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 GO4: Short Pulse Laser-Matter Interactions |
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Chair: Farhat Beg, University of California, San Diego Room: Reunion B |
Tuesday, November 18, 2008 9:45AM - 9:57AM |
GO4.00001: K-Photon and Thermal X-Ray-Emission Measurements from Planar Copper Foil Targets Irradiated by High-Intensity Laser Pulses P.M. Nilson, W. Theobald, J.F. Myatt, C. Stoeckl, P.A. Jaanimagi, J.A. Delettrez, M. Storm, R. Betti, D.D. Meyerhofer, T.C. Sangster, J.S. Green, K.L. Lancaster, P.A. Norreys, F. Beg, R.B. Stephens, M.H. Key K-shell x-ray-spectroscopy measurements of small-mass copper foil targets ($>$20 $\times $ 20 $\times $ 2~\textit{$\mu $}m$^{3})$ irradiated by \textit{I$\lambda $}$^{2} >$ 10$^{18}$ Wcm$^{-2}$\textit{$\mu $}m$^{2}$ laser pulses are presented. K$_{\beta }$/K$_{\alpha }$ variations with increasing energy density using $<$0.5-kJ, 5-ps laser pulses are fully characterized. K{\-}photon yields and bulk-electron temperatures calculated by 3-D numerical target-heating simulations are in good agreement with the experimental measurements. The first observation of a transient ``double-flash'' of x-ray radiation (1 to 2 keV) with increasing energy density indicates a finite-time, thermal-plasma response during the rapid isochoric heating phase. Time-resolved, K-photon emission measurements indicate decompression effects are minimal. This work was supported by U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302. [Preview Abstract] |
Tuesday, November 18, 2008 9:57AM - 10:09AM |
GO4.00002: Intense Laser-to-Fast-Electron Coupling Efficiency in Wedge-Shaped-Cavity Targets W. Theobald, P.M. Nilson, J.F. Myatt, B. Eichman, S. Ivancic, C. Stoeckl, C. Ren, J.A. Delettrez, J.D. Zuegel, T.C. Sangster, V. Ovchinnikov, L. Van Woerkom, R.R. Freeman, R.B. Stephens The interaction of ultra-intense lasers with hollow re-entrant cone targets is an important aspect of the fast-ignition scheme in inertial confinement fusion. Information on the spectrum of the $\sim $MeV electrons and the efficiency with which they are generated is required. We report on experiments of $\sim $5-J, $\sim $0.5-ps, $\sim $1 $\times $ 10$^{19}$ W/cm$^{2}$ laser pulses interacting with wedge-shaped-cavity targets. Spatially and spectrally resolved K-shell emission measurements from small-mass, Cu wedge-shaped-cavity targets with dimensions of $\sim $200 $\times $ 100 $\times $ 20 \textit{$\mu $}m$^{3}$ are reported for various opening angles and laser polarizations and are compared to flat-foil targets in the refluxing geometry.\footnote{P. Nilson \textit{et al}., Phys. Plasmas \textbf{15}, 056308 (2008).} This work was supported by U.S. Department of Energy under DE-FC52-08NA28302 and No. DE-FG02-05ER54839. [Preview Abstract] |
Tuesday, November 18, 2008 10:09AM - 10:21AM |
GO4.00003: MeV electron generation and transport in ultrathin and thick solids Julien Fuchs, Patrizio Antici, Marco Borghesi, Emmanuel d'Humi\`eres, Laurent Gremillet, Thomas Grismayer, Patrick Mora, Erik Lefebvre, Carlo Cecchetti, Ana Mancic, Motoaki Nakatsutsumi, Ariane Piphal, Lorenzo Romagnani, Yasuhiko Sentoku, Oswald Willi, Patrick Audebert We will present recent results regarding MeV electron generation by ultra-intense lasers and their transport through solids. The fast electron density and mean energy are measured via electron spectrometry and optical probing of the fast electron cloud emerging from the target rear side. In particular, our data indicate that the fast electron generation is enhanced when using ultra-thin foils (50 nm) and ultra-high contrast laser pulses. Moreover, strong modulations are found to affect the transverse profile of the electron beam as a result of a magnetic instability developing at the target/vacuum interface. The latter result could have important implications for the Fast Ignition scheme using re-entrant solid cones embedded in fusion capsules. [Preview Abstract] |
Tuesday, November 18, 2008 10:21AM - 10:33AM |
GO4.00004: Generation Mechanisms of Multi-MeV Electrons in Ultra-intense Laser Plasma Interactions and Applications Scott Wilks, H. Chen, W.L. Kruer, A.J. Kemp, P.K. Patel, R. Shepherd, M. Tabak We report on the analysis of recent data taken with a magnetic electron spectrometer on the TITAN laser at LLNL. The experiments consisted of ultra-intense laser pulses incident on solid planar targets at various angles, with and without intentional pre-pulses. We find that a few key parameters, namely the slope and the maximum electron energy, scale with laser intensity in a way that is somewhat different than simple ponderomotive scaling would predict, and discuss various mechanisms that may explain the discrepancy. We then consider two applications of these high energy electrons: as a bremsstrahlung source and a method to generate positrons. LLNL-ABS-405449. [Preview Abstract] |
Tuesday, November 18, 2008 10:33AM - 10:45AM |
GO4.00005: Energetic Electron Spectra with High Ultraintense Radiation on Thick Targets A. Link, R.R. Freeman, D.W. Schumacher, L.D. Van Woerkom, H. Chen, D.S. Hey, M.H. Key, S. Le Pape, A.J. Mackinnon, A.G. MacPhee, P.K. Patel, Y. Ping, T. Bartal, T. Ma, M.S. Wei, F.N. Beg, C.D. Chen, Y.Y. Tsui, K.U. Akli, R.B. Stephens Ultraintense laser interactions with solid density plasma involve significant transfer of energy to electrons at the critical density. The energy and angular distribution of the electrons play an important role in heating the target and are critical to the Fast Ignitor approach to Inertial Fusion Energy. The interaction is complicated by a nonuniform laser intensity, and the measured escaping electron spectrum is modified by surface fields. Experiments were conducted on the Titan Laser at 10$^{20}$ W/cm$^{2}$ to determine the character of the electron distribution and the effect of target charging on the escaping electrons. Results will be presented for escaping electron distributions from .8 to 120 MeV for 1mm thick metal foils. [Preview Abstract] |
Tuesday, November 18, 2008 10:45AM - 10:57AM |
GO4.00006: Characterization of two fast electron populations from the interaction of ultra intense laser and solid target using coherent transition radiation B.I. Cho, J. Osterholz, A.C. Bernstein, G.M. Dyer, A. Karmakar, A. Puhkov, T. Ditmire The transport of energetic electrons generated by the interaction of ultra intense laser and aluminum slab target has been studied by measuring coherent transition radiation (CTR). Two CTR emissions are observed simultaneously, indicating that the fast electrons propagate in two distinct beams, one in the direction of the laser and the other in the direction of the target normal. Analysis of experimental data implies that these two electron beams are produced by different mechanisms, i.e. jxB heating and resonance absorption, and reveals various properties of each electron population such as bunching frequencies, electron temperature, and relative ratio between the number of electrons in the beam. These results are consistent with 3D particle-in-cell simulations. [Preview Abstract] |
Tuesday, November 18, 2008 10:57AM - 11:09AM |
GO4.00007: Spectrum of escaping fast electrons in high intensity laser solid target interaction Mingsheng Wei, Farhat Beg, Richard Stephens, Toshinori Yabuuchi For fast ignition of inertial confinement fusion study, hot electron temperature (T$_{h})$ is a critical parameter required to be determined in experiments. In high intensity laser solid target experiments, the T$_{h}$ is commonly obtained directly from the vacuum electron spectrum measurement. However, whether the vacuum electron spectrum can really represent the spectrum of the fast electrons produced in high intensity laser target interaction is still an open question. In this work, we attempt to address this issue. Hybrid particle-in-cell (PIC) code LSP is used to systematically study the spectrum of the escaping fast electrons under various laser and target parameters. Preliminary results have suggested that the vacuum electron spectrum is strongly modified by sheath field at the target rear surface. With a simple initial one-temperature spectrum for the produced fast electrons, the spectrum of the escaping fast electrons has a reduced temperature. [Preview Abstract] |
Tuesday, November 18, 2008 11:09AM - 11:21AM |
GO4.00008: Electron Generation in Fast Ignition Targets R.B. Stephens, K. Akli, T.J. Bartal, F.N. Beg, S. Chawla, D.P. Higginson, S. Chen, T. Ma, B.J. Westover, M. Wei, T. Yabuuchi, C. Chen, H. Chen, D.S. Hey, M.H. Key, S. LePape, A.J. MacKinnon, A.G. MacPhee, P.K. Patel, R.R. Freeman, V.M. Ovchinnikov, L. Van Woerkom, C. Stoeckl, W. Theobald, Y. Tsui Recent experiments on thin-wall cone/wire targets, have shown 15\% conversion of laser energy to a 40~$\mu$m $\phi$ wire via fast electrons [1]. Electrostatic fields on the cone exterior restrict electron motion unlike in a real fast ignition target where such fields are suppressed by the surrounding blow-off plasma. To eliminate such constraints, we have emulated those conditions with thick-wall ($\sim$200\,$\mu{m}$) aluminum cones in experiments with 150~J, 1~ps laser pulses at the Titan laser facility. The resulting electrons were imaged using $K_{\alpha}$ fluorescence from Cu layers buried in the Al in front of the cone tip and quantified with a single hit CCD camera to get directionality and coupling efficiency. Results will be discussed at the meeting. \vskip6pt \noindent [1] J.A. King, et al., submitted to Phys. Rev. Lett. (2007). [Preview Abstract] |
Tuesday, November 18, 2008 11:21AM - 11:33AM |
GO4.00009: Isochoric heating of reduced-mass targets in short pulse laser experiments at 0.53 $\mu $m Sophie Baton, Patrick Audebert, Michel Koenig, Frederic Perez, Makhlad Chahid, Christophe Rousseaux, Laurent Gremillet, Erik Lefebvre, Jenny Rassuchine, Tom Cowan, Sandrine Gaillard, Ronnie Shepperd, Kirk Flippo Recent works have shown the possibility to heat isochorically mass limited targets by using the electron refluxing. We report on experiment performed at the 100 TW LULI laser facility dedicated to the study of fast electron transport in multilayer reduced mass targets. The targets were composed of 0.2V/5Cu/5Al $\mu $m and varied from 300 to 50 $\mu $m in diameter. They were irradiated by a 300 ps laser pulse at 1.057 $\mu $m and 0.53 $\mu $m that delivered I$\sim $2x10$^{19 }$W/cm$^{2}$ and I$\sim $10$^{19 }$W/cm$^{2}$ respectively to form a warm dense plasma. Emission from the rear side was observed using K-alpha spectroscopy and imaging diagnostics. Spectra including the Al and Cu-K-alpha, and Al He-like emissions show changes as a function of total mass. The data obtained from all diagnostics (K-alpha spectroscopy and imagers on the rear side and the transverse side) show a different behavior depending on the incident wavelength. [Preview Abstract] |
Tuesday, November 18, 2008 11:33AM - 11:45AM |
GO4.00010: Enhanced proton emission and hot electron dynamics inside Cu and Au flat-top cone (FTC) targets Sandrine Gaillard, K. Flippo, J. Workman, D.S. Montgomery, B.J. Albright, J.A. Cobble, J.C. Fernandez, D.C. Gautier, B.M. Hegelich, J.L. Kline, S. Letzring, L. Yin, T.E. Cowan, J. Rassuchine, Y. Sentoku, N. Vutisalchavakul 10 $\mu $m thick Au and Cu FTC targets were shot recently at LANL on the Trident laser at 80-100 J, $\sim $600 fs and $\sim $5x10$^{19 }$W/cm$^{2}$. These initial results are compared to previous experiments on Trident at $\sim $20 J and $\sim $1x10$^{19 }$W/cm$^{2}$ using similar Au FTCs which yielded higher conversion efficiencies (4.5{\%}) and proton energies (30 MeV) compared to 15 $\mu $m Au flat foils (0.75{\%}, 19 MeV) [K. A. Flippo \textit{et al.}, PoP \textbf{14} (2008)]. To elucidate the physics of electron transport and proton beam enhanced emission, Cu-K$\alpha $ imaging and spectroscopy are used to observe the laser absorption and electron heating, and the Cu FTCs are compared to similar funnel-only cones resulting in an enhancement in the electron density toward the cone tip [J. Rassuchine \textit{et al.}, PRL submitted (2008)]. [Preview Abstract] |
Tuesday, November 18, 2008 11:45AM - 11:57AM |
GO4.00011: New scaling laws for ion acceleration from the enhanced 200TW Trident laser at LANL Kirk Flippo, B. Albright, T.E. Cowan, J.C. Fernandez, S.A. Gaillard, D.C. Gautier, K. Harres, B.M. Hegelich, A. Henig, R.P. Johnson, D. Kiefer, J.L. Kline, S. Letzring, F. Nuernberg, M. Roth, J. Rassuchine, M. Schollmeier, R.C. Shah, T. Shimada, J. Workman, L. Yin, N. Vutisalchavakul Recent experiments on the 200 TW Trident have shown the importance ASE and prepulse contrast play in ion acceleration. Ion energies above 50 MeV have been observed even at modest intensities, on par with the Nova Petawatt (Snavely 2002) at half the intensity and energy, with an intrinsic laser ASE contrast of $>$ 10$^{-7}$, yielding efficiencies of greater than 5{\%} into ions above 4 MeV. Scalings for spot size, laser energy, target thickness, and laser pulse duration are presented and compared to other empirical scalings and theory, including the influence of contrast. [Preview Abstract] |
Tuesday, November 18, 2008 11:57AM - 12:09PM |
GO4.00012: Proton Conversion Efficiency Measurements Relevant to Fast Ignition T. Bartal, F.N. Beg, S. Chawla, D. Higginson, M.S. Wei, B. Westover, T. Yabuuchi, M. Foord, D. Hey, M.H. Key, S. LePape, T. Ma, A.J. Mackinnon, A.G. MacPhee, P.K. Patel, K.U. Akli, R.B. Stephens, C.D. Chen, R.R. Freeman, E. Kemp, D. Offermann, V. Ovchinnikov, L. Van Woerkom, Y. Tsui We report on recent experimental and modeling studies of proton conversion efficiencies at laser intensities (10$^{19}-10^{20}$ W/cm$^2$) and pulselengths (0.5 to 5ps) relevant to the Proton Fast Ignition scheme. Experiments were performed on the Titan laser at LLNL using coated targets to investigate the effect of molecular composition on the proton beam flux. Erbium hydride targets provide a proton rich surface layer that minimizes depletion and improves conversion efficiency. The hybrid PIC code LSP (large-scale plasma) is used to self consistently model the processes of electron transport, field generation, and proton acceleration using realistic target parameters (i.e., density, spatial dimensions, and material composition) for these targets. LSP shows enhanced conversion efficiency with high-Z hydride targets due to the dependence of atomic mass on the partition of energy between ion species. *This work was performed under the auspices of the U.S. DOE by LLNL under contracts DEFG02-05ER54834, W-7405-Eng-48 No. DE-FC02-04ER54789 and DE-AC52-07NA27344. [Preview Abstract] |
Tuesday, November 18, 2008 12:09PM - 12:21PM |
GO4.00013: Improvements to the applicability of laser plasma ion acceleration Christopher D. Murphy, Rebecca L. Daskalova, Enam A. Chowdhury, Richard R. Freeman, Andrew Krygier, John Morrison, Linn D. Van Woerkom, Anatoly Maksimchuk, Takeshi Matsuoka, Christopher McGuffey, Karl Krushelnick The study of laser plasma acceleration of ions is a rapidly advancing field due to the potential applications in oncology by proton therapy, astrophysics and neutron production for radiography. At present, most of the experiments use water and oil contaminants on the rear surface of the target as the ion source although this source is not well controlled. In order to advance the progress of laser driven ion sources, the partition of energy between the species present must be studied along with methods of controlling the contaminant content. Preliminary experimental results from an extensive study of the relevant issues will be presented. [Preview Abstract] |
Tuesday, November 18, 2008 12:21PM - 12:33PM |
GO4.00014: Temperature gradients in solid targets irradiated by high intensity short pulse laser Sophia N. Chen, Pravesh K. Patel, Hyun-Kung Chung, Andreas J. Kemp, Sebastien Le Pape, Brian R. Maddox, Scott C. Wilks, Farhat Beg It has been observed that there exists a rapid decrease in thermal temperature in solid targets, as a function of depth, when irradiated by a high intensity short pulse laser. This phenomenon is further investigated using the Titan short pulse laser with intensities greater than 10$^{20}$ W/cm$^2$ and buried layer targets. The longitudinal temperature profile is determined by measuring K-shell spectra from a 0.4 $\mu$m copper tracer layer placed at various depths (ie. 0-1.5 $\mu$m) within the 2.4 $\mu$m thick target. To study origins of K-shell x-rays in both space and time, a model involving hydrodynamics code HYADES and non-LTE atomic code FLYCHK has been developed. In addition, effect of the fast electron population on K-shell spectra is examined. Preliminary simulation results have produced good agreement with experimental measurements. [Preview Abstract] |
Tuesday, November 18, 2008 12:33PM - 12:45PM |
GO4.00015: Electron Acceleration from the Interaction of VULCAN 100TW Laser with Au Foils and its Dependence on Laser Polarisation S.R. Nagel, C. Bellei, S. Kneip, S.P.D. Mangles, C. Palmer, L. Willingale, A.E. Dangor, Z. Najmudin, R.J. Clarke, R. Heathcote, A. Henig, J. Schreiber, A. Saevert, M. Kaluza Electrons as well as ions can be accelerated to high energies (MeV) by high intensity laser interactions with solid targets. An overview of an experiment on the Vulcan laser (pulse length c$\tau\sim 150\mu\rm{m}$, energy on target $\sim 60 \rm{J}$), will be presented. In this experiment electron acceleration from thick overdense plasmas is investigated by conducting thickness scans using Au foil targets ranging from 10 to 100 $\mu$m. The electron spectra, of the most energetic electrons produced in the interaction, are measured along the laser direction and extend up to 40MeV. Surprisingly the electron acceleration depends on target thickness. Simultaneously rear surface proton beam profiles show a dependence of target thickness. Both effects are attributed to electron recirculation. In addition the effects of polarisation was investigated. A decrease in number and effective temperature of energetic electrons is observed for circular polarisation as compared to linear polarisation. [Preview Abstract] |
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