H. X. Vu

Kinetic inflation of stimulated Raman backscatter in regimes of high linear Landau damping

Kinetic simulations and analysis show that backward stimulated Raman scattering (BSRS), in regimes of large linear Landau damping of the primary Langmuir wave, attains levels greatly exceeding the predictions of models based on fixed damping. These regimes are encountered in plasma conditions expected for target designs to be fielded at the National Ignition Facility [J. D. Lindl, Inertial Confinement Fusion (Springer-Verlag, New York, 1998)]. Trapped electrons in the Langmuir wave have the dual effect of reducing its damping, thereby enhancing the BSRS response, and saturating this response by phase detuning, a consequence of the trapping-induced, time-dependent, frequency shift. BSRS, then, occurs as a train of sub-picosecond pulses, arising from the competition between phase detuning and parametric regeneration. A simple three wave parametric model, including the effect of the nonlinear frequency shift and residual nonlinear damping, reproduces these essential features. A similar scenario applies to backward stimulated Brillouin scattering (BSBS). BSRS activity many orders of magnitude above noise level is found for intense laser speckles even when the primary Langmuir wave number times the Debye length is as high as 0.55. The simulation model consistently accounts for the competition of other instabilities, including BSBS, forward stimulated Raman scattering, and the Langmuir decay instability with cavitation.

Evidence of plasma fluctuations and their effect on the growth of stimulated Brillouin and stimulated Raman scattering in laser plasmas

The reflectivity levels of stimulated Brillouin scattering (SBS) in recent large scale length laser plasma experiments is much lower than expected for conditions where the convective gain exponent is expected to be large. Long wavelength velocity fluctuations caused during the plasma formation process, or by parametric instabilities themselves, have been proposed as a mechanism to detune SBS in these experiments and reduce its gain. Evidence of large velocity fluctuation levels is found in the time-resolved SBS spectra from these experiments, and correlates with observed changes in the reflectivity of both SBS and stimulated Raman scattering (SRS). The authors present evidence of fluctuations which increase as the plasma density systematically increases, and discuss their effect on the growth of parametric instabilities.

An analytical and numerical investigation of ion acoustic waves in a two‐ion plasma

The ion acoustic dispersion relation for a plasma containing two distinct ion species is investigated over a wide range of plasma conditions. An approximate general analytic solution to the dispersion relation has been found, and is shown, by comparison to accurate numerical solutions of the individual modes, to be remarkably precise. This solution provides for the first time a systematic account of the totality of ion acoustic modes of the two‐ion system. It has been found that ion acoustic modes consist of two types of modes: (a) at least one, and, at most, two weakly damped modes for which ‖ωI/ωR‖≪1, and (b) an infinity of critically damped modes for which ωI/ωR≂−1. The critically damped modes are organized into two distinct categories: (a) modes for which ‖ω‖/k≳vF (vF is the thermal speed of the fast ion species); and (b) modes for which vS<‖ω‖/k<vF (vS is the thermal speed of the slow ion species). The critically damped modes with ‖ω‖/k≳vF are further organized into three distinct classes: (1) modes ...

Different kλD regimes for nonlinear effects on Langmuir wavesa)

As Langmuir waves (LWs) are driven to large amplitude in plasma, they are affected by nonlinear mechanisms. A global understanding, based on simulations and experiments, has emerged that identifies various nonlinear regimes depending on the dimensionless parameter kλD, where k is the Langmuir wave number and λD is the electron Debye length. The nonlinear phenomena arise due to wave-wave and wave-particle coupling mechanisms, and this basic separation between fluid-like nonlinearities and kinetic nonlinearities depends on the degree to which electron and ion Landau damping, as well as electron trapping, play a role. Previous ionospheric heating experiments [Cheung et al. Phys. Plasmas 8, 802 (2001)] identified cavitation/collapse and Langmuir decay instability (LDI), predominantly wave-wave mechanisms, to be the principal nonlinear effects for driven LWs with kλD<0.1, in agreement with fluid simulations [DuBois et al. Phys. Plasmas 8, 791 (2001)]. In the present research, collective Thomson scattering meas...

Two‐dimensional analysis of the power transfer between crossed laser beams

The power transfer between crossed laser beams made possible by an ion‐acoustic wave is studied. A simple formula is derived for the steady‐state power transfer, which depends on two dimensionless parameters: the ratio of the incident beam intensities and the normalized beamwidth. Numerical simulations show that the transient power transfer is larger than the steady‐state power transfer and usually oscillates in time. The convective depletion of the higher‐frequency beam saturates the power transfer more quickly than the damping of the ion‐acoustic wave.

Nonlinear kinetic simulations of stimulated Brillouin scattering

Numerical simulation results of stimulated Brillouin scattering (SBS) using HERCULES [H. X. Vu, J. Comput. Phys. 124, 417 (1996)], an adiabatic fluid-electron particle-in-cell (PIC) code, are presented. The results of these PIC simulations are compared against fluid simulations, and good agreement is obtained for sufficiently weak laser intensities. When the laser intensity is sufficiently strong for ion trapping to be significant, PIC and fluid simulations differ substantially. The trapping time and nonlinear frequency shift obtained in the PIC simulations are in good agreement with analytical predictions. The SBS reflectivity is shown to be very sensitive to frequency mismatch between the light wave used to seed the instability and the incident laser.

Measurements of laser-plasma instability relevant to ignition hohlraums

The potential for laser-plasma instability is a serious concern for indirect-drive inertial confinement fusion (ICF), where laser beams illuminate the interior of a cavity (called a hohlraum) to produce x-rays for imploding a fusion capsule symmetrically. The speckled nature of laser beams used in ICF is an important factor in laser-plasma instability processes. For example, models which calculate the spatial growth of convective instability by properly accounting for the laser speckles successfully predict the observed onsets of backscattering due to stimulated Brillouin and Raman scattering instabilities (SBS and SRS). Assuming pump depletion as the only saturation mechanism in these models results in very large predicted levels of SBS and SRS backscattering from the long-scale plasmas expected in ignition hohlraums. However, in the long-scale plasmas studied in the Nova and Trident lasers [E. M. Campbell, Rev. Sci. Instrum. 57, 2101 (1986) and N. K. Moncur et al., Appl. Opt. 34, 4274 (1995)], SRS and S...

Quantitative comparison of reduced-description particle-in-cell and quasilinear-Zakharov models for parametrically excited Langmuir turbulence

The effect of kinetic processes on the saturation of parametric instabilities in an electromagnetically driven plasma is investigated. A reduced-description particle-in-cell technique is used as a benchmark to test a new quasilinear-Zakharov model which accounts for electron heating due to Landau damping by coupling the quasilinear diffusion equation to the Zakharov equations. The reduced-description particle-in-cell method utilizes a two-time-scale approximation which significantly reduces the numerical dissipation and ion noise levels. This approach allows accurate modeling of Langmuir and ion acoustic waves in regimes typically studied with Zakharov simulations. The comparison of the two models is performed for the test case of a one-dimensional homogeneous plasma driven by a spatially uniform pump in both the Langmuir decay instability cascade and collapse regimes. Good agreement is found in both weakly and strongly driven regimes for the total Langmuir wave energy and evolved electron velocity distri...

The effect of kinetic processes on Langmuir turbulence

Kinetic processes are shown to be crucial in determining the saturation level of stimulated Raman scattering for regimes relevant to NOVA [Campbell et al., Fusion Technol. 21, 1344 (1992)] and the National Ignition Facility [Lindl, Phys. Plasmas 2, 3933 (1995)]. To investigate these kinetic effects, the Zakharov, quasilinear-Zakharov, and reduced-description particle-in-cell simulation models are compared in the test case of a uniformly driven plasma. Good agreement is observed between all three simulation methods for relatively low primary Langmuir wave numbers (k1λDe∼0.1) in weakly driven regimes. In the strongly driven case, quasilinear diffusion provides an important correction to the Landau damping rate, producing saturation levels in agreement with reduced-description particle-in-cell simulations, in contrast to pure Zakharov simulations, which overestimate the saturation significantly. At higher k1λDe∼0.25, both the quasilinear-Zakharov and pure Zakharov models fail. In this regime, the autocorrela...

Computational study of laser imprint mitigation in foam-buffered inertial confinement fusion targets

Recent experiments have shown that low density foam layers can significantly mitigate the perturbing effects of beam nonuniformities affecting the acceleration of thin shells. This problem is studied parametrically with two-dimensional LASNEX [G. B. Zimmerman and W. L. Kruer, Comments Plasma Phys. Controlled Fusion 2, 51 (1975)]. Foam-buffered targets are employed, consisting typically of 250 A of gold, and 50 μm of 50 mg/cm3 C10H8O4 foam attached to a 10 μm foil. In simulation these were characteristically exposed to 1.2 ns, flat-topped green light pulses at 1.4×1014 W/cm2 intensity, bearing 30 μm lateral perturbations of up to 60% variation in intensity. Without the buffer layers the foils were severely disrupted by 1 ns. With buffering only minimal distortion was manifest at 3 ns. The smoothing is shown to derive principally from the high thermal conductivity of the heated foam. The simulation results imply that (1) the foam thickness should exceed the disturbance wavelength; (2) intensities exceeding ...