摘要:

The effectiveness of incoherence‐induced optical smoothing techniques is studied experimentally and good agreement with theory is found. Using incoherent laser‐drive radiation, the Rayleigh–Taylor instability is investigated in planar foil targets with imposed sinusoidal modulations. Growth rates of up to 60% of the classical value are inferred from the measurements and the nonlinear behavior of the instability is investigated. Growth is observed at all wavelengths tested between 30 and 100 &mgr;m.

ISSN:0899-8221    

DOI:10.1063/1.860940

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The effect of the induced spatial incoherence method of laser beam smoothing on the convective Raman instability in an underdense inhomogeneous plasma has been studied. It is found that only for bandwidths comparable to, or greater than, the time‐averaged homogeneous growth rate, is there significant smoothing effect resulting in a reduction of the backscattered radiation. However, for narrow bandwidths the backscattered radiation computed in the presence of hot spots created by the echelons, turns out to be significantly less than the levels observed in the Naval Research Laboratory experiments. We argue that a combination of three‐dimensional effects and filamentation may account for the observed level of enhanced scattering.

ISSN:0899-8221    

DOI:10.1063/1.860941

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Forward and backward stimulated Raman scattering (SRS) have been investigated in three different plasma geometries: in exponential density profile, in Gaussian density profile, and in quasihomogeneous preformed plasmas. The experiments were performed with the same irradiation setup at 0.53 &mgr;m wavelength and at a laser intensity ∼1015W/cm2. Time‐resolved spectra and energy rates of this instability have been measured in the three experiments. A strong increase of the backward SRS energy rate—up to 1%—with the increasing length of the plasma was observed. From data analysis, backward SRS is explained in terms of absolute instability. On the other hand, forward SRS can be interpreted by considering linear scattering of the incident laser light on the plasma wave driven by backward SRS. This assumption is adequately supported both by the spectral characteristics and by the energy rates of the forward emission light.

ISSN:0899-8221    

DOI:10.1063/1.860942

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Thermal filamentation and self‐focusing of laser light has been shown to be significantly altered in typical inertial confinement fusion (ICF)‐type laser plasmas because of heat flux inhibition associated with nonlocal electron thermal conduction. The conductivity becomes appreciably nonlocal when the temperature scale length is less than about 100 electron mean‐free paths. In contrast to previous flux‐limiter models, this mechanism is also important for small temperature perturbations such as those associated with thermal filamentation, where the heat flux can be orders of magnitude less than the free‐streaming value. Using a three‐dimensional time‐dependent laser–plasma propagation code modified to include nonlocal electron thermal conductivity, the scaling and behavior of nonlocal thermal filamentation are investigated. Both unsmoothed and optically smoothed laser beams are studied. The simulation results are contrasted with earlier results generated with the classical Spitzer–Ha¨rm conductivity, and compared to a perturbation analysis. The filamentation length predicted by the perturbation analysis for unsmoothed and smoothed beams compares favorably to the simulations. This theory is used to predict the behavior of optically smoothed beams in large inhomogeneous plasmas that cannot be adequately resolved by simulation.

ISSN:0899-8221    

DOI:10.1063/1.860976

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The problem of ion expansion when a powerful subpicosecond laser pulse is incident upon a solid target is considered in conditions when the mean electron energy in the plasma is the function of time. It is shown that the problem can be solved for the simple case in which the electron has a power‐law dependence with time during the laser pulse. The solutions which are obtained are significantly different from the well‐known case of isothermal expansion. The ion density profile is much steeper and consequently the ion energy distribution has a very steep gradient in the high‐energy tail. For the case whenvte/vos<1 (vteandvosare the thermal and oscillation velocities of electrons, respectively) profile steepening due to the dynamics of the expansion dominates over that caused by the ponderomotive force. The problem considered is relevant to laser–matter interaction experiments where the electron energy has a strong dependence on time during the laser pulse. Some comparisons with recent experiments are presented.

ISSN:0899-8221    

DOI:10.1063/1.860943

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The problem of thermal instability, having bearing on the formation of astrophysical condensations, is investigated for a hydromagnetic fluid obeying generalized Ohm’s law, both for self‐gravitating and nongravitating configurations. Effects of finite Larmor frequency, resistivity, and finite plasma frequency on the stability of the system are studied and the condition of instabilities are derived for a temperature‐dependent and density‐dependent heat‐loss function. It is found that the condition of instability for propagation parallel to the ambient magnetic field is independent of finite resistivity, Hall current, and electron inertia effects and also of the magnetic field strength. For transverse propagation, however, the instability criterion involves the field strength, resistivity, and electron inertia terms. The Hall current is found to modify the growth rates for generally inclined propagation only. Both monotonically unstable and overstable modes of instability arise in the system depending on the dependence of the heat‐loss function on the local density and temperature.

ISSN:0899-8221    

DOI:10.1063/1.860944

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The magnetic reconnection that occurs during the nonlinear development of the coalescence instability is considered. The structure of the reconnection region at the time of maximum current as a function of the resistivity &eegr; is analyzed in detail using a compressible magnetohydrodynamic fluid code. It is shown that the numerical results concur remarkably well with a simple scaling analysis which predicts the dependence of the reconnection region structure on &eegr;. It is argued that the flow topology is crucial in maintaining the ‘‘fast’’ reconnection rate. The results indicate a flux pileup solution in which the flux annihilation rate is approximately independent of &eegr;, whereas the Ohmic dissipation rate scales as &eegr;−1/3. The possibility that these scalings break down at lower values of &eegr; is discussed.

ISSN:0899-8221    

DOI:10.1063/1.860945

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The motion of an electron in a linearly polarized wiggler with an axial guide field is found to be nonintegrable. There is evidence for chaos from numerical calculations of Poincare´ maps and of nonzero Lyapunov exponents. Resonances can be predicted from a one‐dimensional Hamiltonian perturbed by a small ‘‘time‐dependent’’ quantity.

ISSN:0899-8221    

DOI:10.1063/1.860946

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A fully self‐consistent theory of ferromagnetic waveguide accelerators driven by a relativistic electron beam is developed. The theoretical analysis is based on Faraday’s law, which provides a second‐order partial‐differential equation of the azimuthal magnetic field, under the assumption that &mgr;&egr;≫1. Here &mgr; and &egr; are the permeability and dielectric constant of the waveguide material. The azimuthal magnetic field and axial acceleration field are obtained in forms of integral equations for an arbitrary profile of the drive‐beam currentI(t). In the limit when the conductivity &sgr; of the waveguide material is zero, the acceleration mechanism is similar to the typical wake‐field accelerators. In this limit, the acceleration field is proportional to the square root of the parameter &mgr;/&egr; and can be easily more than 200 MV/m for moderate system parameters. On the other hand, for high conductivity limit, the acceleration mechanism is the magnetic field decay, exhibiting that the electromagnetic fields are a decaying function of the time. With appropriate physical parameters, the acceleration gradient of the magnetic field‐decay accelerator is also very large.

ISSN:0899-8221    

DOI:10.1063/1.860947

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The relativistic electron nonlinear orbit is investigated for a combined electrostatic plasma wave wiggler and an axial guide magnetic field free electron laser near magnetoresonance. Through simplification, the perpendicular orbit equation is reduced to be identical to the wave‐driven nonlinear oscillator equation [R. C. Davidson, Phys. Lett. A126, 21 (1987)] but with different expressions of parameters. It is found that the maximum perpendicular orbit excursion &Dgr;xmaxis in proportion to (aw/&ggr;0)1/3, which is largely determined by the nonlinear effect, whereawis the wiggler strength and &ggr;0is the initial relativistic factor. The analytical results are in good agreement with the numerical calculation for small ratio ofawto &ggr;0.

ISSN:0899-8221    

DOI:10.1063/1.860948

出版商:AIP    

年代:1993

数据来源: AIP