Publications


Hybrid Simulations for the Ion Structure and Dynamics in Dense Plasmas

ATOMIC PROCESSES IN PLASMAS 1161 (2009) 280-285

G Gregori, J Vorberger, DO Gericke


Making relativistic positrons using ultraintense short pulse lasers

Physics of Plasmas 16 (2009)

H Chen, SC Wilks, JD Bonlie, SN Chen, KV Cone, LN Elberson, DF Price, MB Schneider, R Shepherd, DC Stafford, R Tommasini, R Van Maren, P Beiersdorfer, G Gregori, DD Meyerhofer, J Myatt

This paper describes a new positron source using ultraintense short pulse lasers. Although it has been theoretically studied since the 1970s, the use of lasers as a valuable new positron source was not demonstrated experimentally until recent years, when the petawatt-class short pulse lasers were developed. In 2008 and 2009, in a series of experiments performed at the Lawrence Livermore National Laboratory, a large number of positrons were observed after shooting a millimeter thick solid gold target. Up to 2× positrons/s ejected at the back of approximately millimeter thick gold targets were detected. The targets were illuminated with short (∼1 ps) ultraintense (∼1× 10 W/ cm) laser pulses. These positrons are produced predominantly by the Bethe-Heitler process and have an effective temperature of 2-4 MeV, with the distribution peaking at 4-7 MeV. The angular distribution of the positrons is anisotropic. For a wide range of applications, this new laser-based positron source with its unique characteristics may complement the existing sources based on radioactive isotopes and accelerators. © 2009 American Institute of Physics.


High brightness keV harmonics from relativistically oscillating plasma surfaces

European Physical Journal: Special Topics 175 (2009) 57-60

B Dromey, D Adams, S Kar, K Markey, PT Simpson, M Zepf, C Bellei, JS Green, S Kneip, SR Nagel, L Willingale, Z Najmudin, K Krushelnick, DC Carroll, P McKenna, RJ Clarke, D Neely, PA Norreys

X-ray harmonic radiation extending to 3.3 Å, 3.8 keV from Petawatt class laser-solid interactions is presented. The harmonic spectra display a relativistic limit scaling up to ∼3000th order, above which an intensity dependent scaling roll-over is observed. Highly directional beamed emission for harmonic photon energy hv > 1 keV is found to be into a cone angle < 4°, significantly less than that of the incident laser cone (20°). © EDP Sciences and Springer 2009.


Predicting EXAFS signals from shock compressed iron by use of molecular dynamics simulations

High Energy Density Physics 5 (2009) 44-50

A Higginbotham, WJ Murphy, B Nagler, JS Wark, RC Albers, TC Germann, B Lee Holian, K Kadau, PS Lomdahl

Simulated EXAFS signals from ab initio models and configurational averaging of molecular dynamics (MD) data are compared for α-Fe, and configurationally averaged MD EXAFS signals are compared with experimental data for iron shock compressed to pressures above the α-ε{lunate} transition pressure. It is shown that molecular dynamics potentials and ab initio models capable of recreating similar vibrational density of states lead to EXAFS signals in good mutual agreement. The effects of the classical nature of the phonon distribution in the MD and the anharmonicity of the potential give rise to noticeable differences between ab initio models and configurational averaging of MD data. However, the greatest influence on the spectra is the form of the potential itself. We discuss the importance of these effects in simulating EXAFS spectra for shock compressed polycrystalline iron. It is shown that EXAFS is an insensitive probe for determining the nature of the close packed product phase in this system. © 2009 Elsevier B.V.


Evolution of elastic x-ray scattering in laser-shocked warm dense lithium

Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 80 (2009)

NL Kugland, C Constantin, C Niemann, SH Glenzer, AL Kritcher, G Gregori, S Bandyopadhyay, CM Brenner, J Pasley, C Spindloe, CRD Brown, FY Khattak, A Otten, A Pelka, M Roth, D Riley

We have studied the dynamics of warm dense Li with near-elastic x-ray scattering. Li foils were heated and compressed using shock waves driven by 4-ns-long laser pulses. Separate 1-ns-long laser pulses were used to generate a bright source of 2.96 keV Cl Ly- α photons for x-ray scattering, and the spectrum of scattered photons was recorded at a scattering angle of 120° using a highly oriented pyrolytic graphite crystal operated in the von Hamos geometry. A variable delay between the heater and backlighter laser beams measured the scattering time evolution. Comparison with radiation-hydrodynamics simulations shows that the plasma is highly coupled during the first several nanoseconds, then relaxes to a moderate coupling state at later times. Near-elastic scattering amplitudes have been successfully simulated using the screened one-component plasma model. Our main finding is that the near-elastic scattering amplitudes are quite sensitive to the mean ionization state Z̄ and by extension to the choice of ionization model in the radiation- hydrodynamics simulations used to predict plasma properties within the shocked Li. © 2009 The American Physical Society.


Numerical simulations of LWFA for the next generation of laser systems

AIP Conference Proceedings 1086 (2009) 285-290

SF Martins, J Vieira, F Fiúza, RA Fonseca, LO Silva, C Huang, W Lu, WB Mori, R Trines, P Norreys

The development of new laser systems based on OPCPA will push Laser Wakefield Accelerators (LWFA) to a qualitatively new energy range. As in the past, numerical simulations will play a critical role in testing, probing and optimizing the physical parameters and setup of these upscale experiments. Based on the prospective design parameters for the future Vulcan 10 PW OPCPA laser system, we have determined the optimal parameters for a single LWFA stage from theoretical scalings for such system, which predict accelerations to the energy frontier, with self-injected electrons in excess of 10 GeV for a self-guided configuration, and above 50 GeV bunches with externally-injected electrons in a laser-guided configuration. These parameters were then used as a baseline for 3D full scale simulations with OSIRIS and QuickPIC. A 12 GeV self-injected beam was obtained with both codes, in agreement with theoretical predictions for the maximum energy gain and the injected charge. Preliminary results on the laser-guided configuration already confirm the accelerating gradients and the stability of the laser guided propagation for long distances required to reach the higher energies predicted by the theoretical scalings for this scenario. © 2009 American Institute of Physics.


Applications of the wave kinetic approach: From laser wakefields to drift wave turbulence

Physics of Plasmas 16 (2009)

RMGM Trines, R Bingham, CD Murphy, MW Dunlop, JA Davies, R Bamford, PA Norreys, LO Silva, JT Mendoņa, PK Shukla, A Vaivads

Nonlinear wave-driven processes in plasmas are normally described by either a monochromatic pump wave that couples to other monochromatic waves or as a random phase wave coupling to other random phase waves. An alternative approach involves a random or broadband pump coupling to monochromatic and/or coherent structures in the plasma. This approach can be implemented through the wave kinetic model. In this model, the incoming pump wave is described by either a bunch (for coherent waves) or a sea (for random phase waves) of quasiparticles. This approach has been applied to both photon acceleration in laser wakefields and drift wave turbulence in magnetized plasma edge configurations. Numerical simulations have been compared to experiments, varying from photon acceleration to drift mode-zonal flow turbulence, and good qualitative correspondences have been found in all cases. © 2009 American Institute of Physics.


Evidence of anomalous resistivity for hot electron propagation through a dense fusion core in fast ignition experiments

New Journal of Physics 11 (2009)

T Yabuuchi, H Habara, R Kodarna, KA Tanaka, K Mima, A Das, PK Kaw, S Sengupta, GR Kumar, PA Norreys

Anomalous resistivity for hot electrons passing through a dense core plasma is studied for fast ignition laser fusion. The hot electrons generated via the ultra-intense laser pulse and guiding cone interactions are measured after they pass through a dense plasma with a density of 50-100 g cm in a radius of 15-25 m. When significant neutron enhancements are achieved by the ultraintense laser pulse injection, the energy reduction of fast electrons is observed. Also, a reduction in the number of electrons with energy up to 15 MeV can be seen. We offer a new physical mechanism for the stopping of electrons, involving electron magnetohydrodynamic shock formation in the inhomogeneous plasma density region. The dissipation in the shock region can explain electron stopping with energies of the order of 15 MeV. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.


Photon acceleration and modulational instability during wakefield excitation using long laser pulses

Plasma Physics and Controlled Fusion 51 (2009)

RMGM Trines, CD Murphy, KL Lancaster, O Chekhlov, PA Norreys, R Bingham, JT Mendonça, LO Silva, SPD Mangles, C Kamperidis, A Thomas, K Krushelnick, Z Najmudin

The modulational instability that occurs during the interaction of a long laser pulse and its own wakefield in an underdense plasma has been studied experimentally and theoretically. Recent experiments using laser pulses that are several times longer than the wakefield period have yielded transmission spectra that exhibit a series of secondary peaks flanking the main laser peak. These peaks are too closely spaced to be the result of Raman instabilities; their origin was found to be photon acceleration of the laser's photons in the wakefield instead. In the experiments described in this paper, a laser pulse of 50-200 fs containing 300-600 mJ was focused on the edge of a helium gas jet on a 25 νm focal spot. The observed transmission spectra show evidence of both ionization blueshift and modulation by the pulse's wakefield. The transmission spectra have also been modelled using a dedicated photon-kinetic numerical code. The modelling has revealed a direct correlation between the spectral modulations and the amplitude of the excited wakefield. By comparing the measured and simulating spectra, the origin of various spectral characteristics has been explained in terms of photon acceleration. The feasibility of using this effect as a wakefield diagnostic will be discussed. © 2009 IOP Publishing Ltd.


Free-free opacity in warm dense aluminum

High Energy Density Physics 5 (2009) 124-131

SM Vinko, G Gregori, B Nagler, TJ Whitcher, JS Wark, MP Desjarlais, RW Lee, P Audebert

We present calculations of the free-free opacity of warm, solid-density aluminum at photon energies between the plasma frequency at 15 eV and the L-edge at 73 eV, using both density functional theory combined with molecular dynamics and a semi-analytical model in the RPA framework which includes exciton contributions. As both the ion and electron temperature is increased from room temperature to 10 eV, we see a marked increase in the opacity. The effect is less pronounced if only the electron temperature is allowed to increase, while the lattice remains at room temperature. The physical significance of these increases is discussed in terms of intense light-matter interactions on both femtosecond and picosecond time scales. © 2009 Elsevier B.V. All rights reserved.


Perspective for high energy density studies using x-ray free electron lasers

IEEE International Conference on Plasma Science (2009)

RW Lee, S Vinko, T Whitcher, A Nelson, H Chapman, T Döppner, SH Glenzer, B Nagler, J Chalupsky, G Gregori, WJ Murphy, JS Wark, U Zastrau, E Förster, I Uschmann, R Fäustlin, S Bajt, S Düsterer, T Laarmann, S Toleikis, T Tschentscher, R Sobierajski, M Jurek, D Klinger, J Krzywinski, L Juha, T Burian, J Cihelka, V Hajkova, T Bornath, C Fortmann, S Göde, K Meiwes-Broer, A Przystawik, R Redmer, H Reinholz, G Röpke, R Thiele, J Tiggesbäumker, T Dzelzainis, D Riley, M Fajardo, M Kozlova, P Heimann, F Khattak, AR Khorsand, H Lee, P Mercere, K Saksl


Effect of reentrant cone geometry on energy transport in intense laser-plasma interactions

Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 80 (2009)

KL Lancaster, M Sherlock, JS Green, R Heathcote, J Pasley, PA Norreys, CD Gregory, P Hakel, KU Akli, DS Hey, RB Stephens, FN Beg, SN Chen, MS Wei, T Yabuuchi, RR Freeman, K Highbarger, L Van Woerkom, RL Weber, H Habara, R Kodama, MH Key, H Nakamura, M Nakatsutsumi, M Tampo, K Krushelnick, NC Woolsey, M Storm, W Theobald

The energy transport in cone-guided low- Z targets has been studied for laser intensities on target of 2.5× 1020 W cm-2. Extreme ultraviolet (XUV) imaging and transverse optical shadowgraphy of the rear surfaces of slab and cone-slab targets show that the cone geometry strongly influences the observed transport patterns. The XUV intensity showed an average spot size of 65±10 μm for slab targets. The cone slabs showed a reduced spot size of 44±10 μm. The shadowgraphy for the aforementioned shots demonstrate the same behavior. The transverse size of the expansion pattern was 357±32 μm for the slabs and reduced to 210±30 μm. A transport model was constructed which showed that the change in transport pattern is due to suppression of refluxing electrons in the material surrounding the cone. © 2009 The American Physical Society.


Femtosecond X-ray Diffraction: Applications for Laser-Irradiated Materials

ATOMIC PROCESSES IN PLASMAS 1161 (2009) 253-253

JS Wark


Soft X-Ray Thomson scattering in warm dense hydrogen at FLASH

Proceedings of SPIE - The International Society for Optical Engineering 7451 (2009)

RR Fäustlin, S Toleikis, S Düsterer, T Laarmann, H Redlin, F Tavella, T Tschentscher, T Bornath, C Fortmann, S Göde, R Irsig, K-H Meiwes-Broer, A Przystawik, R Redmer, H Reinholz, G Röpke, R Thiele, J Tiggesbäumker, E Förster, I Uschmann, U Zastrau, T Döppner, SH Glenzer, G Gregori, J Mithen, HJ Lee, B Li

We present collective Thomson scattering with soft x-ray free electron laser radiation as a method to track the evolution of warm dense matter plasmas with ∼200 fs time resolution. In a pump-probe scheme an 800 nm laser heats a 20 ∼m hydrogen droplet to the plasma state. After a variable time delay in the order of ps the plasma is probed by an x-ray ultra violet (XUV) pulse which scatters from the target and is recorded spectrally. Alternatively, in a self-Thomson scattering experiment, a single XUV pulse heats the target while a portion of its photons are being scattered probing the target. From such inelastic x-ray scattering spectra free electron temperature and density can be inferred giving insight on relaxation time scales in plasmas as well as the equation of state. We prove the feasibility of this method in the XUV range utilizing the free electron laser facility in Hamburg, FLASH. We recorded Thomson scattering spectra for hydrogen plasma, both in the self-scattering and in the pump-probe mode using optical laser heating. © 2009 SPIE-.


Radiation and hot electron temperature measurements of short-pulselaser driven hohlraums

High Energy Density Physics 5 (2009) 212-215

CRD Brown, SF James, R Edwards, JW Morton, BR Thomas, DJ Hoarty, SJ Rose, FN Beg, DP Higginson, C Constantin, C Niemann, RL Daskalova, RR Freeman, L van Woerkom, D Riley, G Gregori

We have performed measurements of the radiation and the hot electron temperature in sub-millimetre size hohlraums driven by a high intensity short-pulse laser. The results indicate that radiation temperatures ∼80 eV can be obtained with ∼20 J of laser energy delivered on target. Radiation-hydrodynamics simulations indicate an absorption into thermal X-rays of ≲1-2%, with peak temperatures similar to those measured experimentally. Crown Copyright © 2009.


Control of 2ω (527 nm) stimulated raman scattering in a steep density gradient plasma

Physics of Plasmas 16 (2009)

JD Moody, L Divol, DH Froula, SH Glenzer, G Gregori, RK Kirkwood, A MacKinnon, N Meezan, C Niemann, LJ Suter, R Bahr, W Seka

Experiments show that application of laser smoothing schemes including smoothing by spectral dispersion and polarization smoothing effectively suppresses stimulated Raman scattering from a 2ω (527 nm) laser beam in a low average-gain plasma with a steep density gradient. Full-wave simulations reproduce the observed trends in the data and show that the scattering reduction is an indirect result of suppressing active filamentation. © 2009 American Institute of Physics.


Design of a sub 100-femtosecond X-ray Streak Camera

2009 Conference on Lasers and Electro-Optics and 2009 Conference on Quantum Electronics and Laser Science Conference, CLEO/QELS 2009 (2009)

B Li, PP Rajeev, G Gregori, M Benetou, B Dobson, L Pickworth, D Neely, A Cavalleri, P Lau, J Lynn, P Jaanimagi, F Read

The temporal resolution of existing streak cameras are limited by electron transit time dispersion. Here we present a state-of-art design compensating this to achieve a breakthrough of 100fs time resolution. ©2009 IEEE.


Measurements of ionic structure in shock compressed lithium hydride from ultrafast x-ray Thomson scattering.

Phys Rev Lett 103 (2009) 245004-

AL Kritcher, P Neumayer, CR Brown, P Davis, T Döppner, RW Falcone, DO Gericke, G Gregori, B Holst, OL Landen, HJ Lee, EC Morse, A Pelka, R Redmer, M Roth, J Vorberger, K Wünsch, SH Glenzer

We present the first ultrafast temporally, spectrally, and angularly resolved x-ray scattering measurements from shock-compressed matter. The experimental spectra yield the absolute elastic and inelastic scattering intensities from the measured density of free electrons. Laser-compressed lithium-hydride samples are well characterized by inelastic Compton and plasmon scattering of a K-alpha x-ray probe providing independent measurements of temperature and density. The data show excellent agreement with the total intensity and structure when using the two-species form factor and accounting for the screening of ion-ion interactions.


Intense laser-plasma interactions: New frontiers in high energy density physics

Physics of Plasmas 16 (2009)

PA Norreys, RMGM Trines, RA Smith, FN Beg, Y Sentoku, LO Silva

A review is presented here of a number of invited papers presented at the 2008 American Physical Society April meeting [held jointly with High Energy Density Physics/High Energy Density Laboratory Astrophysics (HEDP/HEDLA) Conference] devoted to intense laser-matter interactions. They include new insights gained from wave-kinetic theory into laser-wakefield accelerators and drift wave turbulence interacting with zonal flows in magnetized plasmas; interactions with cluster media for the generation of radiative blast waves; fast electron energy transport in cone-wire targets; numerical investigations into Weibel instability in electron-positron-ion plasmas and the generation of gigabar pressures with thin foil interactions. © 2009 American Institute of Physics.


Measurements of fast electron scaling generated by petawatt laser systems

Physics of Plasmas 16 (2009)

T Tanimoto, H Habara, R Kodama, M Nakatsutsumi, KA Tanaka, KL Lancaster, JS Green, RHH Scott, M Sherlock, PA Norreys, RG Evans, MG Haines, S Kar, M Zepf, J King, T Ma, MS Wei, T Yabuuchi, FN Beg, MH Key, P Nilson, RB Stephens, H Azechi, K Nagai, T Norimatsu, K Takeda, J Valente, JR Davies

Fast electron energy spectra have been measured for a range of intensities between 10 and 10 W cm-2 and for different target materials using electron spectrometers. Several experimental campaigns were conducted on petawatt laser facilities at the Rutherford Appleton Laboratory and Osaka University, where the pulse duration was varied from 0.5 to 5 ps relevant to upcoming fast ignition integral experiments. The incident angle was also changed from normal incidence to 40° in p -polarized. The results confirm a reduction from the ponderomotive potential energy on fast electrons at the higher intensities under the wide range of different irradiation conditions. © 2009 American Institute of Physics.