D. F. DuBois

Statistical properties of laser hot spots produced by a random phase plate

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.

Numerical test of the weak turbulence approximation to ionospheric Langmuir turbulence

The standard weak Langmuir turbulence approach to explain the artificial plasma line in ionospheric radio modification experiments is examined. We compare solutions of a weak turbulence approximation (WTA) derived from a version of the one-dimensional driven and damped Zakharov system of equations (ZSE) with solutions to the same full ZSE. The electromagnetic pump field is modeled as a long-wavelength parametric driving term. We found that from a certain distance below the O mode reflection level the wave number saturation spectra computed from the WTA agree qualitatively with those from the ZSE for weak driving strengths, in the sense that the number of cascade lines increases with increasing pump strength. However, in general, the number of cascades apparent in the WTA solutions is larger than that predicted from the full ZSE. At higher intensities of the driver the saturation spectra from the ZSE differ from the WTA cascade spectra, in that a truncation of the cascade sets in, with a subsequent filling in of the bands between the cascades. This truncation takes place far before the ZSE cascade spectra reach the so-called “Langmuir condensate,”; contrary to earlier conjectures based mainly on dimensional analysis arguments. In the reflection region a qualitatively different process takes place: temporal cycles of large ensembles of localized events; nucleation of cavitons, collapse, and burnout constitute the basic elements of the turbulence in this region of space. No WTA exists for this region. Our findings are discussed with respect to the experiments performed at Arecibo and Tromso, the conclusion being that the ZSE yields results closer to observations than does the WTA, in all regions of space.

Nonlinear saturation of stimulated Raman scattering in laser hot spots

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 ...

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.

Investigation of strong Langmuir turbulence in ionospheric modification

Recent results in the search for strong Langmuir turbulence effects during ionospheric modification experiments performed at the Arecibo Observatory are presented. Indirect evidence of Langmuir wave collapse is obtained through the observation of theoretically predicted “caviton-type” enhanced plasma waves spectra using the 430 MHz incoherent radar at Arecibo. A typical spectrum consists of a “free-mode” peak with a frequency that is significantly higher than the heater frequency, and a broad “caviton continuum” with frequencies below the heater frequency. Free modes are freely propagating Langmuir waves radiated by collapsing cavitons during collapse. The generation and dynamics of these “free modes” will be discussed. Asymmetries between the frequency shifts and strengths of the upshifted and downshifted free-mode lines and their dependence on the time delay following the onset of heating are explained in terms of the radiation of free Langmuir modes by cavitons and the subsequent propagation of free modes down or up the density gradient. Experimental results are compared with theoretical predictions. Results on the transition of “caviton-type” plasma line spectra to the commonly observed “decay-type” spectra will also be presented.

Excitation of strong Langmuir turbulence in the ionosphere: Comparison of theory and observations*

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.

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...

Beam‐plasma instability in the presence of low‐frequency turbulence

General equations are derived for a linear beam‐plasma instability in the presence of low‐frequency turbulence. Within a ’’quasilinear’’ statistical approximation, these equations contain Langmuir waves scattering, diffusion, resonant and nonresonant anomalous absorption, and a ’’plasma laser’’ effect. It is proposed that naturally occurring density irregularities in the solar wind may stabilize the beam‐unstable Langmuir waves which occur during Type III solar radio emissions.

Initial development of ponderomotive filaments in plasma from intense hot spots produced by a random phase plate

Local intensity peaks, hot spots, in laser beams may initiate self‐focusing, in lieu of linear instabilities. If the hot spot power, P, contains several times the critical power, Pc, and if the plasma density, n, is small compared to the critical density, nc, then on a time scale less than an acoustic transit time across the hot spot radius, τia, the hot spot collapses, capturing order unity of the initial hot spot power. The collapse time is determined as a universal function of P/Pc and τia. The focal region moves towards the laser with an initially supersonic speed, and decelerates as it propagates. The power of this back propagating focus decreases monotonically until the critical power is reached. This limiting, shallowest, focus develops on a time scale long compared to τia and corresponds to the focus obtained in a model with adiabatically responding ions. For low‐density plasma nonlinear ion effects terminate collapse and a bound on the transient intensity amplification is obtained as a universal ...