Session GO5: Laser-Plasma Coupling

Optical backscatter measurements from laser plasma interactions in NIF targets

Backscattered light from NIF targets is detected using a full aperture backscatter system (FABS) and a near backscatter imager (NBI). These measurements allow quantification of the coupling efficiency of the NIF laser to the target. Backscatter measurements in ignition scale hohlraum targets are made on two separate groups of 4 beams (a quad) at 30$\,^\circ$ and 50$\,^\circ$ from the hohlraum axis and show primarily SRS with a lower level of SBS. We have added a new capability to the scope of the instrument which now includes backscatter measurements on a 23.5$\,^\circ$ quad. This new measurement will provide greater precision in quantifying the overall laser coupling to the hohlraum. In addition, it will improve our understanding of the cross-beam coupling in ignition targets. We will describe measurements and simulations of backscattered light from NIF targets.   

Laser-plasma interaction physics on the LIL facility

Experiments have been carried out on the LIL (Ligne d'Integration Laser) facility with foam targets in order to study interaction physics in underdense plasma, in the millimeter scale at temperature around 2 keV. The LIL facility, which is a prototype of one quadruplet of the near coming French laser Megajoule (LMJ), has been used to deliver 2.7ns pulse with 12kJ at 3w. Low-density foams (3 to 10 mg/cc) with varying lengths have been used in these experiments in order to understand the physics of parametric instabilities, mainly stimulated Brillouin (SBS) and Raman scattering (SRS) and filamentation. We will present and discuss hydrodynamics simulations carried out with the code FCI2, giving us all the plasma parameters: electron density, velocity and temperature profiles. All these plasma conditions can be used to estimate SBS and SRS linear gain with the postprocessor Piranah and to make comparisons between calculated and experimental spectra. Results will then be presented on the beam propagation through the foam and on the evolution of SBS using our paraxial codes HERA and Harmony.   

LPI Thresholds in Longer Scale Length Plasmas Driven by the Nike Laser*

The Krypton-Fluoride (KrF) laser is an attractive driver for inertial confinement fusion due to its short wavelength (248nm), large bandwidth (1-3 THz), and beam smoothing by induced spatial incoherence. Experiments with the Nike KrF laser have demonstrated intensity thresholds for laser plasma instabilities (LPI) higher than reported for other high power lasers operating at longer wavelengths ($\ge $351 nm). The previous Nike experiments used short pulses (350 ps FWHM) and small spots ($<$260 $\mu $m FWHM) that created short density scale length plasmas (Ln$\sim $50-70 $\mu $m) from planar CH targets and demonstrated the onset of two-plasmon decay (2$\omega _{p})$ at laser intensities $\sim $2x10$^{15 }$W/cm$^{2}$. This talk will present an overview of the current campaign that uses longer pulses (0.5-4.0 ns) to achieve greater density scale lengths (Ln$\sim $100-200 $\mu $m). X-rays, emission near $^{1}$/$_{2}\omega _{o}$ and $^{3}$/$_{2}\omega _{o}$ harmonics, and reflected laser light have been monitored for onset of 2$\omega _{p}$. The longer density scale lengths will allow better comparison to results from other laser facilities. *Work supported by DoE/NNSA and ONR.   

Measurement of Laser Plasma Instability (LPI) Driven Light Scattering from Plasmas Produced by Nike KrF Laser

With short wavelength (248 $nm$), large bandwidth (1$\sim$3 $THz$), and ISI beam smoothing, Nike KrF laser provides unique research opportunities and potential for direct-drive inertial confinement fusion. Previous Nike experiments observed two plasmon decay (TPD) driven signals from CH plasmas at the laser intensities above $\sim$$2\times 10^{15}$ $W/cm^2$ with total laser energies up to 1 kJ of $\sim$350 ps FWHM pulses. We have performed a further experiment with longer laser pulses (0.5$\sim$4.0 ns FWHM) and will present combined results of the experiments focusing on light emission data in spectral ranges relevant to the Raman (SRS) and TPD instabilities. Time- or space-resolved spectral features of TPD were detected at different viewing angles and the absolute intensity calibrated spectra of thermal background were used to obtain blackbody temperatures in the plasma corona. The wave vector distribution in k-space of the participating TPD plasmons will be also discussed. These results show promise for the proposed direct-drive designs.   

Assessing the Two-Plasmon Decay Threat Through Simulations and Experiments on the NIKE Laser System

NIKE is a Krf laser system at the Naval Research Laboratory used to explore hydrodynamic stability, equation of state, and other physics problems arising in IFE research. The comparatively short KrF wavelength is expected to raise the threshold of most parametric instabilities. We report on simulations performed using the FAST3d radiation hydrocode to design TPD experiments that have have allowed us to explore the validity of simple threshold formulas and help establish the accuracy of our simulations. We have also studied proposed high-gain shock ignition designs and devised experiments that can approach the relevant scalelength-temperature regime, allowing us a potential experimental method to study the LPI threat to these designs by direct observation. Through FAST3d studies of shock-ignited and conventional direct-drive designs with KrF (248 nm) and 3rd harmonic (351nm) drivers, we examine the benefits of the shorter wavelength KrF light in reducing the LPI threat.   

Competitive Laser--Plasma Interaction Processes Near Quarter Critical Relevant to Direct-Drive ICF

Long-scale-length plasmas created by two sets of defocused laser beams followed by a more tightly set of focused interaction beams exhibit simultaneous stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) close to $n_{c}$/4 during the presence of the interaction beams. The two-plasmon-decay (TPD) instability is predicted to be above threshold at that time but appears to be suppressed as long as SBS at or very near $n_{c}$/4 is above threshold. The TPD is observed as soon as SBS disappears. SRS at somewhat lower densities appears unaffected. All the instabilities appear to be multiple-beam driven. The operating ranges, threshold conditions, and gains for these instabilities are obtained from laser--plasma interaction codes using input from two-dimensional hydrodynamic simulations and correspond closely to observations. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

Two-Plasmon-Decay Preheat Calculations for OMEGA and Ignition-Scale Direct-Drive Inertial Confinement Fusion

Two-plasmon-decay instability is potentially a source of hot electrons and preheat in both direct- and indirect-drive ICF targets. A model of nonlinear saturation of TPD is developed that relies on two-dimensional extended Zakharov calculations.\footnote{ D. A. Russell and D. F. DuBois, Phys. Rev. Lett. \textbf{86}, 428 (2001).} Hot-electron generation is computed in the saturated state by a test-particle approach and recirculation (an important effect caused by the low \textit{$\rho $R} at the time of instability) is modeled by a particular form of boundary conditions on the test particles.\footnote{ J. F. Myatt\textit{ et al.}, ``The Predicted Dynamics of Hot Electron Heating and Recirculation in Direct-Drive Implosion Experiments,'' in preparation, Phys. Plasmas.} Hot-electron temperature and preheat scalings are presented as a function of density scale length and laser intensity for parameters relevant to OMEGA and the NIF. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

Two-Plasmon-Decay Instability and Stimulated Brillouin Scattering in Direct-Drive ICF Plasmas

In direct-drive inertial confinement fusion (ICF) experiments on the OMEGA Laser System, both the two-plasmon-decay instability (TPD) and the stimulated Brillouin scattering (SBS) are observed.\footnote{C. Stoeckl \textit{et al}., Phys. Rev. Lett. \textbf{90}, 235002 (2003).}$^{,}$\footnote{ W. Seka \textit{et al}., Phys. Plasmas \textbf{16}, 052701 (2009); W. Seka \textit{et al}., ibid. \textbf{15}, 056312 (2008).} The importance of these two instabilities for direct-drive ICF implosions is based on the fact that the TPD may lead to generating fast electrons and that SBS may facilitate the power transfer between the crossing laser beams. On OMEGA and on the National Ignition Facility, the laser--plasma interactions are driven by multiple crossing laser beams, randomized in space with distributed phase plates and randomized in time with smoothing by spectral dispersion. A model has been developed for the saturation of the TPD instability caused by the ion-acoustic modes and applied for crossed-beam interaction conditions. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

Angular Dependence of Two-Plasmon Decay in Multibeam Direct-Drive Irradiation Geometries

Experimental observations of half-harmonic and hard x-ray emission on OMEGA have shown that two-plasmon-decay (TPD) signals depend on the collective rather than the single-beam intensity.\footnote{C. Stoeckl \textit{et al}., Phys. Rev. Lett. \textbf{90}, 235002 (2003).} When a plasmon wave vector forms equal angles with several pump beams it can be driven through the TPD process by all of those beams. Such plasmons will reach nonlinear amplitudes first and therefore are likely to be responsible for the preponderance of hot electrons. Here we study this process as a function of the angle between this common plasmon wave vector and the density gradient. This angle will determine the anisotropy of the hot electrons generated by this plasmon and, therefore, influence their effectiveness in coupling to the compressed target core to cause preheat.\footnote{J. A. Delettrez \textit{et al}., Bull. Am. Phys. Soc. \textbf{53}, 248 (2008).}$^{,}$\footnote{J. Myatt \textit{et al}., Bull. Am. Phys. Soc. \textbf{54 }(15), 145 (2009).} This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

Two-dimensional Vlasov Simulation of Driven, Nonlinear Electron Plasma Waves

In the VALHALLA project at LLNL, we are developing advanced, scalable algorithms for the continuum solution of Vlasov-Maxwell that differ from traditional approaches to continuum Vlasov methods.\footnote{J. Banks and J.Hittinger, sub. to IEEE Trans. Plas. Sci. (Dec 2009), LLNL-JRNL-420843.} Here, continuum solution of the Vlasov-Maxwell system using these techniques is extended to two spatial dimensions and two velocity dimensions. We report Vlasov simulation studies of ponderomotively driven electron plasma waves (EPW) with fixed ions. Motivated plasma waves driven by SRS in light speckles, we consider a driving potential with a finite transverse width. This localization introduces losses as the waves propagate transversely out of the driven region and the particles are only transiently trapped. Linearly, the transverse localization leads to constant phase surfaces that defocus the EPW while nonlinearly, the constant phase surfaces from trapping-induced frequency shifts focus the EPW. We show how these processes are affected by the system length and the boundary conditions.   

Saturation of Two-Plasmon Decay and Ion-Density Fluctuations

Previous particle-in-cell (PIC) simulations showed that saturation of the two-plasmon-decay (TPD) instability correlates with background ion-density fluctuations (IDF).\footnote{ A. B. Langdon, B. F. Lasinski, and W. L. Kruer, Phys. Rev. Lett. \textbf{43}, 133 (1979).}$^{,}$\footnote{ Yan \textit{et al}., Phys. Rev. Lett. \textbf{103}, 175002 (2009).} However, the mechanism that connects IDF to TPD growth is still unknown. We propose a model whereby IDF can modify TPD by coupling two otherwise independent pairs of plasmons. In homogeneous plasmas, this ``4-plasmon'' model shows that a large enough IDF can turn off a range of modes. We will present results from PIC and fluid simulations to illustrate how IDF can stop TPD growth in inhomogeneous plasmas. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

Self-organization and threshold of stimulated Raman scattering

We derive, both theoretically and using an envelope code, threshold intensities for stimulated Raman scattering which compare well with results from Vlasov simulations [1]. To do so, we account for the nonlinear decrease of Landau damping and for the detuning induced by, both, the nonlinear wave number shift and frequency shift of the plasma wave. In particular, we show that the effect of the wave number shift may cancel out that of frequency shift, but only in that plasma region where the laser intensity decreases along the direction of propagation of the scattered wave. Elsewhere, the wave number shift enhances the detuning effect of the frequency shift. \\[4pt] [1] D. B\'enisti, O. Morice, L. Gremillet, E. Siminos and D.J. Strozzi, Phys. Rev. Lett. \textbf{105},~015001 (2010).   

Control of nonlinear optical processes in plasmas, NLOP, using STUD pulses is optimal

If Spike Trains of Uneven Duration and Delay, or STUD pulses are used instead of continuous illumination of the plasma, and the laser pulses become ``on'' for a few inverse growth rates of the fastest instabilities, with comparable delays between the spikes, then we can show that due to a variety of favorable mechanisms, coherent wave-wave interaction caused instabilities can be kept under control. Their growth can be kept linear and unable to turn into runaway processes as they often are in current plasmas, such as Raman scattering on the NIF. In addition, STUD pulses allow the actual true control of crossed beam energy transfer whenever it is desired and its stoppage when it is not desirable. This is achieved by temporal interleaving the pulse trains between cones of beams in indirect drive and in a spatially random subset of overlapping beams in direct drive.