction from the laser and specular directions. This feature could be caused by the formation of a strong static magnetic field along the target surface which traps and holds fast electrons on the surface. In our experiment, the increase in the laser intensity due to relativistic self-focusing in plasma plays an important role for the formation. The strength of the magnetic field is calculated from the bent angle of the electrons, resulting in tens of percent of laser magnetic field, which agrees well with a two-dimensional particle-in-cell calculation. The strong surface current explains the high conversion efficiency on the cone-guided fast ignitor experiments.     

磁场约束下激光诱导等离子体光谱强度演化研究

对磁场约束下激光诱导铜等离子体光谱强度演化进行了实验研究,分析了在磁场约束环境下的等离子体光谱强度演化过程以及激光能量对光谱增强的影响.实验结果表明:在磁场约束下铜等离子体内原子光谱和离子光谱均有所增强,在磁场约束下Cu I 510.55 nm谱线强度时间演化过程中在1.2—5.7μs时间范围内附近出现双峰结构,在距离靶材表面0—1.4 mm空间范围内磁场约束Cu I 510.55 nm光谱增强明显.Cu I510.55 nm和Cu I 515.32 nm光谱增强因子随激光能量的增加呈单调递减变化,激光能量20 mJ时增强因子最大分别为11和8.对磁场约束下等离子体发射光谱强度增强的物理原因进行了探讨.     

Growth of a Gaussian Laser Ripple on a Gaussian Beam in a Collisionless Magnetoplasma and its Effect on the Excitation of Ion-Acoustic Wave

This paper presents an investigation of the growth of a radially symmetrical ripple, superimposed on a Gaussian laser beam in a collisionless magnetoplasma. Here we have presented the effect of magnetic field and the intensity of the laser beam on the growth of ripple in some detail. The effect of presence of ripple on the excitation of an ion-acoustic wave is also investigated. Coupling of a weak ion-acoustic wave with main laser beam is through modified background density. Interesting feature of the analysis is that the combined effect of increased intensity of the laser beam and magnetic field is observed to suppress the growth of the ripple as well as the excitation of the ion-acoustic wave.     

Experimental study of conversion from atomic high—order harmonics to x—ray emissions

There are two physical phenomena in a strong laser intensity. One is the high-order harmonic emission; the other is x-ray emission from optical-field ionized plasmas. The experiment of conversion from high-order harmonics to x-ray emissions was given with a 105fs Ti:sapphire laser by adjusting laser intensities. The ingredient in plasma was investigated by the numerical simulations.Our experimental results suggested that the free electrons have detrimental effects on harmonic generation but are favourable for x-ray emission from optical-field ionized plasmas. If we want to obtain more intense harmonic signals as a coherent light source in the soft x-ray region, we must avoid the production of free electrons in plasmas. At the same time, if we want to observe x-rays for the development of high-repetition-rate table-top soft x-ray lasers, we should strip all atoms in the plasmas to a necessary ionized stage by the optical-field-ionization in the field of a high-intensity laser pulse.     

Self-Focusing and de-Focusing of Intense Left- and Right-Hand Polarized Laser Pulse in Hot Magnetized Plasma in the Presence of an External Non-Uniform Magnetized Field

In this paper, self-focusing of an intense circularly polarized laser beam in the presence of a non-uniform positive guide magnetic field with slope constant parameter δ in hot magnetized plasma, using Maxwell’s equations and relativistic fluid momentum equation is investigated. An envelope equation governing the spot-size of laser beam for both of left- and right-hand polarizations has been derived, and the effects of the plasma temperature and magnetic field on the electron density distribution of hot plasma with respect to variation of normalized laser spot-size has been studied. Numerical results show that self-focusing is better increased in the presence of an external non-uniform magnetic field. Moreover, in plasma density profile, self-focusing of the laser pulse improves in comparison with no non-uniform magnetic field. Also, with increasing slope of constant parameter of the non-uniform magnetic field, the self-focusing increases, and subsequently, the spot-size of laser pulse propagated through the hot magnetized plasma decreases.     

Observation of the decay dynamics and instabilities of megagauss field structures in laser-produced plasmas

Monoenergetic proton radiography was used to make the first measurements of the long-time-scale dynamics and evolution of megagauss laser-plasma-generated magnetic field structures. While a 1-ns 10(14) W/cm2 laser beam is on, the field structure expands in tandem with a hemispherical plasma bubble, maintaining a rigorous 2D cylindrical symmetry. With the laser off, the bubble continues to expand as the field decays; however, the outer field structure becomes distinctly asymmetric, indicating instability. Similarly, localized asymmetry growth in the bubble interior indicates another kind of instability. 2D LASNEX hydrosimulations qualitatively match the cylindrically averaged post-laser plasma evolution but even then it underpredicts the field dissipation rate and of course completely misses the 3D asymmetry growth.     

Quadruple Gaussian Laser Beam Profile Dynamics in Collisionless Magnetized Plasma

This paper presents an investigation of self-focusing of a quadruple Gaussian laser beam in collisionless magnetized plasma. The nonlinearity due to ponderomotive force which arises on account of nonuniform intensity distribution of the laser beam is considered. The nonlinear partial differential equation governing the evaluation of complex envelope in the slowly varying envelope approximation is solved using a paraxial formalism. The self-focusing mechanism in magnetized plasma, in the presence of self-compression mechanism will be analyzed in contrast to the case in which it is absent. It can be observed that, in case of ponderomotive nonlinearity, the self-compression mechanism obstructs the pulse self-focusing above a certain intensity value. The effect of an external magnetic field is to generate pulses with smaller spot size and shorter compression length. The lateral separation parameter and the initial intensity of the laser beam play a crucial role on focusing and compression parameters. Also, the three-dimensional analysis of pulse propagation is presented by coupling the self-focusing equation with the self-compression one.     

Numerical Study of Instabiities in Magnetized Inhomogeneous Plasmas under the Effect of Ionization

Influence of ionization is studied on two stream instability (TSI) in an inhomogeneous plasma in the presence of obliquely applied magnetic field. In addition to the usual TSI, a new type of instability is found to occur in this system, which is driven by the magnetic field and survives for relatively longer wavelength of oscillations. The growth rates of both the instabilities are enhanced by the magnetic field but their magnitudes attain a minimum value at certain angles of the wave propagation depending upon the wavelength of oscillations. At a critical value of ionization rate there is a sudden fall in the growth of both the instabilities, the reason of which is understood as the Landau damping. A further enhancement in the ionization suppresses the usual TSI whereas the magnetic field‐driven instability attains much lower growth. This new type of instability grows faster in the plasma having heavier ions, but shows a weak dependence on the charge of the ions. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)