Bulletin of the American Physical Society
50th Annual Meeting of the Division of Plasma Physics
Volume 53, Number 14
Monday–Friday, November 17–21, 2008; Dallas, Texas
Session TO4: Long and Short Pulse Laser Matter Interactions and Transport |
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Chair: Denise Hinkel, Lawrence Livermore National Laboratory Room: Reunion B |
Thursday, November 20, 2008 9:30AM - 9:42AM |
TO4.00001: Two-Plasmon Decay Driven by Multiple Obliquely Incident Laser Beams R.W. Short Theoretical studies of the two-plasmon-decay (TPD) instability in inhomogeneous plasmas have generally considered the absolute instability of a single beam normally incident on the plasma.\footnote{ C. S. Liu and M. N. Rosenbluth, Phys. Fluids \textbf{19}, 967 (1976); A. Simon \textit{et al}., Phys. Fluids \textbf{26}, 3107 (1983).} However, experimental observations of 3/2 harmonic and hard x-ray emission on OMEGA have shown that TPD signals depend on combined rather than single-beam intensity.\footnote{ C. Stoeckl \textit{et al}., Phys. Rev. Lett. \textbf{90}, 235002 (2003).} Here the Fourier transform technique used to study the absolute instability in Ref. 1 is extended to the convective regime driven by multiple obliquely incident beams. The integrated gain is calculated for a plasma wave passing through the quarter-critical interaction region; a gain of exp(2\textit{$\pi $}) is conventionally taken as the threshold for convective instability, while a divergent gain indicates the onset of absolute instability. Results will be presented for the geometry and plasma conditions relevant to OMEGA experiments. 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 20, 2008 9:42AM - 9:54AM |
TO4.00002: Modeling of Two-Plasmon-Decay Instability in OMEGA Plasmas A.V. Maximov, J.F. Myatt, R.W. Short, W. Seka, C. Stoeckl, J.A. Delettrez The direct-drive inertial confinement fusion experiments on the OMEGA Laser System have identified the two-plasmon-decay (TPD) instability as a source of fast electrons that are important for target implosions.\footnote{ C. Stoeckl \textit{et al}., Phys. Rev. Lett. \textbf{90}, 235002 (2003).} In OMEGA experiments, TPD is driven by multiple laser beams that are spatially incoherent because of random phase plates. For this regime, the TPD instability thresholds and growth rates are calculated and compared with the results of the three-wave TPD model.\footnote{ A. Simon\textit{ et al}., RPhys. Fluids \textbf{26, }3107 (1983).} The saturation of the TPD instability caused by low-frequency plasma perturbations takes into account the perturbations driven by the laser beams and by the beating of plasma waves, including the Langmuir decay instability. The composition of multispecies plasmas modifies the properties of the low-frequency density perturbations and, therefore, the level of the TPD saturation. 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 20, 2008 9:54AM - 10:06AM |
TO4.00003: Laser-Plasma Interactions on NIKE and the Fusion Test Facility Lee Phillips, James Weaver Recent proposed designs for a Fusion Test Facility (FTF) (Obenchain et al., Phys. Plasmas 13 056320 (2006)) for direct-drive ICF targets for energy applications involve high implosion velocities combined with higher laser irradiances. The use of high irradiances increases the likelihood of deleterious laser plasma instabilities (LPI) but the proposed use of a 248 nm KrF laser to drive these targets is expected to minimize the LPI risk. We examine, using simulation results from NRL's FAST hydrocode, the proposed operational regimes of the FTF in relation to the thresholds for the SRS, SBS, and 2-plasmon instabilities. Simulations are also used to help design and interpret ongoing experiments being conducted at NRL's NIKE facility for the purpose of generating and studying LPI. Target geometries and laser pulseshapes were devised in order to create plasma conditions with long scalelengths and low electron temperatures that allow the growth of parametric instabilities. These simulations include the effects of finite beam angles through the use of raytracing. [Preview Abstract] |
Thursday, November 20, 2008 10:06AM - 10:18AM |
TO4.00004: Positron Creation Using the TITAN Short Pulse Laser Hui Chen, S.C. Wilks, E. Liang, J. Myatt, K. Cone, L. Elberson, D.D. Meyerhofer, M. Schneider, R. Shepherd, D. Stafford, R. Tommasini, P. Beiersdorfer Using ultra-intense lasers to generate positrons was theorized some time ago[1] and demonstrated in principle in two previous experiments[2] where small numbers of positrons were measured. Recently, new experiments were performed on the LLNL Titan laser to study positron creation, where the laser pulse length, pre-plasma condition, target material and thickness were varied. Using newly built positron spectrometers, copious positron production was observed with good signal-to-background ratio. Hot electron spectra (out to 100 MeV) and bremsstrahlung photons were measured simultaneously to further constrain models for the experiment. This talk will present detailed experimental results and their comparison with theory and previous experimental data. [1] Shearer et al, PRA,(1973);Liang, AIP Conf. Proc.(1994); Shkolnikov et al, APL,(1997), Liang, Wilks and Tabak, PRL(1998); Nakashima and Takabe, PoP,(2002); Myatt et al,PRE (2008).[2] Cowan et al, LPB(1999); Gahn et al, APL(1998) [Preview Abstract] |
Thursday, November 20, 2008 10:18AM - 10:30AM |
TO4.00005: X-ray Polarization Spectroscopy to Study Hot Electron Transport in High Intensity Laser Produced Plasma H. Nishimura, Y. Inubushi, Y. Okano, S. Fujioka, T. Kai, T. Kawamura, D. Batani, A. Morace, R. Redaelli, C. Fourment, J. Santos, G. Malka, A. Boscheron, A. Casner, M. Koenig, T. Jhozaki, H. Nagatomo, K. Mima X-ray polarization spectroscopy was used to study anisotropy of hot electron velocity distribution functions (VDFs) in a plasma generated at 10$^{18}$W/cm$^{2}$. Chlorinated triple-layer targets were irradiated and polarization degree of Cl Hea line was measured as a function of overcoat thickness. The polarization degree, nearly zero at the surface, becomes negative then positive, and finally becomes zero with increase in the thickness. This result indicate that VDF in under-dens region is affected with laser field, and that in over-dense region is with acceleration along the laser propagation. Depolarization seen in the surface and dense region is consistent with predictions with a time-dependent atomic kinetics code [1]. \newline [1] T. Kawamura, et al., PRL \textbf{99}, 115003 (2007) [Preview Abstract] |
Thursday, November 20, 2008 10:30AM - 10:42AM |
TO4.00006: Anisotropic third-harmonic plasmonic resonances in expanding nanoclusters Xiaoming Wang, Xiaohui Gao, Bonggu Shim, Alexey Arefiev, Boris Breizman, Mike Downer We present experiments showing that optical third-harmonic generation (THG) from expanding argon nanoclusters by time-delayed 80 fs probe pulses exhibits strong transient polarization anisotropy for several hundred femtoseconds after cluster ionization and heating by a linearly polarized pump, even though linear optical properties remain isotropic. We then theoretically model THG anisotropy by extending our previous model of cluster nonlinear response as a collective probe-driven oscillation of a cold electron core in the potential of a positive ion background of nonuniform density [1] to the case of arbitrary relative polarizations of the pump and probe. The anisotropy of THG is determined only by the angular dependence of the ion density nonuniformity and not by the entire ion density profile. Consequently, even a weak anisotropy of the ion density nonuniformity, that would not alter the isotropic linear optical properties of the cluster, can produce the observed two-fold stronger THG with perpendicular, compared to parallel, pump-probe polarizations. [1] M. V. Fomyts'kyi et al., Phys. Plasmas 11, 3349 (2004). [Preview Abstract] |
Thursday, November 20, 2008 10:42AM - 10:54AM |
TO4.00007: Controlled Interactions of Femtosecond Light Filaments in Air Bonggu Shim, Samuel Schrauth, Christopher Hensley, Pui Hui, Luat Vuong, Amiel Ishaaya, Alexander Gaeta We report on the controlled interactions of two copropagating long-range light filaments in air generated by ultrafast, high-power laser pulses with parallel linear polarizations. In experiments, 800 nm, 50 fs Ti:Sapphire laser pulses (0.5 TW peak power, 10-Hz repetition rate) are interferometrically combined with a motorized delay stage precisely controlling the relative phase (time delays) and separation between beams. Using two collinear beams, each with approximately 20GW peak power and 800 um radius spot sizes, separated by 1 mm, we observe that the self-channeling beams demonstrate fusion, repulsion and energy transfer depending on the relative phase between beams. We compare our experimental results with simulations based on the time-averaged (2D+1) Nonlinear Schrodinger equation. We also present pulse splitting and spectrum measurements of two interacting air filaments. Our method can be applicable to precise control in long range propagation of femtosecond light filaments including remote sensing. [Preview Abstract] |
Thursday, November 20, 2008 10:54AM - 11:06AM |
TO4.00008: Femtosecond imaging of surface heat transport in solid target excited at relativistic intensity Hans Langhoff, Mike Downer, Ben Bowes, John Nees, Bixue Hou We present an extension of recent fs microscopy experiments [1] in which a planar Al target is excited by a 24 fs pump focused to intensity up to $3 \times 10^{18} W/cm^2$ in a 1 micron radius spot; subsequent heat propagation along the target surface appears as a region of reduced reflectivity that is imaged by a delayed probe pulse. Isotropic expansion of the surface heated region out to 15 microns within 500 fs is observed for pump intensities above $10^{18} W/cm^2$. We present a theoretical model in which the pump drives hot electrons into the target via a mixture of $j \times B$ heating and resonance absorption (RA), and a return current heats the target. Ultrafast expansion of the heated surface layer is explained by two-dimensional diffusive motion of returning electrons that undergo damped oscillations between vacuum and a sub-surface layer confined by positive surface charges. Isotropy of the observed expansion is consistent with dominance of RA over $j \times B$ heating, indicating prepulse heating is important. [1] B. Bowes et al., Opt. Lett. 31, 116 (2006). [Preview Abstract] |
Thursday, November 20, 2008 11:06AM - 11:18AM |
TO4.00009: Characterization of cluster/monomer ratio in pulsed supersonic gas jets Xiaohui Gao, Bonggu Shim, Xiaoming Wang, Mike Downer While Rayleigh scatter and interferometry are standard methods for determining average cluster size and total atomic density, respectively, in cluster gas jets, determination of cluster mass fraction has required additional input from gasdynamic simulations. Here we determine cluster mass fraction experimentally with fs-time-resolved measurement of refractive index using frequency domain interferometery (FDI) after ionization and heating by a pump pulse. The essence of this method is that the negative index contribution of monomer plasma appears immediately after ionization by the pump, whereas the positive contribution of clustered plasma becomes significant only after clusters expand to a Mie resonance condition, enabling separation of monomer and cluster densities in the time domain. This method allows us to investigate various influences (nozzle geometry, temperature, etc.) on cluster fraction, which varies widely in nominally identical gas jets, and is a critical parameter in realizing phase-matched harmonic generation at high laser intensity, which would lead to an efficient table-top soft X-ray source. [Preview Abstract] |
Thursday, November 20, 2008 11:18AM - 11:30AM |
TO4.00010: Toward ultra high intensities laser plasma interaction on solid target at ALLS Sylvain Fourmaux, St\'ephane Payeur, S\'ebastien Buffechoux, Cristina Serbanescu, Jean Claude Kieffer We demonstrate the implementation of laser beam wavefront correction on the 200 TW laser system at the Advanced Laser Light Source facility. Ultra high intensities higher than 10$^{20}$ W.cm$^{-2}$ is reported. To our knowledge, this laser system is the first 100 TW scale to combine simultaneously ultra high intensity, higher than 10$^9$ laser pulse contrast ratio and 10 Hz high repetition rate. These characteristics are necessary to be able to implement practical high mean flux applications of secondary source resulting form the interaction of such high intensity laser pulse with solid targets. We will present the experimental program planned with this system next year. [Preview Abstract] |
Thursday, November 20, 2008 11:30AM - 11:42AM |
TO4.00011: Interactions at the Laser-Plasma Interface at Intensity of 10$^{22}$ W/cm$^{2}$ T. Matsuoka, C. McGuffey, S.S. Bulanov, F. Dollar, Y. Horovitz, E. Power, J. Easter, J. Nees, V. Chvykov, G. Kalintchenko, P. Rousseau, V. Yanovsky, A. Maksimchuk, K. Krushelnick Understanding of interaction at the interface between an ultra-intense laser pulse and plasma is important for high harmonics generation (HHG), generation of attosecond pulses, and in fast ignitor research. The experiments have been conducted for various materials and with different laser polarizations by use of the HERCULES laser facility at the University of Michigan, recently upgraded to 300 TW (10$^{22}$ W/cm$^{2})$. Near field images of the specular reflected laser light show filamented structure. The specular reflectivity decreases with increasing laser intensity, indicating high absorption of the laser pulse. The images show that the divergence angle increases with the laser intensity indicating deformation of the interface. Measured HH in the EUV range suggest that prepulses in the laser pulse produce preplasma with a few $\mu $m scale length. Results were consistent with 2D-PIC simulations, showing that absorption of the laser is strongly dependent on plasma scale length. [Preview Abstract] |
Thursday, November 20, 2008 11:42AM - 11:54AM |
TO4.00012: The effects of magnetic fields and non-classical transport on long pulse laser-plasma interactions Christopher Ridgers, Robert Kingham, Alexander Thomas Predictive simulation of long-pulse laser plasma interactions is crucially dependent on accurate modelling of heat transport in the presence of magnetic fields and the dynamics of these fields. The applicability of Braginskii's transport theory to such interactions has been determined using the first fully kinetic, nanosecond time-scale, Vlasov-Fokker-Planck simulation code including self-consistent magnetic fields and hydrodynamic plasma expansion. Recently D.H. Froula et al have experimentally investigated the heat flow in a laser-gas jet plasma in the presence of externally applied B-fields. We have shown that for the largest magnetic fields externally applied in such experiments (12T), the rate of advection of the B-field is dramatically enhanced by the Nernst effect and leads to the re-emergence of non-locality even if the initial value of the magnetic field strength is sufficient to localize the transport. [Preview Abstract] |
Thursday, November 20, 2008 11:54AM - 12:06PM |
TO4.00013: Vlasov-Fokker-Planck modelling of magnetic field generation by laser speckles in nanosecond laser plasma interactions Alexander Thomas, Robert Kingham, Christopher Ridgers The magnetic fields that can develop in laser heated plasmas can be significant, in that their presence affects the magnitude and direction of the particle fluxes, e.g. electron heat flux, and therefore the long time evolution of the system. This evidently has consequences for inertial fusion energy applications, as the coupling of the laser beams with the walls or pellet and the development of hot spots are all critical. Presented here are the first fully kinetic two-dimensional Vlasov-Fokker-Planck simulations of nanosecond laser-plasma interactions to include self-consistent magnetic fields, hydrodynamic plasma expansion and anisotropic electron pressure. Inverse bremsstrahlung laser heating and non-local electron transport lead to the development of the collisional analogue of the Weibel electromagnetic instability. The instability is seeded by an initial linear growth of magnetic fields generated by the anisotropic electron distribution of laser speckles. This can lead to the generation of significant magnetic fields over the coherence time-scale of the speckles, which affects the heat transport and hydrodynamics for longer time-scales. [Preview Abstract] |
Thursday, November 20, 2008 12:06PM - 12:18PM |
TO4.00014: The development of a Krook model for nonlocal transport in laser produced plasmas I. Basic theory Wallace Manheimer, Denis Colombant, Valeri Goncharov We examine the Krook model as a means of quantifying the problem of nonlocal transport of electron energy in laser produced plasmas. The result is an expression for the nonlocal electron energy flux q. The roles of both flux limitation and preheat are clearly delineated. Furthermore, it develops a test for the validity of this model. This is a physics based ``first principles'' model that can be economically incorporated into a fluid simulation. [Preview Abstract] |
Thursday, November 20, 2008 12:18PM - 12:30PM |
TO4.00015: The development of a Krook model for nonlocal transport in laser produced plasmas II. Comparisons with Fokker Planck, experiment and other models Denis Colombant, Wallace Manheimer The Krook model described in the previous talk has been incorporated into a fluid simulation. These fluid simulations are then compared with Fokker Planck simulations and also with a recent NRL Nike experiment. We also examine several other models for electron energy transport that have been used in laser fusion research. As regards comparison with Fokker Planck simulation, the Krook model gives better agreement than the other models, especially in the time asymptotic limit. As regards the NRL experiment, all models except one give reasonable agreement. [Preview Abstract] |
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