Bulletin of the American Physical Society
54th Annual Meeting of the APS Division of Plasma Physics
Volume 57, Number 12
Monday–Friday, October 29–November 2 2012; Providence, Rhode Island
Session TO5: Laser-plasma Coupling at Long Scale Lengths |
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Chair: Pierre Michel, Lawrence Livermore National Laboratory Room: 552AB |
Thursday, November 1, 2012 9:30AM - 9:42AM |
TO5.00001: Multibeam Two-Plasmon Decay: Experimental Signatures and Diagnostic Applications W. Seka, D.H. Edgell, D.H. Froula, J. Katz, J.F. Myatt, J. Zhang, R.W. Short, D.T. Michel, A.V. Maximov, V.N. Goncharov Images and spectra at \textit{$\omega $}/2 and 3\textit{$\omega $}/2 along with 2-D and 3-D Zakharov simulations indicate that the two-plasmon-decay (TPD) instability operates primarily as a multibeam instability in direct-drive--implosion experiments. Simulations indicate that the instability enters the nonlinear regime within $\sim $1 ps and covers a large phase space volume within 10 ps, far beyond linear gain predictions. This largely explains the preponderance of 3\textit{$\omega $}/2 spectra seen in the past 40 years. The \textit{$\omega $}/2 and 3\textit{$\omega $}/2 images and spectra indicate the localized areas on the target surface where the TPD instability operates. The \textit{$\omega $}/2 spectra can be used as powerful $T_{e}$ measurement at $n_{c}$/4 as originally proposed in 1985. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302. [Preview Abstract] |
Thursday, November 1, 2012 9:42AM - 9:54AM |
TO5.00002: Two-Plasmon-Decay Electron-Divergence Measurements in Direct-Drive Implosions on OMEGA D.H. Froula, B. Yaakobi, A.A. Solodov, M.J. Bonino, J.F. Myatt, J. Fooks The divergence of electrons generated by two-plasmon decay in the coronal plasma created in direct-drive experiments on OMEGA was shown to be isotropic. These experiments show that the total hot-electron energy that reaches the fuel (``preheat'') is reduced by a factor of 5. Varying-diameter molybdenum (Mo) spheres (200 $\mu$m to 860 $\mu$m diameter) were mounted inside 50-$\mu$m-wall-thickness, 860-$\mu$m-diam CH shells containing 1 atm of N$_{2}$ gas. Sixty laser beams with an on-target overlapped intensity of 10$^{15}$ W/cm$^{2}$ generate hot electrons that penetrate through the CH shell. The electrons that intercept the Mo ball are stopped. The associated Mo K$_{\alpha}$ yield is measured using an absolutely calibrated x-ray spectrometer. The K$_{\alpha}$ yield is shown to scale with the surface area of the Mo balls indicating that the electron divergence is isotropic. Monte Carlo simulations are used to determine the total energy in hot electrons. The simulations indicate that the effect of scattering in the cold CH shell is small. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302. [Preview Abstract] |
Thursday, November 1, 2012 9:54AM - 10:06AM |
TO5.00003: Collisional Effects on Hot-Electron Generation in Two-Plasmon-Decay Instability in Inertial Confinement Fusion J. Li, R. Yan, C. Ren, A.V. Maximov, W.B. Mori, F.S. Tsung Recent particle-in-cell (PIC) and fluid simulations for the study of the collisional effects on hot-electron generation in two-plasmon-decay (TPD) instability in the regime relevant to experiments on the OMEGA Laser System are presented. The collision package in the PIC code \textit{OSIRIS} has been benchmarked and shown to produce proper electron--ion collision rates in PIC simulations. Collisions have been found to significantly reduce TPD hot-electron generation. This reduction is partially caused by collisional suppression of the nonlinear TPD modes away from the quarter-critical surface that were found to form the first stage of hot-electron acceleration.\footnote{R. Yan \textit{et al.}, Phys. Rev. Lett. \textbf{108}, 175002 (2012).} This work was supported by the U.S. Department of Energy under Cooperative Agreement Nos. DE-FG02-06ER54879 and DE-FC52-08NA28302, by NSF PHY-0903797, and by NSFC 11129503. [Preview Abstract] |
Thursday, November 1, 2012 10:06AM - 10:18AM |
TO5.00004: A Three-Dimensional Zakharov Model of the Two-Plasmon-Decay Instability in Inhomogeneous Plasmas Driven by Multiple Laser Beams J. Zhang, J.F. Myatt, A.V. Maximov, R.W. Short, D.F. DuBois, D.A. Russell, H.X. Vu A three-dimensional extended Zakharov model of the two-plasmon-decay instability\footnote{D. F. DuBois, D. A. Russell, and H. A. Rose, Phys. Rev. Lett. \textbf{74}, 3983 (1995); D. A. Russell and D. F. DuBois, Phys. Rev. Lett. \textbf{86}, 428 (2001).} is described and its validity verified in the regime of linear instability by comparison with multibeam growth-rate calculations. The nonlinear saturated state described by the model is shown to be sensitive to the geometry and polarization of the driving laser beams. Several cases relevant to spherical and polar-drive targets on OMEGA are shown and the implications for future experiments on the National Ignition Facility are discussed. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302. [Preview Abstract] |
Thursday, November 1, 2012 10:18AM - 10:30AM |
TO5.00005: Mitigating Two-Plasmon-Decay Hot-Electron Generation Through the Modification of Langmuir and Ion-Acoustic Wave Dissipation in Directly Driven Targets J.F. Myatt, J. Zhang, V.N. Goncharov, A.V. Maximov, R.W. Short, D.F. DuBois, D.A. Russell, H.X. Vu The deleterious effects of two-plasmon-decay (TPD) instability (mainly preheat caused by hot-electron generation) can be mitigated by reducing the TPD growth rate through an increase in Langmuir wave (LW) collisional damping and by encouraging nonlinear saturation at low LW amplitudes. Both processes are investigated using a quasilinear-Zakharov model of TPD. It is shown that the lowest level of nonlinear saturation is achieved for mid-$Z$ materials with weak ion-acoustic wave (IAW) damping. Possible explanations for these results are presented including a reduction in the threshold for the Langmuir decay instability, more-favorable nucleation of LW's in density cavities, and easier ponderomotive excitation of IAW turbulence. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302. [Preview Abstract] |
Thursday, November 1, 2012 10:30AM - 10:42AM |
TO5.00006: The Effects of Beam Polarization and Orientation on Convective and Absolute Two-Plasmon Decay Driven by Multiple Laser Beams R.W. Short, J.F. Myatt, J. Zhang, A.V. Maximov, D.T. Michel, D.H. Froula There is now much evidence that two-plasmon decay in direct-drive geometries is a collective process, in which a given set of decay waves is driven by two or more laser beams.\footnote{C. Stoeckl\textit{ et al.}, Phys. Rev. Lett. \textbf{90}, 235002 (2003).}$^{,}$\footnote{T. Michel \textit{et al.}, ``Experimental Demonstration of the Two-Plasmon-Decay Common-Wave Process,'' to be published in Physical Review Letters.} Since the single-beam decay is maximized on a hyperbola lying in the plane of polarization of the beam, maximum gain for the multibeam process is constrained to the vicinity of the intersection of the hyperbolas corresponding to the beams involved. As a result, the nature of the decay depends on the relative orientations and polarizations of the beams. It is found that when the polarizations of two beams lie in the plane of their wave vectors, they drive a collective mode with a large-plasmon wave vector \textbf{k}, while when they are polarized out of this plane the collective mode is at small \textbf{k}. In the latter case the instability can be absolute. For more general polarizations or when polarization smoothing is used both types of decay may be present. The small-\textbf{k} instability is less affected by imperfect symmetry of the beams, and its gain increases relative to the large-\textbf{k} instability as the number of beams increases. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302. [Preview Abstract] |
Thursday, November 1, 2012 10:42AM - 10:54AM |
TO5.00007: PIC Simulations of Stimulated Raman Scattering Due to Interacting Laser Speckles W.B. Mori, B.J. Winjum, F.S. Tsung The laser beams in inertial confinement fusion experiments consist of a distribution of high-intensity speckles, a percentage of which are above-threshold for stimulated Raman scattering (SRS). SRS can also be driven in below-threshold speckles due to inter-speckle interactions via waves and particles. We present 2D PIC simulations with the code OSIRIS showing conditions for which scattered light waves, plasma waves, and hot electrons generated in above-threshold speckles drive SRS in neighboring, below-threshold speckles. Through tailored two-speckle simulations in which we control the relative placement and polarizations of the speckles, we isolate inter-speckle SRS driven by each of these three intermediary elements. Scattered light is the most efficient mechanism for inter-speckle SRS, though all three can stimulate SRS in below-threshold speckles. We also present simulations of multi-speckle ensembles illustrating differences in plasma wave activity and reflectivity levels when the polarizations are not all uniform, limiting interactions via scattered light. [Preview Abstract] |
Thursday, November 1, 2012 10:54AM - 11:06AM |
TO5.00008: Self-organized coherent stimulated Raman scattering and scaling with $k \lambda_{D}$ in multi-speckled laser beams L. Yin, B.J. Albright, H.A. Rose, R. Kirkwood, J.L. Kline, D.S. Montgomery, K.J. Bowers, B. Bergen Stimulated Raman Scattering (SRS) continues to be a laser plasma instability of concern for laser-driven fusion experiments. Recently, the key nonlinear physics governing SRS onset and saturation has been identified in multi-speckled laser beams. Hot electrons from intense speckles, produced during SRS daughter electron plasma wave bowing and filamentation, seed and enhance the growth of SRS in neighboring speckles by reducing Landau damping. Trapping-induced nonlinearity and speckle interaction through transport of hot electrons, backscatter, and sidescatter SRS waves enable the system of speckles to self-organize and exhibit coherent, sub-ps SRS bursts with more than 100\% instantaneous reflectivity, consistent with an SRS transverse coherence width much larger than a speckle width [L. Yin, et al.,PRL,108, 245004 (2012)]. SRS reflectivity is found to saturate above a threshold laser intensity [L. Yin, et al., PoP, 19, 056304 (2012)] at a level of reflectivity that depends upon $k \lambda_{D}$: higher $k \lambda_{D}$ (e.g., as obtained by raising electron temperature) leads to lower SRS. We are exploring the efficacy of novel approaches to lowering SRS by raising $k \lambda_{D}$ in hohlraum plasmas. [Preview Abstract] |
Thursday, November 1, 2012 11:06AM - 11:18AM |
TO5.00009: Toggling between single and multi-beam effects on Stimulated Raman Scattering in a NIF hohlraum plasma J.D. Moody, D.J. Strozzi, L. Divol, P. Michel, J. Ralph, R.L. Berger, R.K. Kirkwood, H. Robey, O.L. Landen, S. LePape, S. Ross, B.J. MacGowan, E.A. Williams, S.H. Glenzer, A. Nikroo We have developed a method for studying single and multi-beam laser-plasma interactions (LPI) in a NIF hohlraum plasma. This method utilizes toggling combinations of beams on and off during the time of high (partly saturated) stimulated Raman backscattering (SRS) and measuring the effects on the SRS. We find that during the high-intensity part of the laser pulse SRS saturates at about 10 $-$ 20\% reflectivity for single and multi-beam interactions. In addition, we can place limits on the cross-beam energy transfer and show that re-amplification is small due to multiple beam effects. Spectral measurements indicate that toggling beams creates a $\le 10$\% change in the plasma temperature. These results are important for developing models of multi-beam intereactions. We will describe the backscatter measurements and simple models used to constrain the multi-beam effects. [Preview Abstract] |
Thursday, November 1, 2012 11:18AM - 11:30AM |
TO5.00010: Modeling of NIF Laser-Plasma Interaction Experiments with Single and Multiple Beams D.J. Strozzi, J.D. Moody, H.F. Robey, L. Divol, P. Michel, R.L. Berger, E.A. Williams, D.E. Hinkel, D.C. Eder We discuss modeling of recent NIF ignition hohlraum experiments, in which selected beams were toggled on or off, in order to study laser-plasma interactions. Backscattered light, namely Raman scattering from the inner cones and Brillouin scattering from the outers, was the main diagnostic. One pair of experiments determined the effect of turning off the outer cones on inner-cone Raman. This revealed cross-beam energy transfer levels ($\sim35$\% of outer-cone power) roughly in accord with radiation-hydrodynamic modeling and capsule symmetry measurements. A modest upper bound was also placed on inner-cone Raman backscatter re-amplification by the outers (total gain $<$ 1). We find decent agreement between the measured and calculated (gain) Raman spectra, and minor changes in the spectrum when the outers are turned off. These experiments provide an effective power scaling of inner-beam SRS, which indicates via a simple Tang model of pump depletion that SRS is in a partly saturated regime. More recent experiments will be discussed, including those which examined the effect of toggling off the inner cones on outer-cone Brillouin. [Preview Abstract] |
Thursday, November 1, 2012 11:30AM - 11:42AM |
TO5.00011: Studies of bandwidth dependence of laser plasma instabilities driven by the Nike laser J. Weaver, D. Kehne, S. Obenschain, V. Serlin, A.J. Schmitt, J. Oh, R.H. Lehmberg, C.M. Brown, J. Seely, U. Feldman Experiments at the Nike laser facility of the Naval Research Laboratory are exploring the influence of laser bandwidth on laser plasma instabilities (LPI) driven by a deep ultraviolet pump (248 nm) that incorporates beam smoothing by induced spatial incoherence (ISI). In early ISI studies with longer wavelength Nd:glass lasers (1054 nm and 527 nm),\footnote{Obenschain, PRL 62(1989);Mostovych, PRL 62(1987);Peyser, Phys. Fluids B 3(1991).} stimulated Raman scattering, stimulated Brillouin scattering, and the two plasmon decay instability were reduced when wide bandwidth ISI ($\delta\nu/\nu\sim$0.03-0.19\%) pulses irradiated targets at moderate to high intensities ($10^{14}-10^{15} W/cm^{2}$). The current studies will compare the emission signatures of LPI from planar CH targets during Nike operation at large bandwidth ($\delta\nu\sim$1THz) to observations for narrower bandwidth operation ($\delta\nu\sim$0.1-0.3THz). These studies will help clarify the relative importance of the short wavelength and wide bandwidth to the increased LPI intensity thresholds observed at Nike. New pulse shapes are being used to generate plasmas with larger electron density scale-lengths that are closer to conditions during pellet implosions for direct drive inertial confinement fusion. [Preview Abstract] |
Thursday, November 1, 2012 11:42AM - 11:54AM |
TO5.00012: Controlling Scattering Instabilities and Adapting to Unknown and Changing Plasma Conditions Using STUD Pulses Bedros Afeyan, Stefan H\"uller We will show the results of changing STUD pulse configurations in order to maintain strict control of parametric instabilities in high energy density plasmas (HEDP). Nonlinear optical processes (NLOP) in HEDP respond to changing plasma conditions which are unknown and not easily knowable by standard experimental procedures. Adapting to changing and unknown plasma conditions is one feature of STUD pulses which is absent in other beam conditioning techniques. We demonstrate this by simulating long enough that plasma conditions change, instability gains are altered and new STUD pulse configurations become necessary. Two such configurations are spliced together or run independently and compared. All available methods of changing STUD pulse characteristics are explored, such as duty cycle (20{\%} vs 50{\%}) and modulation period (cutting hot spots in half and into quarters) as well as phase scrambling and number of spikes before the spatial distribution of hot spots is randomized (1, 2, 3 and infinity). [Preview Abstract] |
Thursday, November 1, 2012 11:54AM - 12:06PM |
TO5.00013: Estimating the Stimulated Backscatter Risks from NIF Ignition Hohlraums with Beryllium and Diamond Ablators Richard Berger, J.D. Moody, P. Michel, J. Milovich, D.E. Hinkel, D.J. Strozzi, R.P. Town, A.B. Langdon, R. Olson Since the beginning of the NIF ignition experiments, CH has been the standard capsule ablator in hohlraums with gold and uranium walls. The optimum laser pulse shape for CH consists of a $\sim 16 \rm ~ns$ low-power foot followed by 3-4 ns high-power drive pulse with 300-500 TW at peak power. During this high power pulse, $\sim 30 \%$ of the laser energy is backscattered from the inner $30^{\circ}$ and $23^{\circ}$ beams primarily as stimulated Raman scatter. New capsules with Beryllium and Diamond ablators, now being designed for NIF experiments, use much shorter pulses with higher foot powers but similar high-power drive pulses. Using plasma conditions from rad-hydro modeling, linear gain calculations, and pF3D simulations for the post-shot CH experiments and the pre-shot Beryllium and Diamond designs, we will present our expectations for SRS and SBS for these new designs. [Preview Abstract] |
Thursday, November 1, 2012 12:06PM - 12:18PM |
TO5.00014: Laser-Plasma Interaction Experiments in MM-Long Gas-Filled Hohlraums on the LIL Facility Paul-Edouard Masson-Laborde, Pascal Loiseau, Christophe Rousseaux, Michel Casanova, Denis Teychenne, Gael Huser, Marie-Christine Monteil In 2011, a laser-plasma interaction campaign has been conducted using gas-filled hohlraums on the LIL facility, which is a prototype of one quadruplet of the upcoming french laser megajoule. In order to mimic plasma conditions relevant for ignition in the indirect drive scheme, different targets have been designed: one relevant to the gold bubble expansion seen by the outer beams in ignition hohlraum, which could be sensitive to the stimulated Brillouin scattering (SBS), and one relevant to the inner beams with long distance of propagation and sensitive to the stimulated Raman scattering (SRS). In this talk, we will discuss this last configuration. While different optical smoothing techniques have been used during the campaign, two different lengths for the target have also been used: 4mm and 1.5mm long both with 6ns pulse long and a maximum energy of 15kJ. Large levels of SRS have been obtained in this campaign with a backscattered energy fraction of 20\% of the incident laser energy. Calculated spectra will be compared to experimental results and discussed with paraxial simulations carried out with the code \textsc{Hera}. Finally, PIC simulations based on the plasma conditions of the cavity will be discussed in order to understand experimental SRS spectrum. [Preview Abstract] |
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