Mode-coupling assisted electron accelerations by a plasma wave

The acceleration of electrons by a plasma wave in the presence of density ripple in plasma has been investigated. Plasma density ripple can excite higher harmonics of different phase velocities of the fundamental plasma wave. The combined role of the different harmonics of the plasma wave contributes significantly in electrons energy gain during acceleration by the fundamental plasma wave. Our calculation shows that the plasma electrons gain considerable energy during the acceleration by the plasma waves in the presence of a density ripple in plasma. The initial electron energy and the ripple density play an important role for the acceleration of an electron.     

Evolution of a langmuir wave in a weakly inhomogeneous plasma with a positive concentration gradient

Spatial evolution of a Langmuir wave excited by external sources in a weakly inhomogeneous electron plasma without external sources is considered for a small positive gradient of the plasma concentration in the direction of propagation of the wave. At the first state of the evolution, the dispersion of the wave is close to linear. When the phase velocity is doubled, the second stage of the evolution begins. The wave loses its individuality and becomes a hybrid of two waves. Its profile acquires the shape of an alternating sequence of fragments of these waves. The wave dispersion is determined by the dispersion of each fragment. In the course of evolution, the spacing between the equilibrium values of the wave fragments increases; as a result, the wave decays into two waves, which are also loaded by trapped electrons. Prior to decay, the humps of the wave become steeper; as a result, at the instant of the decay, the wave is transformed into a sequence of solitons with different polarities.     

Novel features of non-linear Raman instability in a laser plasma

The electron phase space evolution in a non-relativistic and homogeneous laser plasma generated by a nanosecond laser in a near infrared region in the presence of stimulated Raman scattering is investigated by a numerical simulation. The mechanism of electron acceleration in the potential wells of the plasma wave accompanying the Raman back-scattering is analyzed in a 1D Vlasov-Maxwell model. The dominant wave modes are both the backward and the forward propagating Raman waves, each accompanied by a daughter electrostatic wave. In addition to a strong interaction of plasma electrons with the primary electrostatic wave in the case of back-scattering, a cascading is observed consisting in a secondary scattering of the primary Raman back-scattered wave. This phenomenon reduces the Raman reflectivity and causes an acceleration of electrons against the direction of the heating laser beam. Moreover, the strong trapping in the primary electrostatic wave generated by the Raman back-scattering leads due to the trapped particle instability to a significant spectral broadening of the original plasma wave and a subsequent intermittent behaviour of the scattering process. The high phase velocity electrostatic daughter wave of the forward Raman scattering cannot trap the electrons directly, but there is an indication of non-resonant quasi-modes combined of this wave and of the simultaneously existing electrostatic daughter wave accompanying the Raman back-scattering. The transform method is used for a solution of the set of partial differential equations, which consists of the Vlasov equation and of the full set of Maxwell equations in a 1D approximation. A simplified Fokker-Planck collision term is added to overcome the numerical instabilities during the simulation. The model has relevance to a long scale plasma geometry, such as occurring in the indirect drive experiments near the light entrance holes of target hohlraum.     

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     

Observation of transition from a reactive-medium instability to an inverse Landau damping in a beam-plasma system

The wave excited by a coaxial probe in a beam-plasma system usually propagates as two waves which are independent of each other, i.e., a damped Trivelpiece mode of the plasma and a growing space charge wave of the beam (referred to a reactive-medium instability). While, when the beam velocity is equal to or slightly larger than the phase velocity of the plasma wave, the inverse Landau damping of the wave is observed, instead of the damped Trivelpiece mode.     

Cross-focusing of two laser beams in a plasma

Cross-focusing of two copropagating laser beams in a plasma is investigated using paraxial ray theory. If the lasers have a frequency difference equal to the electron plasma frequency, they can drive a large amplitude plasma wave. The ponderomotive force due to the plasma wave forces the plasma electrons outwards thereby generating a parabolic density profile giving rise to cross-focusing. The results show a decrease in threshold for focusing by two orders of magnitude as compared to focusing due to the ponderomotive force of the laser beams.     

Plasma Induced Wiggler Magnetic Field in a Cavity

The transformation of an elliptically polarized standing wave in a cavity by a suddenly and uniformly created plasma is discussed. Theoretical expressions for the plasma induced wiggler magnetic field as well as the frequency-upshifted standing wave are derived. By choosing appropriate values of the source wave parameters and plasma parameters, one can get wiggler magnetic field of desired magnitude, direction and wiggler wavelength. A few representative results are discussed.     

Feasibility of a plasma-filled inverse free electron laseraccelerator

A high power millimeter (mm) wave, in the presence of a magnetic wiggler, produces a large longitudinal ponderomotive force that can accelerate electrons. When a plasma of density n~n_cr, where n _cr is critical density, is introduced in the interaction region, the ponderomotive force resonantly drives a plasma wave that accelerates electrons to higher energies. However, propagation of the mm wave requires a guide magnetic field; O-mode requiring less field than the X-mode. The plasma wave in this situation goes over to the upper hybrid (UH) mode. A parabolic plasma density profile with minimum on axis provides guiding for the mm wave as well as the UH wave, the latter being more strongly localized than the former. The UH wave, for typical parameters, can accelerate electrons to several tens of megaelectronvolts     

Features of the generation of a collisionless electrostatic shock wave in a laser-ablation plasma

The appearance of a density bump is experimentally revealed in an electrostatic shock wave during the ablation of an aluminum foil by a femtosecond laser pulse. The numerical simulation shows that this phenomenon can be explained by the generation of a packet of ion acoustic waves under the action of high-energy electron flows in a collisionless plasma. It is found that, for the formation and maintenance of the dense plasma layer in the shock wave, the contributions of accelerated ions overtaking it and wave-captured ions of the background plasma formed by a nanosecond laser prepulse in the process of ablation are significant.     

On the momentum of quasi-monochromatic waves in a plasma

The formulae for the momentum of quasi-monochromatic wave packets of transverse and longitudinal waves in a plasma without a magnetic field are derived including the terms of the second order in the amplitude of the electromagnetic field. The well-known increase of the momentum of the transverse wave penetrating into the plasma is given by the momentum (transported with the group velocity of the wave) of the averaged motion of the plasma. The laws of energy and momentum conservation lead simply to some results of the theory of the wave decay.Nademlýnská 600, Praha 9, Czechoslovakia.The authors thank K. Jungwirth for valuable discussions.     

General solution of a system of equations for acoustic wave propagation in a partially ionized gas in an external electric field

In this paper a system of equations describing the propagation of acoustic waves in multicomponent partially ionized plasma in an external electric field is solved generally, when these waves are generated by an external neutral sound wave in plasma. The equations are solved by means of Laplace transformation. It has been found that a sound wave generates a group of waves with different frequencies and wave vectors. For the sake of comparison, the solution without an external field and the linear solution in the form of waves having the same frequencies as the frequency of the excitation wave are given.     

Finite-Difference Time-Domain Analysis of Wave Propagation in a Thin Plasma Layer

The propagation property of an electromagnetic wave in with the finite-difference time-domain method. The effects a thin plasma layer at high pressure is investigated of the non-uniformity of plasma distribution, and the frequency of incident wave on the propagation property of the electromagnetic wave are discussed. Numerical results indicate that the phase shift and the reflectivity of wave are sensitive to plasma density distribution, and reflectivity is lower at the middle band of frequency for different plasma distributions.     

Passage of a Plane Shock Wave through the Region of a Glow Gas Discharge

Technical Physics - The interaction of a plane shock wave (M = 5) with an ionized plasma region formed before the arrival of a shock wave by a low-current glow gas discharge is considered...     

Observation of the decay instability of a beam plasma wave

The spontaneous decay of a beam plasma wave into an ion acoustic and an electron plasma wave has been observed during the interaction of a monoenergetic electron beam with a helium and a neon plasma.     

Penetration of an ordinary wave into a weakly inhomogeneous magnetoplasma at oblique incidence

A theory is given describing the propagation of high-frequency electromagnetic waves in a plane-stratified weakly inhomogeneous plasma. The density gradient is supposed to be perpendicular to the external magnetic field and the wave vector is expected not to be generally parallel to the plane given by both the preceding vectors. The analysis points out that the ordinary wave can penetrate through the plasma resonance region if the direction of vacuum wave vector is chosen appropriately. Analytical expressions for the reflecion and transmission coefficients are obtained and their dependence on the direction cosines of the wave vector of the incident is studied. The paper further shows in outline that, after transmission through the plasma resonance, the ordinary wave is transformed into an extraordinary wave and the latter is reflected back to the region of the hybrid resonance. In this region the extraordinary wave is fully transformed into the Bernstein modes.     

Theoretical analysis of a relativistic travelling wave tube filled with plasma

A cold and uniform plasma-filled travelling wave tube with sinusoidally corrugated slow wave structure is driven by a finite thick annular intense relativistic electron beam with the entire system immersed in a strong longitudinal magnetic field. By means of the linear field theory, the dispersion relation for the relativistic travelling wave tube (RTWT) is derived. By numerical computation, the dispersion characteristics of the RTWT are analysed in different cases of various geometric parameters of the slow wave structure and plasma densities. Also the gain versus frequency for three different plasma densities and the peak gain of the tube versus plasma density are analysed. Some useful results are obtained on the basis of the discussion.     

Generation of wakefields in a dielectric structure filled with plasma

The effect of plasma on the amplitude of the wakefield excited in a dielectric structure by a relativistic electron bunch train is studied. The structure under study is a dielectric cylindrical waveguide with an axial drift channel filled with plasma. The dependences of the amplitude of a longitudinal electric field on the plasma density are obtained for the following three cases: the parameters of the dielectric structure and bunches are fixed; the inside or outside radius of the dielectric tube changes according a change in the plasma frequency, and the bunch repetition frequency is adjusted to the plasma frequency and the frequency of the first radial mode of a dielectric wave. It is shown that, when the eigenwave frequencies are adjusted to the bunch repetition frequency via a change in the structure radii, the maximum of the accelerating field is determined by a plasma wave, and there is a plasma density range where a dielectric wave significantly contributes to the total field amplitude. In the case of changing the outside radius, this range is substantially wider.     

Analysis of the Evolution of a Supersonically Expanding Plasma

A supersonically expanding arc plasma in argon is analyzed both experimentally and theoretically. The plasma is created in a cascaded arc and extracted through a hole in the anode. It emanates in a large vacuum system, where it expands supersonically. This expansion is limited by a shock wave. After the shock wave a subsonic plasma beam is created. A quasi one-dimensional model, based on the conservation of mass, momentum and energy is presented. The shock wave is treated as a discontinuity. The electron density, the gas velocity and the gas temperature are measured as a function of the position in the expansion by means of Stark broadening and Doppler spectroscopy. The model calculations agree well with the measurements, especially in the first part of the supersonic flow.