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The Arecibo high-power, high-frequency (HF) facility and 430 MHz radar are used to examine the temporal development of the HF-induced Langmuir and ion turbulences from 1 ms to many minutes after the turn-on of the HF beam in the F region. All HF observations begin in a smooth, stratified, stable plasma. “Cold start” HF transmissions are employed to avoid remnant irregularities from prior HF transmissions. HF-excited plasma line (HFPL) and ion line echoes are used to monitor the evolution of the turbulence. In the evening/nighttime the HFPL develops in three reproducible stages. Over time scales of 0 to 10–20 ms (possibly 40 ms), the smooth plasma conditions are maintained, and the results are consistent with theoretical models of the excitation of strong Langmuir turbulence near HF reflection. This entails the initiation of the so-called “caviton production cycle.” The turbulence from the parametric decay instability is detected at lower altitudes where the radar wave vector matches those of the HF-enhanced waves. The data suggests that the two processes coexist in the region in between. After ~40 ms the “overshoot process” begins and consists of a downward extension of the HFPL from the HF reflection region to heights ~1.1 km below followed by a retreat back to the reflection region. The whole overshoot process takes place over a time scale of ~3 s. Thereafter the echo remains near HF reflection for 20–90 s after HF turn-on. The HFPL echo subsequently breaks up into patches because of the formation of large-scale electron density structures in the plasma. New kinetic models indicate that suprathermal electrons excited in the plasma by, for example, caviton burn-out serve to regulate plasma turbulence in the modified ionospheric volume.

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.

The linear stability of multiple coherent laser beams with respect to two-plasmon-decay instability in an inhomogeneous plasma in three dimensions has been determined. Cooperation between beams leads to absolute instability of long-wavelength decays, while shorter-wavelength shared waves are shown to saturate convectively. The multibeam, in its absolutely unstable form, has the lowest threshold for most cases considered. Nonlinear calculations using a three-dimensional extended Zakharov model show that Langmuir turbulence created by the absolute instability modifies the convective saturation of the shorter-wavelength modes, which are seen to dominate at late times.

In experiments where ground-based VHF/UHF radars are used to diagnose Langmuir turbulence driven by powerful HF waves transmitted from the ground into the ionosphere, there has been a severe observability aspect: The excited Langmuir wave vector spectrum must contain a component satisfying the actual radar Bragg condition. In order to explain earlier observations, in particular those made by the Arecibo facility, it was proposed long ago that the actual wave number spectra were widened by refraction in small-scale irregularities, or ducts. This mechanism of improved observability is discussed in a broad approach in this paper. The main contribution is a mathematical derivation of the theory of parametric decay instability in the presence of magnetic-field-aligned ducts. This implies that the decay is into discrete duct modes. The threshold is slightly higher and the growth rate is slightly lower than for a corresponding homogeneous medium. Moreover, this tendency is more pronounced for the higher duct modes, which are those that may satisfy conditions for observability. The mathematical derivation is based on the driven and damped Zakharov model with ducts included. The model is run numerically, confirming the approximate decay instability theory and demonstrating improved observability. The numerics also demonstrate higher Langmuir energy density inside the ducts than outside.

This is the first of two papers comprising a theoretical and observational study of new, altitude-resolved, observations at Arecibo of Langmuir turbulence induced in the ionosphere by a powerful high-frequency (hf) heater operated at very low duty cycles. As shown in paper II [Cheung et al., Phys. Plasmas 8, 802 (2001)], higher power enabled the first observation at Arecibo of the well-developed decay-cascade features in the Thomson scatter radar power spectrum at the unmodified matching altitudes. New theoretical predictions are presented here for the parameters of these observations emphasizing the altitude and pump power dependence of the radar spectra and the time dependence of the spectra from the decaying spectra following heater switch-off. Further details of the strong turbulence signatures from higher altitudes are also presented. At the lower matching altitudes the increase, with hf power, of the angular width of the well-developed decay-cascade spectrum allows these spectral features to come in...

This is the second of two papers comprising a theoretical and observational study of new, altitude-resolved, observations at Arecibo of Langmuir turbulence induced in the ionosphere by a new, more powerful, high frequency heater operated at very low duty cycles. Altitude resolution of 150 m in incoherent scatter radar spectra is made possible by the coded-long-pulse method. Here we present the first observation at Arecibo of the well-developed parametric decay instability and the Langmuir decay instability cascade features in the Thomson scatter radar power spectrum, of the plasma line, at the unmodified matching altitudes under near-cold start conditions. The dependence of the plasma line spectra on altitude, pump power, and density scale length have been studied. The temporal growth and saturation of the spectra during heating and the decay of the spectra in the afterglow of heating has also been studied in detail. Comparisons are made here with the theoretical predictions of the companion paper I [DuBois et al., Phys. Plasmas 8, 791 (2001)]. From these comparisons and a comparison with recent observations at both Arecibo and Tromso/, we conclude that all the predictions of modern Langmuir turbulence theory for the radar spectral signatures of the turbulence in a smooth ionosphere have now been verified.

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.

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 distributions. Electron heating significantly decreases saturation levels in strongly driven regimes by increasing the Landau damping rate, bringing the quasilinear-Zakharov simulations in much closer agreement with the reduced-description particle-in-cell simulations than standard Zakharov simulations, which do not account for the evolution of the electron distribution.

Three multi-dimensional plasma simulation models for simulating parametric instabilities occurring in laser-produced plasmas are presented. These models are based on a common framework, henceforth referred to as reduced-description, in which the electromagnetic field is represented by three separate temporal envelopes in order to model parametric instabilities with high-frequency and low-frequency daughter waves. Because the temporal envelope representation is used, the explicit dependence on the laser time scale is removed from the simulations models. The time step of the simulation is then restricted by either the electron timescale or the ion timescale, depending on whether parametric instabilities that result in high-frequency daughter waves are important. These simulation models are the basic building blocks of a comprehensive, ongoing model development program at Los Alamos National Laboratory, and have been implemented in multi-dimension on the Accelerated Strategic Computing Initiative (ASCI) parallel computer. Test simulations of stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), the Langmuir decay instability (LDI), and the interaction thereof, are presented.

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...

A fully kinetic, reduced-description particle-in-cell (RPIC) model is presented in which deviations from quasineutrality, electron and ion kinetic effects, and nonlinear interactions between low-frequency and high-frequency parametric instabilities are modeled correctly. The model is based on a reduced description where the electromagnetic field is represented by three separate temporal envelopes in order to model parametric instabilities with low-frequency and high-frequency daughter waves. Because temporal envelope approximations are invoked, the simulation can be performed on the electron time scale instead of the time scale of the light waves. The electrons and ions are represented by discrete finite-size particles, permitting electron and ion kinetic effects to be modeled properly. The Poisson equation is utilized to ensure that space-charge effects are included. The RPIC model is fully three dimensional and has been implemented in two dimensions on the Accelerated Strategic Computing Initiative (ASCI) parallel computer at Los Alamos National Laboratory, and the resulting simulation code has been named ASPEN. We believe this code is the first particle-in-cell code capable of simulating the interaction between low-frequency and high-frequency parametric instabilites in multiple dimensions. Test simulations of stimulated Raman scattering, stimulated Brillouin scattering, and Langmuir decay instability are presented.

Two-dimensional simulation studies are reported of the nonlinear development of stimulated Raman scattering (SRS) from a compact laser hot spot using a reduced model, which includes saturation by pump depletion, Langmuir wave decay cascades, Langmuir wave collapse, and ponderomotive density profile modification. The needle-like intensity distribution in a speckle arising from a random phase plate processed laser beam promotes backscatter SRS. The dependence of the saturated reflectivity and (the comparable in magnitude) absorptivity, on ion acoustic wave and Langmuir wave damping, laser power, electron density, and temperature is studied. There are regimes in which the ponderomotive potential (as well as the Ohmic dissipation) of the induced Langmuir turbulence exceeds that of the localized laser pump. The results support the conclusion that the Langmuir wave Landau damping must be determined by an electron velocity distribution modified by quasilinear and Ohmic heating to account for SRS observed at low ...

We introduce two new and complementary simulation models, which have domains of quantitative agreement, for the self-consistent evolution of the electron distribution, Langmuir decay cascades, and Langmuir collapse. In our reduced-description particle-in-cell (RPIC) simulations, Langmuir decay cascades and cycles of collapse are seen for the first time in particle simulation. The velocity distribution in our Zakharov model is evolved using quasilinear diffusion plus Cerenkov emission loss and agrees in expected domains with the RPIC results. These techniques promise to solve long known problems in the standard simulations of laser-plasma instabilities. {copyright} {ital 1999} {ital The American Physical Society}

New measurements of stimulated electromagnetic emissions (SEE) in low duty cycle heating experiments performed at the HIPAS Observatory are presented. Two distinct types of spectra, a weak diffuse type and a stronger type with deep frequency modulations, were observed. These results have been compared with numerical predictions from a 1D driven and damped Zakharov model and are found to be consistent with Langmuir collapse processes (strong spectra) and Langmuir turbulence of the coexistence type (weak spectra). Through this joint experimental and numerical study, evidence of strong Langmuir turbulence via SEE measurements has been demonstrated. {copyright} {ital 1997} {ital The American Physical Society}

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...

Los Alamos National Laboratory is contributing to the core science and technology of the inertial confinement fusion program leading to the National Ignition Facility. Short summaries of a sample o...

Scattering of laser light by stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) is a concern for indirect drive inertial confinement fusion (ICF). The hohlraum designs for the National Ignition Facility (NIF) raise particular concerns due to the large scale and homogeneity of the plasmas within them. Experiments at Nova have studied laser–plasma interactions within large scale length plasmas that mimic many of the characteristics of the NIF hohlraum plasmas. Filamentation and scattering of laser light by SBS and SRS have been investigated as a function of beam smoothing and plasma conditions. Narrowly collimated SRS backscatter has been observed from low density, low‐Z, plasmas, which are representative of the plasma filling most of the NIF hohlraum. SBS backscatter is found to occur in the high‐Z plasma of gold ablated from the wall. Both SBS and SRS are observed to be at acceptable levels in experiments using smoothing by spectral dispersion (SSD).

Two important effects of long wavelength velocity and density perturbations on stimulated Brillouin scattering (SBS) have been identified: (i) nonlinear interaction of the SBS‐produced ion acoustic wave with velocity perturbations provides detuning of the SBS interaction, formation of ion acoustic wave satellites and its additional damping; (ii) harmonic generation of long wavelength density perturbations enhances the spectral density of ion acoustic fluctuations over a wide range of wavevectors, including SBS resonant sound waves. The first effect reduces the magnitude of the SBS gain, and the second one enhances the nonthermal noise level for SBS. Long wavelength ion acoustic perturbations reduce SBS reflectivity more dramatically than short wavelength harmonics produced by the SBS driven nonlinear sound wave.

The onset of stimulated Brillouin scattering (SBS) has been examined by varying the effective F number (lens focal length/incident beam diameter) of the optical system used to irradiate a performed 1 mm long, longitudinally uniform hydrocarbon (CH) plasma. SBS onset occurs at higher average laser intensity for smaller F, in qualitative agreement with theory. Quantitative comparison between theory and experiment, and implications for the National Ignition Facility [Phys. Plasmas 2, 2480 (1995)] are discussed.

Progress made in the last few years in the calculation of the saturation spectra of parametric instabilities which involve Langmuir daughter waves will be reviewed. These instabilities include the ion acoustic decay instability, the two plasmon decay instability (TPDI), and stimulated Raman scattering (SRS). In particular we will emphasize spectral signatures which can be directly compared with experiment. The calculations are based on reduced models of driven Langmuir turbulence. Thomson scattering from hf-induced Langmuir turbulence in the unpreconditioned ionosphere has resulted in detailed agreement between theory and experiment at early times. Strong turbulence signatures dominate in this regime where the weak turbulence approximation fails completely. Recent experimental studies of the TPDI have measured the Fourier spectra of Langmuir waves as well as the angular and frequency spectra of light emitted near 3/2 of the pump frequency again permitting some detailed comparisons with theory. Thomson scattering measurements of the Langmuir wave spectra from SRS are consistent with the saturation by secondary and tertiary decay of the primary SRS Langmuir waves. Scaling laws derived from a local model of SRS saturation are compared with full simulations and recent Nova experiments.

Numerical solutions of the Zakharov equations for a plasma driven above the electron plasma frequency by a long‐wavelength radiation pump can be applied to both ionospheric modification experiments and laboratory laser‐plasma interactions. A key difference between these two environments is the much larger collisional damping of Langmuir waves near the critical density in laser plasmas. Zakharov equation simulations in one and two dimensions reveal a significant change in the character of the saturated turbulence state of the electromagnetic ion‐acoustic decay instability for pump strengths near threshold as the collisional damping is increased to values appropriate to certain (low‐intensity) laboratory laser‐plasma experiments [Phys. Fluids B 3, 1983 (1991)]. The linear‐instability is then characterized by the coupling of the up‐ and downshifted Langmuir decay modes. A new turbulence regime differing from existing models of both weak and strong turbulence is found, which is characterized by a sequence of narrow peaks in the Langmuir frequency spectrum and a nonlinearly broadened wave‐vector spectrum centered near the linearly most unstable modes. Results in this regime may be relevant to second harmonic emission experiments [Phys. Fluids B 3, 1983 (1991)].

The interaction of a powerful, subpicosecond laser pulse with plasma is modeled within the capacitor model by means of one‐dimensional electrostatic particle code. The temporal profile of the laser pulse and the growth of the electron energy by several orders of magnitude were taken into account providing adequate temporal and spatial resolution. It is shown that initially, a coherent structure is excited, and growth of plasma waves results in wave breaking and acceleration of electrons followed with decreasing intervals between them, accelerating electrons in both directions. Finally the system evolves to a state of moderate Langmuir turbulence where E2/(16πnT)≪1 due to the strong heating of plasma electrons and the decrease of excited plasma wave fields.

A nonlinear, reduced description of the saturated steady-state turbulence resulting from the two-plasmon decay instability is shown to lead to Langmuir wave and 3/2 {omega}{sub 0} radiation spectra which are in semiquantitative agreement with recent experimental observations of Meyer {ital et} {ital al}. [Phys. Rev. Lett. {bold 71}, 2915 (1993)]. Secondary Langmuir wave decay of the primary unstable waves is shown to be an important process for moderate laser intensities while strong turbulence effects are observed for higher intensities.

Vlasov simulations are developed to study the one‐dimensional modulational instability and Langmuir collapse in a plasma driven by an external field. Both electrons and ions in the plasma are described by the Vlasov equations. The external driving field energy density is ∼10−3 times the initial electron thermal energy density. The plasma has a mass ratio of mi/me=1836, and a temperature ratio of Ti/Te=0.1 or 1.0, where mi, me, Ti, and Te are the ion mass, electron mass, ion temperature, and electron temperature. The modulational instability is saturated by the formation of Langmuir cavitons which later collapse and burn out due to the strong wave–particle interactions. The particle heating during a single Langmuir collapse is small under this weak driving field. The accelerated electron and ion jet‐like streams in phase space as well as the nucleation of cavitons are observed during the evolution. Zakharov equations with the same parameters and similar initial conditions are also simulated, and the result...

A reduced description of strong Langmuir turbulence is derived from the Vlasov–Poisson equations by using an oscillation center transformation that decomposes the distribution function into envelope components which vary slowly on the plasma frequency time scale. The familiar high‐frequency response is identified in the context of a basic ordering of small quantities. A generalization of the ion acoustic wave equation to the regime of equal electron and ion temperatures which is valid in this ordering is derived using a three‐pole approximant for the collisionless ion susceptibility. The resulting extended Zakharov model is solved numerically in two dimensions for parameters relevant to radio‐frequency heating of the ionosphere, producing energy and power spectra of fluctuations that are qualitatively similar to experimental observations. A new local quasilinear equation for the spatial and temporal evolution of the slowly varying part of the oscillation center distribution function is proposed.

Langmuir collapse in a radiation‐driven plasma is simulated in one dimension using both the hybrid Vlasov model and the modified Zakharov model. In the hybrid model, the electrons are described by the Vlasov equation and the ions are described by the fluid equations. Two numerical runs with different parameters are performed: (a) a weakly driven case, and (b) a strongly driven case. The comparisons of the results between the two models for each case are carried out in detail. It is found that the dynamic behavior of the plasma can be changed dramatically by varying the external driving field. The electron temperature increases only ∼10% in the weakly driven case, while it increases ∼200% in the strongly driven case during a single Langmuir collapse. The two models agree with each other during the first collapse after the saturation of modulational instability. Dynamic differences appear after the first collapse, caused by the electron heating in the hybrid model, which is not accounted for in the standard isothermal Zakharov model.

Convective instabilities in the strongly damped regime are shown to exhibit essential nonlinear behavior due to laser hot spots when the average laser intensity [l angle][ital I][r angle] approaches a critical threshold value [ital I][sub [ital c]]. The onset of this nonlinear regime is formally signaled by the divergence of the average convective amplification [l angle][ital A][r angle]as [l angle][ital I][r angle][r arrow][ital I][sub [ital c]]. An independent hot spot model of random phase plate optics predicts that [l angle][ital A][r angle][similar to]1/([ital I][sub [ital c]][minus][l angle][ital I][r angle])[sup 2]. A saturated hot spot model of nonlinear stimulated Brillouin scattering (SBS) predicts a rapid turn on and saturation of SBS reflectivity with laser intensity and optic [ital f] number.

The predictions of models of strong Langmuir turbulence (SLT) are compared with recent space‐ and time‐resolved radar observations of the power spectra of turbulence induced in the ionosphere by powerful high‐frequency (HF) waves. Distinct signatures of caviton dynamics, not predicted by the weak turbulence approximation, are seen in the observations. An improved model of the low‐frequency fluctuations for equal electron and ion temperatures is presented as well as a discussion of density profile modification by the induced turbulence.

Numerical studies of a model of stimulated Raman scattering (SRS) coupled to Langmuir turbulence show that once the collisional threshold is exceeded for parametric decay of the SRS daughter Langmuir wave the noncollisional dissipation resulting from Langmuir collapse can strongly saturate the Langmuir wave and the resulting SRS reflectivity. The model provides information on the total rate of hot electron generation. Scaling laws for the reflectivity as a function of laser and plasma parameters are found.

A quantitative theory of laser hot spots, which control plasma instabilities in real laser–plasma interactions, is presented in the case of random phase plate (RPP) optics. It is shown that the probability density of intense hot spots with intensity I, Phot(I), is given by Phot(I)∼(I/I02)exp(−I/I0) where I0 is the average intensity, and that the detailed amplitude and phase variation of the laser field in the vicinity of an intense hot spot is uniquely specified by the optics and is deterministic. These hot spots may be the source of below threshold stimulated Raman scattering (SRS) and its variation with I0 is shown to be super exponential. A brief preview of a quantitative nonlinear theory of hot‐spot‐induced laser filamentation is presented.

The linear stability of a system of multiple overlapping laser beams is analyzed in terms of a linear dispersion relation that is valid with sufficient bandwidth. Each beam may or may not have spatial incoherence. Two beams can always act in concert to produce a linear instability equivalent to that obtained from a single beam with an intensity equal to the sum of the two beam intensities. There is a threshold intensity Icp for all beams to produce a collective ponderomotive instability equivalent to that of a single beam with the same total intensity. The collective thermal filamentation threshold, based upon classical transport, is one based on total power Pct and it seems to be easily accessible.

A class of collective three‐wave parametric instabilities resulting from the overlap of N, phase‐independent, converging laser beams, each with finite frequency bandwidth and modest spatial incoherence is considered. A statistical analysis is employed based on the random phase approximation (RPA) to derive the thresholds, growth rates, and spatial gain coefficients for these instabilities. When the laser beams are symmetrically distributed in a converging conical array it is argued that collective parametric instabilities can be excited in which one of the daughter waves propagates along the symmetry axis of the cone and is coupled equally to all the pump beams and to the other daughter waves that lie in another cone determined by the matching conditions. For a sufficiently large number of beams the collective modes have lower thresholds than the single‐beam instabilities. The collective thresholds are greater than or equal to the incoherent thresholds for a single beam containing N times the single‐beam power. To be effective the spatial extent of the beam overlap region must be long enough to allow significant convective amplification or absolute instability. A potentially dangerous collective instability, resulting from stimulated Raman scattering (SRS), has a common electron plasma wave along the axis of symmetry and has a low threshold for coherent beams. This common plasma wave may be detrimental to laser fusion as a potential source of hot electrons. Sufficient bandwidth and angular divergence of the beams can suppress this instability relative to a more benign collective mode where the scattered light is the common mode along the symmetry axis.

Features in certain laser-plasma and ionospheric experiments are identified with the basic properties of Langmuir wave turbulence. Also, a model of caviton nucleation is presented which leads to certain novel scaling predictions.

We report results of two-dimensional numerical solutions of Zakharov's model for strong Langmuir turbulence excited by powerful electromagnetic pump waves at subcritical densities, as in hf modification of the ionisphere and laser-plasma interactions. The results show that truncated parametric decay cascades can coexist with caviton collapse for moderate pump power. A high level of ion density fluctuations, driven primarly by caviton collapse, produces energy and power spectra which are not consistent with conventional weak turbulence theory.

Recent studies based on long time computer simulations of Langmuir turbulence as described by Zakharov's model will be reviewed. These show that for strong to moderate ion sound damping the turbulent energy is dominantly in non-linear "caviton" excitations which are localized in space and time. A local caviton model will be presented which accounts for the nucleation-collapse-burnout cycles of individual cavitons as well as their space-time correlations. This model is in detailed agreement with many features of the electron density fluctuation spectra in the ionosphere modified by powerful HF waves as measured by incoherent scatter radar. Recently such observations have verified a prediction of the theory that "free" Langmuir waves are emitted in the caviton collapse process. These observations and theoretical considerations also strongly imply that cavitons in the heated ionosphere, under certain conditions, evolve to states in which they are ordered in space and time. The sensitivity of the high frequency Langmuir field dynamics to the low frequency ion density fluctuations and the related caviton nucleation process will be discussed.

The parametric coupling involving backward stimulated scattering of a laser and electron beam acoustic modes (BAM) is described as observed in particle-in-cell (PIC) simulations. The BAM modes evolve from Langmuir waves (LW) as the electron velocity distribution is nonlinearly modified to be non-Maxwellian by backward stimulated Raman scattering (BSRS). With a marginal damping rate, BAM can be easily excited and allow an extended chirping in frequency to occur as later SRS pulses encounter modified distributions. Coincident with the emergence of this non-Maxwellian distribution is a rapid increase in BSRS reflectivities with laser intensities. Both the reflectivity scaling with laser intensity and the observed spectral features from PIC simulations are consistent with recent Trident experiments.

Simulations are reported of the Thomson scatter spectrum of electrostatic waves (ESWs) excited in single laser hot spots by backward stimulated Raman scattering (BSRS). Under conditions similar those in the recent experiments of Kline et al. [Phys. Rev. Lett. 94, 175003 (2005)], a spectral streak, resulting from the trapping-induced frequency shift of the ESW, is found for high wave-number ESWs, similar to the observations. This shift and parametric frequency matching lead to isolated BSRS pulses. Modes with acoustic dispersion, resulting from the trapping-modified electron velocity distribution, can enhance the frequency range of the streak.

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.

The geometric aspects of HF-generated Langmuir turbulence in the ionosphere and its detection by radars are theoretically discussed in a broad approach, including local modelling (damped and driven Zakharov system), basic parametric instabilities, polarization and strength of the driving electric field, and radar configurations. Selected examples of numerical results from the local model are presented and discussed in relation to recent experiments, with emphasis on recent experiments at the EISCAT facilities. Anisotropic aspects of the cavitation process in the magnetized plasma are exhibited. Basic processes of cascades and cavitation are by now well identified in these experiments, but a few problems of the detailed agreement between theory and experiments are pointed out.

Most recent experiments on the excitation of the two plasmon-decay (TPD) instability involve a three-dimensional (3D) array of overlapping laser beams. Our recent two dimensional (2D) simulations suggested that Langmuir cavitation and collapse are important nonlinear saturation mechanisms for TPD. There are important quantitative differences in the Langmuir collapse process in 2D and 3D. To address these and other issues, we have developed a 3D Zakharov code. It has been applied to study the evolution of TPD from absolute instabilities (arising from 3D laser geometries) to the nonlinear state (J. Zhang et al., Phys. Rev. Lett. (submitted)). The present paper concentrates on the nonlinear saturated state excited by the collective action of two crossed laser beams with arbitrary polarizations. Remarkable agreement between 3D and 2D simulations is found for several averaged physical quantities when the beams are polarized in their common plane. As in the previous 2D simulations, we find: (a) the collective, initially convectively unstable triad modes dominate after a sub-picosecond burst, (b) Langmuir cavitation and collapse are important nonlinearities, and (c) that the statistics of intense cavitons are characteristic of a Gaussian random process. The 3D steady-state saturated Langmuir energy level is about 30% higher than in 2D. The auto-correlation functions of the Langmuir envelope field and of the low-frequency electron density field yield the spatial shape of the strongest collapsing cavitons which are 3D ellipsoids whose orientation depends on the laser polarizations. This tilting of the caviton's strongest electric field direction away from the normal to the target surface is a major new 3D result. This tilting may deflect the hot electron flux and thereby mitigate target preheat.

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.

The linear instability analysis of stimulated Brillouin backscatter (SBS) is revisited to study the effect of nonlocal electron heat transport that enhances the density modulation over that caused by the ponderomotive pressure. It is found that the intensity threshold is lowered by a factor of roughly the inverse of 1+1000 (ne/nc)ln Λ(Z3/2/TeV2)/λμ, where TeV is the electron temperature in eV and λμ is the laser wavelength in microns, provided that λμTeV2(nc/ne)≫50 ln Λ√Z. A similar reduction in the threshold for Langmuir decay is found, which, in the case of stimulated Raman scattering (SRS), may lead to a reduced saturation level for SRS.

We describe a method of performing salvage prostate brachytherapy on patients whose initial implant was suboptimal. This technique uses an image correlation algorithm only previously used to fuse postimplant magnetic resonance and computed tomographic (CT) images. Here, the initial postimplant CT and the second preimplant volume study are coregistered to plan delivery of the salvage implant.Two early-stage patients had salvage implants performed with this technique, in which only a limited number of sources were visible on the ultrasound images. The dosimetric results of the first implant were displayed on the preplan generated for the second procedure. The planned total dose then was visualized prior to salvage implant.The implants were performed without complication. Rectum and urethra doses remained acceptable. In each case, the improvement in coverage of the gland was dramatic (V80 coverage improved from 65.2% and 47.3% to 93.1% and 92.2%, respectively), precluding the need for further intervention.Coregistration of the postimplant CT scan to an ultrasound volume study can be quantifiably and reliably performed. The resulting image set can be used to guide needle placement during a second salvage implant to achieve much improved dosimetric coverage of the gland.

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.

Using the reduced-description particle-in-cell (RPIC) method, we study the coupling of backward stimulated Raman scattering (BSRS) and backward stimulated Brillouin scattering (BSBS) in regimes where the reflectivity involves the nonlinear behavior of particles trapped in the daughter plasma waves. The temporal envelope of a Langmuir wave (LW) obeys a Schrodinger equation where the potential is the periodic electron density fluctuation resulting from an ion-acoustic wave (IAW). The BSRS-driven LWs in this case have a Bloch wave structure and a modified dispersion due to the BSBS-driven spatially periodic IAW, which includes frequency band gaps at kLW∼kIAW/2∼k0 (kLW, kIAW, and k0 are the wave number of the LW, IAW, and incident pump electromagnetic wave, respectively). This band structure and the associated Bloch wave harmonic components are distinctly observed in RPIC calculations of the electron density fluctuation spectra and this structure may be observable in Thomson scatter. Bloch wave components gro...

The reflectivity of a laser due to stimulated Raman scattering (SRS) from long scale-length hohlraum plasmas is shown to depend on the damping of ion-acoustic waves. This dependence is observed in plasmas with either low or high ionization states. Since the SRS process itself is unrelated to acoustic waves, these data are evidence of a nonlinear coupling of SRS to other parametric processes involving daughter acoustic waves. {copyright} {ital 1996 The American Physical Society.}

Two-plasmon-decay (TPD) instability is identified as a potential source of target preheat in direct-drive-implosion experiments on OMEGA. A physical model of electron heating is developed that relies on extended Zakharov simulations to predict the nonlinearly saturated Langmuir wave spectrum. Hot electron generation is estimated via a test-particle approach. It is noted that because of the relatively low areal density of the targets during the time of TPD instability, hot-electron recirculation and reheating are potentially important effects. This is modeled by a particular form of boundary conditions on the test particles. Such boundary conditions might prove useful in other kinetic simulations of particle heating where recirculation is a possibility.