Generation and control of ultrafast pulse trains for quasi-phase-matching high-harmonic generation

Journal of the Optical Society of America B: Optical Physics 27 (2010) 763-772

T Robinson, K O'Keeffe, SM Hooker, M Zepf, B Dromey

Two techniques are demonstrated to produce ultrashort pulse trains capable of quasi-phase-matching highharmonic generation. The first technique makes use of an array of birefringent crystals and is shown to generate high-contrast pulse trains with constant pulse spacing. The second technique employs a grating-pair stretcher, a multiple-order wave plate, and a linear polarizer. Trains of up to 100 pulses are demonstrated with this technique, with almost constant inter-pulse separation. It is shown that arbitrary pulse separation can be achieved by introducing the appropriate dispersion. This principle is demonstrated by using an acousto-optic programmable dispersive filter to introduce third- and fourth-order dispersions leading to a linear and quadratic variation of the separation of pulses through the train. Chirped-pulse trains of this type may be used to quasi-phase-match high-harmonic generation in situations where the coherence length varies through the medium. © 2010 Optical Society of America.

Electronic structure of an XUV photogenerated solid-density aluminum plasma.

Phys Rev Lett 104 (2010) 225001-

SM Vinko, U Zastrau, S Mazevet, J Andreasson, S Bajt, T Burian, J Chalupsky, HN Chapman, J Cihelka, D Doria, T Döppner, S Düsterer, T Dzelzainis, RR Fäustlin, C Fortmann, E Förster, E Galtier, SH Glenzer, S Göde, G Gregori, J Hajdu, V Hajkova, PA Heimann, R Irsig, L Juha, M Jurek, J Krzywinski, T Laarmann, HJ Lee, RW Lee, B Li, KH Meiwes-Broer, JP Mithen, B Nagler, AJ Nelson, A Przystawik, R Redmer, D Riley, F Rosmej, R Sobierajski, F Tavella, R Thiele, J Tiggesbäumker, S Toleikis, T Tschentscher, L Vysin, TJ Whitcher, S White, JS Wark

By use of high intensity XUV radiation from the FLASH free-electron laser at DESY, we have created highly excited exotic states of matter in solid-density aluminum samples. The XUV intensity is sufficiently high to excite an inner-shell electron from a large fraction of the atoms in the focal region. We show that soft-x-ray emission spectroscopy measurements reveal the electronic temperature and density of this highly excited system immediately after the excitation pulse, with detailed calculations of the electronic structure, based on finite-temperature density functional theory, in good agreement with the experimental results.

Generation and control of chirped, ultrafast pulse trains

Journal of Optics A: Pure and Applied Optics 12 (2010)

K O'Keeffe, T Robinson, SM Hooker

A method for generating non-uniformly spaced (chirped) trains of high-energy, high-contrast, femtosecond pulses is described and demonstrated. In this method a temporally stretched laser pulse is passed through an acousto-optic programmable dispersive filter (AOPDF), a birefringent plate, and a linear polarizer. It is demonstrated that linear and nonlinear variation of the pulse separation within the train may be controlled by changing respectively the third-and fourth-order dispersion introduced by the AOPDF. Programmable, non-uniform pulse trains of this type may find applications in quasi-phase matching high-harmonic generation. © 2010 IOP Publishing Ltd.

Bragg diffraction using a 100 ps 17.5 keV x-ray backlighter and the Bragg diffraction imager


BR Maddox, H-S Park, J Hawreliak, A Elsholz, R Van Maren, BA Remington, A Comley, JS Wark

Relativistic quasimonoenergetic positron jets from intense laser-solid interactions.

Phys Rev Lett 105 (2010) 015003-

H Chen, SC Wilks, DD Meyerhofer, J Bonlie, CD Chen, SN Chen, C Courtois, L Elberson, G Gregori, W Kruer, O Landoas, J Mithen, J Myatt, CD Murphy, P Nilson, D Price, M Schneider, R Shepherd, C Stoeckl, M Tabak, R Tommasini, P Beiersdorfer

Detailed angle and energy resolved measurements of positrons ejected from the back of a gold target that was irradiated with an intense picosecond duration laser pulse reveal that the positrons are ejected in a collimated relativistic jet. The laser-positron energy conversion efficiency is ∼2×10{-4}. The jets have ∼20 degree angular divergence and the energy distributions are quasimonoenergetic with energy of 4 to 20 MeV and a beam temperature of ∼1  MeV. The sheath electric field on the surface of the target is shown to determine the positron energy. The positron angular and energy distribution is controlled by varying the sheath field, through the laser conditions and target geometry.

Probing near-solid density plasmas using soft x-ray scattering

Journal of Physics B: Atomic, Molecular and Optical Physics 43 (2010)

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

X-ray scattering using highly brilliant x-ray free-electron laser (FEL) radiation provides new access to probe free-electron density, temperature and ionization in near-solid density plasmas. First experiments at the soft x-ray FEL FLASH at DESY, Hamburg, show the capabilities of this technique. The ultrashort FEL pulses in particular can probe equilibration phenomena occurring after excitation of the plasma using ultrashort optical laser pumping. We have investigated liquid hydrogen and find that the interaction of very intense soft x-ray FEL radiation alone heats the sample volume. As the plasma establishes, photons from the same pulse undergo scattering, thus probing the transient, warm dense matter state. We find a free-electron density of (2.6 ± 0.2) × 10 cm and an electron temperature of 14 ± 3.5 eV. In pump-probe experiments, using intense optical laser pulses to generate more extreme states of matter, this interaction of the probe pulse has to be considered in the interpretation of scattering data. In this paper, we present details of the experimental setup at FLASH and the diagnostic methods used to quantitatively analyse the data. © 2010 IOP Publishing Ltd.

Phonon instabilities in uniaxially compressed fcc metals as seen in molecular dynamics simulations

PHYSICAL REVIEW B 81 (2010) ARTN 092102

G Kimminau, P Erhart, EM Bringa, B Remington, JS Wark

Observation of ultrafast nonequilibrium collective dynamics in warm dense hydrogen.

Phys Rev Lett 104 (2010) 125002-

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

We investigate ultrafast (fs) electron dynamics in a liquid hydrogen sample, isochorically and volumetrically heated to a moderately coupled plasma state. Thomson scattering measurements using 91.8 eV photons from the free-electron laser in Hamburg (FLASH at DESY) show that the hydrogen plasma has been driven to a nonthermal state with an electron temperature of 13 eV and an ion temperature below 0.1 eV, while the free-electron density is 2.8x10{20} cm{-3}. For dense plasmas, our experimental data strongly support a nonequilibrium kinetics model that uses impact ionization cross sections based on classical free-electron collisions.

Soft X-ray scattering using FEL radiation for probing near-solid density plasmas at few electron volt temperatures

High Energy Density Physics 6 (2010) 15-20

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

We report on soft X-ray scattering experiments on cryogenic hydrogen and simple metal samples. As a source of intense, ultrashort soft X-ray pulses we have used free-electron laser radiation at 92 eV photon energy from FLASH at DESY, Hamburg. X-ray pulses with energies up to 150 μJ and durations 15-50 fs provide interaction with the sample leading simultaneously to plasma formation and scattering. Experiments exploiting both of these interactions have been carried out, using the same experimental setup. Firstly, recording of soft X-ray inelastic scattering from near-solid density hydrogen plasmas at few electron volt temperatures confirms the feasibility of this diagnostics technique. Secondly, the soft X-ray excitation of few electron volt solid-density plasmas in bulk metal samples could be studied by recording soft X-ray line and continuum emission integrated over emission times from fs to ns. © 2009 Elsevier B.V.

Metal deformation and phase transitions at extremely high strain rates

MRS BULLETIN 35 (2010) 999-1006

RE Rudd, TC Germann, BA Remington, JS Wark

Inferring the electron temperature and density of shocked liquid deuterium using inelastic X-ray scattering

Journal of Physics: Conference Series 244 (2010)

SP Regan, PB Radha, TR Boehly, VN Goncharov, RL McCrory, DD Meyerhofer, TC Sangster, VA Smalyuk, T Doeppner, SH Glenzer, OL Landen, P Neumayer, K Falk, G Gregori

An experiment designed to launch laser-ablation-driven shock waves (10 to 70 Mbar) in a planar liquid-deuterium target on the OMEGA Laser System and to diagnose the shocked conditions using inelastic x-ray scattering is described. The electron temperature (T ) is inferred from the Doppler-broadened Compton-downshifted peak of the noncollective (α = 1kλ > 1) x-ray scattering for T > T . The electron density (n ) is inferred from the downshifted plasmon peak of the collective (α > 1) x-ray scattering. A cylindrical layer of liquid deuterium is formed in a cryogenic cell with 8-μm-thick polyimide windows. The polyimide ablator is irradiated with peak intensities in the range of 10 to 10 W/cm and shock waves are launched. Predictions from a 1-D hydrodynamics code show the shocked deuterium has a thickness of ∼0.1 mm with spatially uniform conditions. For the drive intensities under consideration, electron density up to ∼5 × 10 cm and electron temperature in the range of 10 to 25 eV are predicted. A laser-irradiated saran foil produces Cl Ly emission. The spectrally resolved x-ray scattering is recorded at 90° for the noncollective scattering and at 40° for the collective scattering with a highly oriented pyrolytic graphite (HOPG) crystal spectrometer and an x-ray framing camera. © 2010 IOP Publishing Ltd.

A proposal for testing subcritical vacuum pair production with high power lasers

ArXiv (2010)

G Gregori, DB Blaschke, PP Rajeev, H Chen, RJ Clarke, T Huffman, CD Murphy, AV Prozorkevich, CD Roberts, G Röpke, SM Schmidt, SA Smolyansky, S Wilks, R Bingham

We present a proposal for testing the prediction of non-equilibrium quantum field theory below the Schwinger limit. The proposed experiments should be able to detect a measurable number of gamma rays resulting from the annihilation of pairs in the focal spot of two opposing high intensity laser beams. We discuss the dependence of the expected number of gamma rays with the laser parameters and compare with the estimated background level of gamma hits for realistic laser conditions.

Plasma emission spectroscopy of solids irradiated by intense XUV pulses from a free electron laser

High Energy Density Physics 6 (2010) 109-112

TWJ Dzelzainis, J Chalupsky, M Fajardo, R Fäustlin, PA Heimann, V Hajkova, L Juha, M Jurek, FY Khattak, M Kozlova, J Krzywinski, RW Lee, B Nagler, AJ Nelson, FB Rosmej, R Soberierski, S Toleikis, T Tschentscher, SM Vinko, JS Wark, T Whitcher, D Riley

The FLASH XUV-free electron laser has been used to irradiate solid samples at intensities of the order 1016 W cm-2 at a wavelength of 13.5 nm. The subsequent time integrated XUV emission was observed with a grating spectrometer. The electron temperature inferred from plasma line ratios was in the range 5-8 eV with electron density in the range 1021-1022 cm-3. These results are consistent with the saturation of absorption through bleaching of the L-edge by intense photo-absorption reported in an earlier publication. © 2009 Elsevier B.V. All rights reserved.

All-optical steering of laser-wakefield-accelerated electron beams

Physical Review Letters 105 (2010)

A Popp, J Osterhoff, Z Major, R Hörlein, M Fuchs, R Weingartner, F Krausz, F Grüner, S Karsch, J Vieira, M Marti, RA Fonseca, SF Martins, LO Silva, TP Rowlands-Rees, SM Hooker

We investigate the influence of a tilted laser-pulse-intensity front on laser-wakefield acceleration. Such asymmetric light pulses may be exploited to obtain control over the electron-bunch-pointing direction and in our case allowed for reproducible electron-beam steering in an all-optical way within an 8 mrad opening window with respect to the initial laser axis. We also discovered evidence of collective electron-betatron oscillations due to off-axis electron injection into the wakefield induced by a pulse-front tilt. These findings are supported by 3D particle-in-cell simulations. © 2010 The American Physical Society.

X-ray polarization spectroscopy from ultra-intense interactions

Journal of Physics: Conference Series 244 (2010)

N Booth, J Pasley, E Wagenaars, JN Waugh, NC Woolsey, G Gregori, B Li, L Gizzi, P Koester, L Labate, T Levato, R Clarke, P Gallegos, PP Rajeev, M Makita, D Riley

Detailed knowledge of fast electron energy transport following the interaction of ultrashort intense laser pulses is a key subject for fast ignition. This is a problem relevant to many areas of laser-plasma physics with particular importance to fast ignition and X-ray secondary source development, necessary for the development of large-scale facilities such as HiPER and ELI. Operating two orthogonal crystal spectrometers set at Bragg angles close to 45° determines the X-ray s- and p- polarization ratio. From this ratio, it is possible to infer the velocity distribution function of the fast electron beam within the dense plasma. We report on results of polarization measurements at high density for sulphur and nickel buried layer targets in the high intensity range of 10 - 10 Wcm . We observe at 45° the Ly-α doublet using two sets of orthogonal highly-orientated pyrolytic graphite (HOPG) crystals set in 1 order for sulphur and 3 order for nickel. © 2010 IOP Publishing Ltd.

Transport of laser accelerated proton beams and isochoric heating of matter

Journal of Physics: Conference Series 244 (2010)

M Roth, I Alber, M Günther, K Harres, F Nürnberg, A Otten, A Pelka, M Schollmeier, J Schütrumpf, V Bagnoud, A Tauschwitz, C Brown, G Gregori, J Mithen, R Clarke, R Heathcote, B Li, G Schaumann, H Daido, M Tampo, J Fernandez, K Flippo, S Gaillard, C Gauthier, D Offermann, S Glenzer, A Kritcher, N Kugland, S Lepape, M Makita, D Riley, C Niemann, AN Tauschwitz

The acceleration of intense proton and ion beams by ultra-intense lasers has matured to a point where applications in basic research and technology are being developed. Crucial for harvesting the unmatched beam parameters driven by the relativistic electron sheath is the precise control of the beam. We report on recent experiments using the PHELIX laser at GSI, the VULCAN laser at RAL and the TRIDENT laser at LANL to control and use laser accelerated proton beams for applications in high energy density research. We demonstrate efficient collimation of the proton beam using high field pulsed solenoid magnets, a prerequisite to capture and transport the beam for applications. Furthermore we report on two campaigns to use intense, short proton bunches to isochorically heat solid targets up to the warm dense matter state. The temporal profile of the proton beam allows for rapid heating of the target, much faster than the hydrodynamic response time thereby creating a strongly coupled plasma at solid density. The target parameters are then probed by X-ray Thomson scattering (XRTS) to reveal the density and temperature of the heated volume. This combination of two powerful techniques developed during the past few years allows for the generation and investigation of macroscopic samples of matter in states present in giant planets or the interior of the earth. © 2010 IOP Publishing Ltd.

Screening of ionic cores in partially ionized plasmas within linear response

Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 81 (2010)

DO Gericke, J Vorberger, K Wünsch, G Gregori

We employ a pseudopotential approach to investigate the screening of ionic cores in partially ionized plasmas. Here, the effect of the tightly bound electrons is condensed into an effective potential between the (free) valence electrons and the ionic cores. Even for weak electron-ion coupling, the corresponding screening clouds show strong modifications from the Debye result for elements heavier than helium. Modifications of the theoretically predicted x-ray scattering signal and implications on measurements are discussed. © 2010 The American Physical Society.

Static ion structure factor for dense plasmas: Semi-classical and ab initio calculations

High Energy Density Physics 6 (2010) 305-310

V Schwarz, B Holst, T Bornath, C Fortmann, W-D Kraeft, R Thiele, R Redmer, G Gregori, HJ Lee, T Döppner, SH Glenzer

We calculate the static structure factor of dense multi-component plasmas. Large scale ab initio finite-temperature DFT molecular dynamics simulations are performed in order to cover the region where a consistent quantum treatment for the electrons is inevitable. Especially, the behavior at small wave numbers k can be inferred from the relation to the isothermal compressibility. Alternatively, the static structure factor is obtained by solving the integral equations for the pair correlation functions within the hypernetted chain (HNC) scheme. For this purpose we derive new effective two-particle quantum potentials for the interactions between the charge carriers from the full two-particle Slater sum by accounting for bound states. Comparison to the ab initio molecular dynamics simulations enables us to determine the short-range behavior of the effective electron-ion quantum potentials. Results for the static structure factor are presented for beryllium plasmas at solid density and at threefold compression. © 2009 Elsevier B.V.

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 W/ cm . 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 and Ly 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.

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.