Publications


Low frequency structural dynamics of warm dense matter

Physics of Plasmas 16 (2009)

G Gregori, DO Gericke

Measurements of the microscopic response of warm dense matter have been demonstrated by multi-keV inelastic x-ray scattering using laser-based sources. These techniques have been used to study the high frequency electron correlations (plasmons) in low to mid- Z plasmas. The advent of fourth generation light sources will provide high fluxes of narrowband and coherent x rays that will allow to look at the low frequency correlations (the ion-acoustic waves). In this paper we present an analysis of such low frequency modes by calculating the frequency dependent ion-ion structure factor. Our model includes all the relevant multibody contributions arising from strong coupling and nonideal plasma effects. In particular, the ion-ion structure factor is obtained within the memory function formalism by satisfying a finite number of sum rules. This work could be used as a basis to a direct experimental test of dense plasma model as soon as keV free electron laser sources will become available. © 2009 American Institute of Physics.


Recent fast electron energy transport experiments relevant to fast ignition inertial fusion

Nuclear Fusion 49 (2009)

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

A number of experiments have been undertaken at the Rutherford Appleton Laboratory that were designed to investigate the physics of fast electron transport relevant to fast ignition inertial fusion. The laser, operating at a wavelength of 1054 nm, provided pulses of up to 350 J of energy on target in a duration that varied in the range 0.5-5 ps and a focused intensity of up to 1021 W cm-2. A dependence of the divergence of the fast electron beam with intensity on target has been identified for the first time. This dependence is reproduced in two-dimensional particle-in-cell simulations and has been found to be an intrinsic property of the laser-plasma interaction. A number of ideas to control the divergence of the fast electron beam are described. The fractional energy transfer to the fast electron beam has been obtained from calibrated, time-resolved, target rear-surface radiation temperature measurements. It is in the range 15-30%, increasing with incident laser energy on target. The fast electron temperature has been measured to be lower than the ponderomotive potential energy and is well described by Haines' relativistic absorption model. © 2009 IAEA, Vienna.


Temperature profiles derived from transverse optical shadowgraphy in ultraintense laser plasma interactions at 6 × 1020 W cm? 2

Physics of Plasmas 16 (2009)

KL Lancaster, J Pasley, JS Green, D Batani, S Baton, RG Evans, L Gizzi, R Heathcote, C Hernandez Gomez, M Koenig, P Koester, A Morace, I Musgrave, PA Norreys, F Perez, JN Waugh, NC Woolsey

A variety of targets with different dimensions and materials was irradiated using the VULCAN PW laser [C. N. Danson, Nucl. Fusion 44, S239 (2004)]. Using transverse optical shadowgraphy in conjunction with a one-dimensional radiation hydrodynamics code it was possible to determine a longitudinal temperature gradient. It was demonstrated for thick targets with a low Z substrate and a thin higher Z tracer layer at the rear that the boundary between the two materials was Rayleigh-Taylor unstable. By including a simple bubble growth model into the calculations it was possible to correct for the associated behavior with regard to temperature. The resulting temperature gradient was in good agreement with the previously published data using two different methods of determining the temperature. © 2009 American Institute of Physics.


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


Turning solid aluminium transparent by intense soft X-ray photoionization

Nature Physics 5 (2009) 693-696

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

Saturable absorption is a phenomenon readily seen in the optical and infrared wavelengths. It has never been observed in core-electron transitions owing to the short lifetime of the excited states involved and the high intensities of the soft X-rays needed. We report saturable absorption of an L-shell transition in aluminium using record intensities over 10 16 W cm 2 at a photon energy of 92 eV. From a consideration of the relevant timescales, we infer that immediately after the X-rays have passed, the sample is in an exotic state where all of the aluminium atoms have an L-shell hole, and the valence band has approximately a 9 eV temperature, whereas the atoms are still on their crystallographic positions. Subsequently, Auger decay heats the material to the warm dense matter regime, at around 25 eV temperatures. The method is an ideal candidate to study homogeneous warm dense matter, highly relevant to planetary science, astrophysics and inertial confinement fusion. © 2009 Macmillan Publishers Limited. All rights reserved.


Free-free opacity in warm dense aluminum

High Energy Density Physics 5 (2009) 124-131

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

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.


Guiding of relativistic electron beams in solid targets by resistively controlled magnetic fields

Physical Review Letters 102 (2009)

S Kar, APL Robinson, DC Carroll, O Lundh, K Markey, P McKenna, P Norreys, M Zepf

Guided transport of a relativistic electron beam in solid is achieved experimentally by exploiting the strong magnetic fields created at the interface of two metals of different electrical resistivities. This is of substantial relevance to the Fast Ignitor approach to fusion energy production, since it allows the electron deposition to be spatially tailored-thus adding substantial design flexibility and preventing inefficiencies due to electron beam spreading. In the experiment, optical transition radiation and thermal emission from the target rear surface provide a clear signature of the electron confinement within a high resistivity tin layer sandwiched transversely between two low resistivity aluminum slabs. The experimental data are found to agree well with numerical simulations. © 2009 The American Physical Society.


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

Physics of Plasmas 16 (2009)

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

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.


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

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.


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

Optics InfoBase Conference Papers Optical Society of America (2009)

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

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 OSA/CLEO/IQEC 2009.


Ion structure in dense plasmas: MSA versus HNC

Journal of Physics A: Mathematical and Theoretical 42 (2009)

K Wünsch, J Vorberger, G Gregori, DO Gericke

We present results for the ionic structure in dense, moderately to strongly coupled plasmas using two models: the mean spherical approximation (MSA) and the hypernetted chain (HNC) approach. While the first method allows for an analytical solution, the latter has to be solved iteratively. Independent of the coupling strength, the results show only small differences when the ions are considered to form an unscreened one-component plasma (OCP) system. If the electrons are treated as a polarizable background, the different ways to incorporate the screening yield, however, large discrepancies between the models, particularly for more strongly coupled plasmas. © 2009 IOP Publishing Ltd.


The 10 PW OPCPA vulcan laser upgrade

Optics InfoBase Conference Papers Optical Society of America (2009)

O Chekhlov, J Collier, RJ Clark, C Hernandez-Gomez, A Lyachev, P Matousek, IO Musgrave, D Neely, PA Norreys, I Ross, Y Tang, TB Winstone, BE Wyborn


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

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-.


Ultrashort pulse filamentation and monoenergetic electron beam production in LWFAs

Plasma Physics and Controlled Fusion 51 (2009)

AGR Thomas, SPD Mangles, CD Murphy, AE Dangor, PS Foster, JG Gallacher, DA Jaroszynski, C Kamperidis, K Krushelnick, KL Lancaster, PA Norreys, R Viskup, Z Najmudin

In the experiments reported here, the filamentation of ultrashort laser pulses, due to non-optimal choice of focusing geometry and/or electron number density, has a severely deleterious effect on monoenergetic electron beam production in laser wakefield accelerators. Interactions with relatively small focal spots, w0 < λp/2, and with pulse length cτ ≈ λp, incur fragmentation into a large number of low power filaments. These filaments are modulated with a density dependent size of, on average, close to λp. The break-up of the driving pulse results in shorter interaction lengths, compared with larger focal spots, and broad energy-spread electron beams, which are not useful for applications. Filamentation of the pulse occurs because the strongly dynamic focusing (small f-number) of the laser prevents pulse length compression before reaching its minimum spot-size, which results in non-spherical focusing gradients. © 2009 IOP Publishing Ltd.


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


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.


A dual-channel, curved-crystal spectrograph for petawatt laser, x-ray backlighter source studies

Review of Scientific Instruments 80 (2009)

W Theobald, C Stoeckl, PA Jaanimagi, PM Nilson, M Storm, DD Meyerhofer, TC Sangster, D Hey, AJ MacKinnon, H-S Park, PK Patel, R Shepherd, RA Snavely, MH Key, JA King, B Zhang, RB Stephens, KU Akli, K Highbarger, RL Daskalova, L Van Woerkom, RR Freeman, JS Green, G Gregori, K Lancaster, PA Norreys

A dual-channel, curved-crystal spectrograph was designed to measure time-integrated x-ray spectra in the ∼1.5 to 2 keV range (6.2-8.2 Å wavelength) from small-mass, thin-foil targets irradiated by the VULCAN petawatt laser focused up to 4× 10 20 W/ cm 2. The spectrograph consists of two cylindrically curved potassium-acid-phthalate crystals bent in the meridional plane to increase the spectral range by a factor of ∼10 compared to a flat crystal. The device acquires single-shot x-ray spectra with good signal-to-background ratios in the hard x-ray background environment of petawatt laser-plasma interactions. The peak spectral energies of the aluminum He α and Ly α resonance lines were ∼1.8 and ∼1.0 mJ/eV sr (∼0.4 and 0.25 J/Å sr), respectively, for 220 J, 10 ps laser irradiation. © 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, LO Silva, JT Mendoņa, PK Shukla, CD Murphy, MW Dunlop, JA Davies, R Bamford, A Vaivads, PA Norreys

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.


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, G Gregori, DD Meyerhofer, J Myatt, DF Price, MB Schneider, R Shepherd, DC Stafford, R Tommasini, R Van Maren, P Beiersdorfer

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× 1010 positrons/s ejected at the back of approximately millimeter thick gold targets were detected. The targets were illuminated with short (∼1 ps) ultraintense (∼1× 1020 W/ cm2) 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.