3/2omega0 radiation from the laser-driven two-plasmon decay instability in an inhomogeneous plasma

We present the results of the first reduced model simulations of the nonlinear development of the two-plasmon decay instability in an inhomogeneous plasma, including properties of the 3/2 harmonic emission. A sharp increase in radiation and Langmuir turbulence fluctuation levels occurs above a threshold laser intensity that depends on initial fluctuation levels. We study the competition between the linear propagation of Langmuir waves in the density gradient and the nonlinear saturation due to the Langmuir decay instability. The secondary decay Langmuir waves can provide the dominant source of the radiation and are essential to explain experiments.     

Resonant decay of a Langmuir wave in the presence of dust grains in a cylindrical plasma

The resonant parametric decay of a Langmuir wave into a backward propagating Langmuir wave and an ion acoustic (IA) wave is studied in a cylindrical dusty plasma. The analysis shows that the frequency of the IA mode decreases with the parameter δ_c (where δ_c is the ratio of the ion density to the electron density) for negatively charged dust grains. The growth rate of the resonance decay instability (RDI) and the threshold required for its onset also decrease with δ_c and are strongly dependent on the electron to ion temperature ratio for both positively and negatively charged dust grains. The results obtained also illustrate the dependence of the threshold of the resonance decay instability (μ_th) on the plasma cylinder radius.     

Transient enhancement and detuning of laser-driven parametric instabilities by particle trapping

Kinetic simulations of backward stimulated Raman scattering (BSRS), where the Langmuir wave coherence time is greater than the bounce time for trapped electrons, yield transient reflectivity levels far above those predicted by fluidlike models. Electron trapping reduces the Langmuir wave damping and lowers the Langmuir wave frequency, and leads to a secular phase shift between the Langmuir wave and the BSRS beat ponderomotive force. This phase shift detunes and saturates BSRS and a similar effect, due to ion trapping, is the saturation mechanism for backward stimulated Brillouin scattering. Competition with forward SRS is discussed.     

First observation of ion acoustic waves produced by the langmuir decay instability

Thomson scattering measurements are presented which demonstrate conclusively the occurrence of the Langmuir decay instability (LDI) in a laser-produced plasma experiment. Both products of the instability, the ion acoustic wave and the electron plasma wave, were simultaneously observed and identified with their spectral characteristics. The secondary decay of the LDI-generated electron plasma wave, into another Langmuir wave and an ion acoustic wave, has been observed for the first time. The connection with growth and saturation of the stimulated Raman instability is discussed.     

Stimulated Raman backscattering from an ultrashort laser interacting with underdense plasmas

We present experimental results of the stimulated Raman backscattering instability (BSRS) in an ultrashort intense (45fs, 5.7×10¹⁷Wcm^-2) laser pulse interacting with an optically ionized helium gas. We have studied the stimulated Raman backscattering reflectivity and the phenomenon of the transition from strongly coupled BSRS to weakly coupled BSRS. We have obtained a good agreement between our experimental results and the theory of BSRS.     

Two-stream instabilities in degenerate quantum plasmas

The quantum effects on the plasma two-stream instability are studied by the dielectric function approach. The analysis suggests that the instability condition in a degenerate dense plasma deviates from the classical theory when the electron drift velocity is comparable to the Fermi velocity. Specifically, for a high wave vector comparable to the Fermi wave vector, a degenerate quantum plasma has larger regime of instability than predicted by the classical theory. A regime is identified, where there are unstable plasma waves with frequency 1.5 times of a normal Langmuir wave.     

Nonlocal theory of stimulated Raman backscattering in a strongly magnetized plasma

A nonlocal theory of stimulated Raman backscattering (BSRS) parametric instability of a large amplitude electromagnetic (EM) mode in a strongly magnetized plasma e.g., one encountered in a plasma filled backward wave oscillator, is reported. The EM mode is unstable to parametric instability in a magnetized plasma and decays into a Trivelpiece-Gould (TG) mode and a sideband EM mode. The growth rate of instability (Γ) scales proportional to three-fourth power of plasma density. For a typical BWO the growth rate is ∼10⁸ s^-1     

Oscillating two-stream instability of a plasma wave in a plasmachannel

A long wavelength Langmuir wave (ω₀, k₀), propagating through a parabolic plasma density channel, can decay into a low-frequency mode (ω,k&oarr;) and two short wavelength Langmuir wave sidebands (ν_1,2,k&oarr;_1,2 ), via two-stream instability where ω_1,2=ω∓ω₀ and k&oarr;_1,2=k&oarr;∓k&oarr;₀. Depending on the mode number n, the growth rate maximizes in the range γ_max≃0.1ω_pi-0.4ω_pi for the range of k from 0.1(ω_pi/c_s) to 0.2(ω_pi/c_x) for ν₀~ν_th where ν₀ and ν_th are the oscillatory and thermal velocities of electrons, ω_pi is the ion plasma frequency on the axis, and c_s is the sound speed. The growth rate increases with the width a of the plasma density channel. It decreases with the mode number. The instability may be relevant to laser based charged particle accelerators     

New scaling law for the decay exponent of bimolecular reactions in unbounded transitional flows

We propose a new scaling law for global kinetics of the stoichiometric reaction A+B-->P in unsteady, transitional flows. We find in the nonlinear flow regime the decay as approximately t(-alpha) where alpha is related to a space-time scaling parameter psi as alpha proportional, variant psi(m), for the considered parameter range m=0.067. In the linear flow regime, we find that the maximum is alpha approximately 2/3 for psi approximately 1. The proposed scaling law should be useful for linking dynamical subgrid processes with reaction kinetics in a variety of transitional flow systems.     

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基于多光子非线性Compton散射模型,研究了Compton散射下等离子体中强朗缪尔湍动对调制不稳定性的影响。将入射光和Compton散射光作为形成强朗缪尔湍动和调制不稳定性的新机制,给出了强朗缪尔湍动、色散和调制不稳定性时间增长率所满足的修正方程,并进行了数值模拟。结果表明,与Compton散射前相比,Compton散射使等离子体内产生了更为剧烈的坍塌,坍塌后期形成的强朗缪尔湍动,使等离子体界面附近的调制不稳定性的时间增长率显著增大,调制不稳定性发展得更快,光场峰值增加得更强,并使整体激光场出现明显的成丝现象。     

Radiation guiding in a plasma wave wiggler free-electron laser

The radiation guiding of a plasma wave wiggler free-electron laser (FEL) in the Compton regime was examined. It was found that a Langmuir wave supported by a plasma cylinder acts as a wiggler for the generation of high-frequency coherent radiation when an annular relativistic electron beam passes through it. The radiation mode in the Compton regime tends to be localized close to the radius of the beam. A normal-mode analysis of this process revealed that the growth rate of the instability increases as the square root of the beam current. The treatment presented is restricted to the case where the radial width of the FEL radiation mode is larger than the beam radius, but smaller than the waveguide radius     

Linear and Nonlinear Wavetransformation and Absorption in the Inhomogeneous Plasma Capacitor

The parametric decay process in inhomogeneous layers existing near the plasma boundaries or in front of antennas and probes in a plasma has been investigated. The linear enhancement of the pump field near ω = ω_p, the threshold fieldstrength, the wavenumber selection rules and the influence of spontaneous low frequency fluctuations are discussed in detail using a one-dimensional model of the inhomogeneous plasma capacitor. According to this model the instabilities appear in the layers with maximum linear transformation and (linear) absorption. In addition, a strong nonlinear part of absorption in the presence of the instability has been observed. The level of the spontaneous low frequency fluctuations influences strongly the spectrum of the parametrically excited ion waves. The experiments show a redistribution of the transferred ion acoustic wave energy over the whole wave continuum up to ω_pi, if a sufficient strong spontaneous fluctuation level exists in the plasma. It is impossible, however, to excite ion acoustic turbulence by the decay of the high frequency pump field under the present conditions. The conditions for the linear field enhancement are disturbed by the action of the ponderomotive forces changing the density profile near the critical point before reaching the strong pump amplitude being necessary for the excitation of a cascade of decay processes.     

Langmuir turbulence and modulational instability

In this paper we discuss the turbulent state of an unmagnetized, completed ionized hydrogen plasma for the case when the electron temperature is much higher than the ion temperature so that the main modes to be considered are the Langmuir and ion-sound modes, while the transverse, electromagnetic mode can for most purposes be neglected. We give in the introduction a brief discussion of the quasi-linear theory of weak Langmuir turbulence as developed by Tsytovich and coworkers. We discuss why the peak of the resulting spectrum occurs at a finite wavenumber; the reason is that the removal of energy through electron collisions is faster than either thermalization of the ion-component or the transfer to yet lower wavenumbers through scattering by ions. In section 2 we sketch Zakharov's derivation of the basic equations which in the linear form describe the three modes mentioned earlier and which describe the nonlinear modulational instability (M.I.). In section 3 we give the linear analysis of the equations derived in section 2 and show that the decay instability of finite-amplitude Langmuir waves is fully covered by these equations. We also discuss the general dispersion relations for perturbations of such finite-amplitude waves and show that they lead to both the M.I. and the decay instability. We evaluate the growth rates for various kinds of perturbations. It is shown that all our results are valid only provided the energy density of the Langmuir modes W is less than the kinetic energy density nT_e of the electrons. In section 4 we discuss under what circumtances in the linear approximation the M.I. dominates over other dissipative mechanisms and we show that it may well be the dominant mechanism for the case of strong turbulence, when $\text{W/nT}{}_{\mathrm{e}}\text{≡}\text{W}\text{?}\text{> = m}{}_{\mathrm{e}}\text{/m}{}_{\mathrm{i}}$. Section 5 is devoted to a discussion of the one-dimensional variant of our basic equations. We give a brief discussion of various soliton-bearing equations such as the Kortweg-de Vries equation (KdV), the sine-Gordon equation (SG), and the non-linear Schrödinger equation (NLS).We discuss the possible connection between the number of polynomial conserved densities (p.c.d.) which lead to integrals of motion and the number of solitons in the strict sense of the term which can occur in a single exact solution of the equation. We also discuss the evolution of a system of Langmuir solitons in terms of their interactions with one another and with ion-sound. In section 6 we first of all show that in three-dimensions soliton-like structures are unstable against adiabatic collapse. We then discuss the dynamics of such collapsing “cavitons” and show that the theory of collapsing cavitons predicts that the turbulent energy should increase as the $\text{2}\text{3}$ power of the pumping rate. We discuss the fact that it may be difficult to avoid overcrowding of cavitons which would violate their independence. We finally briefly discuss the few relevant experiments and mention some ideas for future research.     

Laser–plasma interaction physics in the context of fusion

Of vital importance for Inertial Confinement Fusion (ICF) are the understanding and control of the nonlinear processes which can occur during the propagation of the laser pulses through the underdense plasma surrounding the fusion capsule. The control of parametric instabilities has been studied experimentally, using the LULI six-beam laser facility, and also theoretically and numerically. New results based on the direct observation of plasma waves with Thomson scattering of a short wavelength probe beam have revealed the occurence of the Langmuir decay instability. This secondary instability may play an imporant role in the saturation of stimulated Raman scattering. Another mechanism for reducing the growth of the scattering instabilities is the so-called `plasma-induced incoherence'. Namely, recent theoretical studies have shown that the propagation of laser beams through the underdense plasma can increase their spatial and temporal incoherence. This plasma-induced beam smoothing can reduce the levels of parametric instabilities. One signature of this process is a large increase of the spectral width of the laser light after propagation through the plasma. Comparison of the experimental results with numerical simulations shows an excellent agreement between the observed and calculated time-resolved spectra of the transmitted laser light at various laser intensities.     

受激布里渊散射的非线性增强和饱和

针对典型激光聚变等离子体参数条件，利用弗拉索夫程序研究非均匀流等离子体中受激布里渊散射的非线性行为。在动理学效应占主导的参数区域，观察到受激布里渊散射激发的离子声波由于非线性动理学频移和非均匀流空间失谐相互补偿引起的离子声波自共振增长，这会导致受激布里渊水平量级的增强；提出用光束时间去相干抑制这种绝对增长。在流体非线性占主导的参数区域，观察到由于离子声波谐波导致的孤立波产生、离子加热以及受激布里渊散射饱和现象。     

Regimes and spectra of čerenkov beam instability in a nonlinear plasma

We study the nonlinear evolution of an arbitrary initial disturbance due to the development of Čerenkov beam instability in a magnetized plasma-beam system of finite transverse size. Singleparticle, collective, and aperiodic regimes of this instability are considered. We calculate the nonlinear spatial spectra of the waves excited at different development stages of the beam instability in a plasma for the cases of quasi-monochromatic, pulsed, and noise initial disturbances. We analyze the formation and decay of regular structures in the beam and plasma at the developed nonlinear stage of the process. We find that plasma nonlinearity leads to the transfer of disturbance energy to the short-wave region of the spectrum. We show that, due to the development of beam instability, noise initial disturbance tends to become more monochromatic, whereas the shape of a pulsed one tends to remain unchanged. Transformation of monochromatic spatial disturbances into quasi-monochromatic plasma waves due to the instability development is analyzed. Institute of General Physics of the Russian Academy of Sciences, Moscow, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 10, pp. 958–976, October 1999.     

Nonlinear theory of resonant beam-plasma interaction: Nonrelativistic case

Analytic and numerical methods are used to study the nonlinear dynamics of the resonant interaction between a dense nonrelativistic electron beam and a plasma in a spatially bounded system. Regimes such as collective (Raman) and single-particle (Thomson) Cherenkov effects are considered. It is shown that in the first case, the motion of both the beam and plasma electrons exhibits significant nonlinearities. However, because of the weak coupling between the beam and the plasma, the nonlinear dynamics of the instability can be studied analytically and it can be strictly shown that saturation of instability is caused by a nonlinear shift of the radiation frequency and loss of resonance. In the second case, the nonlinear instability dynamics can only be studied numerically. In this regime, at low beam densities significant nonlinearity is only observed in the motion of the beam electrons while the plasma remains linear and saturation of the instability is caused by trapping of beam electrons in the field of the beam-excited plasma wave.     

Dynamics of the magnetospheric cyclotron ELF/VLF maser in the backward-wave-oscillator regime. II. The influence of the magnetic-field inhomogeneity

We study the influence of the magnetic-field inhomogeneity on the nonlinear dynamics of the absolute instability of whistler-mode waves in the Earth’s magnetosphere in the presence of a step-like deformation in the distribution function of energetic electrons. Development of this instability, implying the transition of the magnetospheric cyclotron maser to the regime of a backward-wave oscillator (BWO), was proposed earlier as a generation mechanism of magnetospheric chorus emissions. We analyze the results of numerical simulations of the simplified nonlinear equations describing the magnetospheric-BWO dynamics in the case of low efficiency of wave-particle interactions. We found that the case of an inhomogeneous magnetic field where the system length is much greater than the length characterizing the linear stage of the BWO regime has important specific features compared with the case of a homogeneous medium. The main feature of the nonlinear wave dynamics in the magnetospheric BWO in an inhomogeneous magnetic field consists in the fact that for a sufficiently large excess over the generation threshold, a sequence of separate wave packets, i.e., discrete elements, is formed. The frequency within each packet varies in time, and these discrete elements are close in their properties to the chorus elements observed in the magnetosphere. The results of calculations confirm the quantitative estimates of parameters of chorus emissions, which were performed earlier on the basis of the BWO model. ^†Deceased Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 11, pp. 977–987, November 2008.     

Kinetic modulational instability of Langmuir turbulence

The envelope modulational instability of a spectrum of random-phase Langmuir waves is examined in the regime where the group velocities are comparable to the ion thermal speed such that the ion kinetic effects are important.     

Observation of pulsed fast electron precipitations and the cyclotron generation mechanism of burst activity in a decaying ECR discharge plasma

We have detected and investigated quasi-periodic series of pulsed energetic electron precipitations in the decaying plasma of a pulsed ECR discharge in a mirror axisymmetric magnetic trap. The observed particle ejections from the trap are interpreted as the result of resonant interaction between energetic electrons and a slow extraordinary wave propagating in the rarefied plasma across the external magnetic field. We have been able to explain the generation mechanism of the sequences of pulsed precipitations at the nonlinear instability growth phase in terms of a cyclotron maser model in which the instability threshold is exceeded through a reduction in electromagnetic energy losses characteristic of the plasma decay.