非均匀等离子体中受激Raman散射非线性行为

鉴于实际中等离子体不均匀性和非周期性边界条件，建立了受激Ｒａｍａｎ散射（ＳＲＳ）和受激Ｂｒｉｌｌｏｕｒｉｎ散射（ＳＢＳ）非线性耦合模型。ＳＢＳ对ＳＲＳ的影响主要表现在两方面：１）Ｌａｎｇｍｕｉｒ波与离子声波的非线性相互作用，２）ＳＢＳ与ＳＲＳ的竞争。本文研究了离子声衰变不稳定性、离子声波对Ｌａｎｇｍｕｉｒ波的非共振散射两种非线性过程在ＳＲＳ发展过程中的作用，给出一维不均匀等离子体中ＳＲＳ发展图象。Ｌａｎｇｍｕｉｒ波向短波转换，从而被强烈阻尼是抑制ＳＲＳ的重要机制。文中给出了ＳＢＳ／ＳＲＳ耦合过程中决定ＳＲ 关键词：     

Trapped-particle instability leading to bursting in stimulated Raman scattering simulations

Nonlinear, kinetic simulations of stimulated Raman scattering (SRS) under laser-fusion conditions present a bursting behavior. Different explanations for this regime have been given in previous studies: saturation of SRS by increased nonlinear Landau damping [K. Estabrook et al., Phys. Fluids B 1, 1282 (1989)]], and detuning due to the nonlinear frequency shift of the plasma wave [H. X. Vu et al., Phys. Rev. Lett. 86, 4306 (2001)]]. Another mechanism, also assigning a key role to the trapped electrons is proposed here: the breakup of the plasma wave through the trapped-particle instability.     

Linear Fluid Theory for Weakly Inhomogeneous Plasmas with Strong Correlations

Linearized fluid equations for the collective modes of weakly inhomogeneous plasmas including strong coupling effects are derived from a recent kinetic theory based on an extended Singwi‐Tosi‐Land‐Sjölander (STLS) ansatz [H. Kählert, G. J. Kalman, and M. Bonitz, Phys. Rev. E 90 , 011101(R) (2014)]. The equations are analogous to the equations of linearized elasticity theory with space dependent elastic moduli that correspond to those of a bulk system with the local fluid density. The identification of the latter as infinite‐frequency elastic moduli shows that the present version of the extended STLS theory accounts for the elastic properties of the fluid but does not capture the viscous behavior. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Observation of a transition from fluid to kinetic nonlinearities for langmuir waves driven by stimulated Raman backscatter

Thomson scattering is used to measure Langmuir waves (LW) driven by stimulated Raman scattering (SRS) in a diffraction limited laser focal spot. For SRS at wave numbers klambda(D) less similar 0.29, where k is the LW number and lambda(D) is the Debye length, multiple waves are detected and are attributed to the Langmuir decay instability (LDI) driven by the primary LW. At klambda(D) greater similar 0.29, a single wave, frequency-broadened spectrum is observed. The transition from the fluid to the kinetic regime is qualitatively consistent with particle-in-cell simulations and crossing of the LDI amplitude threshold above that for LW self-focusing.     

一维动理学数值模拟激光与等离子体的相互作用

利用一维(1D3V)、显式、全电磁、相对论粒子模拟代码研究动理学范畴内激光与等离子体相互作用中的受激拉曼散射,给出了粒子代码的控制方程及其数值离散的详细方案,研究表明:动理学效应在受激拉曼散射不稳定性中十分重要;时问平均的反射率在阈值强度处跃升,在更高的激光强度处达到饱和;受激拉曼背向散射周期性地在次皮秒内爆发,离子效应延迟背向拉曼散射的发生;电子俘获导致了背向拉曼散射出现爆发;Langmuir波的非线性频移使得背向散射达到饱和。     

Modification of spatial and temporal gains of stimulated brillouin and raman scattering by polarization smoothing

Significant reductions of stimulated Brillouin (SBS) and Raman (SRS) scattering are measured by adding polarization smoothing (PS) to a random phase plate smoothed beam. The associated plasma waves, as measured by Thomson scattering, are spatially and temporally modified and reveal that the interplay between SBS and SRS has to be taken into account to understand the effect of the smoothing. The results also support the numerical simulations [S. Huller et al., Phys. Plasmas 5, 2706 (1998); R. Berger et al., Phys. Plasmas 6, 1043 (1999)] predicting a reduction of self-focusing with PS, resulting in a decrease of the maximum laser intensity and consequently of SBS and SRS gains.     

New paradigm for the isotope scaling of plasma transport paradox

Most tokamak experimental results [Nucl. Fusion 33, 1205 (1993)]] and basic physics experiments [Phys. Rev. Lett. 89, 095001 (2002)]] in the Columbia Linear Machine indicate dependence of the ion thermal conductivity on the isotopic mass close to chi( perpendicular ) approximately A(-0.5)(i), i.e., inverse gyro-Bohm. This is in stark contradiction to most present theoretical models predicting Bohm (A(0)(i)) or gyro-Bohm (A(0.5)(i)) scaling. A series of experiments designed to explore the physics basis of this scaling appears to lead to a new model for this scaling based on 3-wave coupling of two ion temperature gradient radial harmonics and an ion acoustic wave.     

Quantum kinetic theory of trapped particles in a strong electromagnetic field

The idea of treating quantum systems by semiclassical representations using effective quantum potentials (forces) has been successfully applied in equilibrium by many authors, see e.g. [D. Bohm, Phys. Rev. 85 (1986) 166 and 180; D.K. Ferry, J.R. Zhou, Phys. Rev. B 48 (1993) 7944; A.V. Filinov, M. Bonitz, W. Ebeling, J. Phys. A 36 (2003) 5957 and references cited therein]. Here, this idea is extended to nonequilibrium quantum systems in an external field. A gauge-invariant quantum kinetic theory for weakly inhomogeneous charged particle systems in a strong electromagnetic field is developed within the framework of nonequilibrium Green’s functions. The equation for the spectral density is simplified by introducing a classical (local) form for the kinetics. Nonlocal quantum effects are accounted for in this way by replacing the bare external confinement potential with an effective quantum potential. The equation for this effective potential is identified and solved for weak inhomogeneity in the collisionless limit. The resulting nonequilibrium spectral function is used to determine the density of states and the modification of the Born collision operator in the kinetic equation for the Wigner function due to quantum confinement effects.     

Reply to “On the Morse oscillator with a kinetic coupling” by Fernández

Having analyzed Fernández's approach for the Morse oscillator with a kinetic coupling [Phys. Lett. A 229 (1997) 262] and contrasting it with our original paper [Phys. Lett. A 213 (1996) 226], we point out that a new representation is implicitly used by Fernández. The dynamic phase difference of two wave functions obtained via two different approaches is due to different representations adopted in these two papers respectively.     

On the interaction of strange attractors

This paper deals with the dynamics of diffusively coupled strange attractors. Such interaction tends to equalize their instantaneous states and, for large coupling constant, results in a homogeneous state that is chaotic in time. The stability of this state depends on the relation between the Lyapunov exponent and the coupling constant. Statistical properties are determined for weakly inhomogeneous disturbances near a stable homogeneous regime. The inhomogeneous state beyond the stability threshold is treated by using the mean-field approximation. We show that both cases of soft (supercritical) and hard (subcritical) excitation of the inhomogeneous state may occur.     

Emergent O(4) symmetry at the phase transition from plaquette-singlet to antiferromagnetic order in quasi-two-dimensional quantum magnets

Recent experiments [Guo et al., Phys. Rev. Lett. 124 206602(2020)] on thermodynamic properties of the frustrated layered quantum magnet SrCu_2(BO_3)_2-the Shastry–Sutherland material-have provided strong evidence for a lowtemperature phase transition between plaquette-singlet and antiferromagnetic order as a function of pressure. Further motivated by the recently discovered unusual first-order quantum phase transition with an apparent emergent O(4) symmetry of the antiferromagnetic and plaquette-singlet order parameters in a two-dimensional "checkerboard J-Q" quantum spin model[Zhao et al., Nat. Phys. 15 678(2019)], we here study the same model in the presence of weak inter-layer couplings. Our focus is on the evolution of the emergent symmetry as the system crosses over from two to three dimensions and the phase transition extends from strictly zero temperature in two dimensions up to finite temperature as expected in SrCu_2(BO_3)_2.Using quantum Monte Carlo simulations, we map out the phase boundaries of the plaquette-singlet and antiferromagnetic phases, with particular focus on the triple point where these two ordered phases meet the paramagnetic phase for given strength of the inter-layer coupling. All transitions are first-order in the neighborhood of the triple point. We show that the emergent O(4) symmetry of the coexistence state breaks down clearly when the interlayer coupling becomes sufficiently large, but for a weak coupling, of the magnitude expected experimentally, the enlarged symmetry can still be observed at the triple point up to significant length scales. Thus, it is likely that the plaquette-singlet to antiferromagnetic transition in SrCu_2(BO_3)_2 exhibits remnants of emergent O(4) symmetry, which should be observable due to additional weakly gapped Goldstone modes.     

Double exchange models: self consistent renormalisation

We propose a scheme for constructing classical spin Hamiltonians from Hunds coupled spin-fermion models in the limit J_H/t →∞. The strong coupling between fermions and the core spins requires self-consistent calculation of the effective exchange in the model, either in the presence of inhomogeneities or with changing temperature. In this paper we establish the formalism and discuss results mainly on the “clean” double exchange model, with self consistently renormalised couplings, and compare our results with exact simulations. Our method allows access to system sizes much beyond the reach of exact simulations, and we can study transport and optical properties of the model without artificial broadening. The method discussed here forms the foundation of our papers [Phys. Rev. Lett. 91, 246602 (2003), and Phys. Rev. Lett. 92, 126602 (2004)].     

Saturation of backward stimulated scattering of a laser beam in the kinetic regime

Stimulated Raman (SRS) and Brillouin scattering (SBS) are examined in the kinetic regime using particle-in-cell simulations. Wave front bowing of electron-plasma waves (ion-acoustic waves) from trapped particle nonlinear frequency shift is observed in the SRS (SBS) regime for the first time. Self-focusing from trapped particle modulational instability (TPMI) is shown to occur in 2D and 3D SRS simulations. The key physics of SRS saturation is identified as a combination of wave front bowing, TPMI, and self-focusing: Bowing marks the beginning of SRS saturation and self-focusing terminates SRS. Ion-acoustic wave bowing also contributes to SBS saturation. Velocity diffusion by transverse modes and rapid loss of hot electrons in regions of small transverse extent formed from self-focusing dissipate wave energy and increase Landau damping, despite trapping that reduces Landau damping initially.     

Plasmon Dispersion of a Weakly Degenerate Nonideal One‐Component Plasma

Classical Molecular Dynamics simulations (MD) for a one‐component weakly degenerate plasma are presented. Using an effective quantum pair potential (Kelbg potential), the dynamic structure factor and the dispersion of Langmuir waves are computed. The influence of the coupling strength Γ and degree of degeneracy ρΛ³ on these properties is discussed. The results are compared with predictions of mean‐field theories.     

Potential Formation in Front of an Electrode Close to the Plasma Potential Studied by PIC Simulation

Potential formation in front of an electrode that has the potential which is close to the plasma potential is studied by particle‐in‐cell (PIC) simulations. The code BIT1 [D. Tskhakaya, R. Schneider, J. Comp. Phys., 225 , 829 (2007)] is used for this purpose. It is shown that this code is very appropriate for our analysis because of its ability to create charged particles by uniform volume production in the entire system and to maintain in the same time the Maxwellian electron velocity distribution function prescribed in the input file. It turns out that some modifications of the code are necessary in order to achieve small sheath potential drops and to detect the cutoff in the electron velocity distribution function. The modifications of the code are described. The small sheath potential drop is achieved by introducing a finite reflectivity of the electrons at the left electrode, while the expected cutoffs of the electron velocity distribution function are found by tracking the reflected electrons as separate particle species. The simulation results are compared to the theoretical model of Jeli? [N. Jeli?, Phys. Plasmas, 18 , 113504 (2011)]. The matching is very good and this is a sign that the PIC simulations are very appropriate tool for the analysis of this type of problems. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Meandering Spiral Waves Induced by Time-Periodic Coupling Strength

Effects of time-periodic coupling strength (TPCS) on spiral waves dynamics are studied by numerical computations and mathematical analyses. We find that meandering or drifting spirals waves, which are not observed for the case of constant coupling strength, can be induced by TPCS. In particular, a transition between outward petal and inward petal meandering spirals is observed when the period of TPCS is varied. These two types of meandering spirals are separated by a drifting spiral, which can be induced by TPCS when the period of TPCS is very close to that of rigidly rotating spiral. Similar results can be obtained if the coupling strength is modulated by a rectangle wave. Furthermore, a kinetic model for spiral movement suggested by Di et al., [Phys. Rev. E 85 (2012) 046216] is applied for explaining the above findings. The theoretical results are in good qualitative agreement with numerical simulations.     

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.     

Backscatter reduction using combined spatial, temporal, and polarization beam smoothing in a long-scale-length laser plasma

Spatial, temporal, and polarization smoothing schemes are combined for the first time to reduce to a few percent the total stimulated backscatter of a NIF-like probe laser beam (2x10(15) W/cm (2), 351 nm, f/8) in a long-scale-length laser plasma. Combining temporal and polarization smoothing reduces simulated Brillouin scattering and simulated Raman scattering (SRS) up to an order of magnitude although neither smoothing scheme by itself is uniformly effective. The results agree with trends observed in simulations performed with the laser-plasma interaction code F3D simulations [R. L. Berger et al., Phys. Plasma 6, 1043 (1999)].     

Meandering Spiral Waves Induced by Time-Periodic Coupling Strength

Effects of time-periodic coupling strength （TPCS） on spiral waves dynamics are studied by numerical computations and mathematical analyses. We find that meandering or drifting spirals waves, which are not observed for the case of constant coupling strength, can be induced by TPCS. In particular, a transition between outward petal and inward petal meandering spirals is observed when the period of TPCS is varied. These two types of meandering spirals are separated by a drifting spiral, which can be induced by TPCS when the period of TPCS is very close to that of rigidly rotating spiral. Similar results can be obtained if the coupling strength is modulated by a rectangle wave. Furthermore, a kinetic model for spiral movement suggested by Diet al., [Phys. Rev. E 85 （2012） 046216] is applied for explaining the above findings. The theoretical results are in good qualitative agreement with numerical simulations.