Publications


Simulations of neon irradiated by intense X-ray laser radiation

High Energy Density Physics 7 (2011) 111-116

O Ciricosta, H Chung, R Lee, JS Wark

We present simulations of the charge states produced by the interaction of intense X-ray laser radiation with a neon gas. We model the results of a recent experiment (Young et al., Nature 466, 56 (2010)), where mJ pulses of X-rays, with photon energies ranging from 800 to 2000 eV and pulse lengths ranging from 70 to 340 fs were incident on neon atoms at intensities of up to 10 18 W cm -2. Simulations using an adapted version of the SCFLY collisional-radiative code, which included the effect of electron collisions and a simple self-consistent temperature model, result in charge state distributions that are in good agreement with the experimental data. We calculate the electron temperature of the system during the evolution of the plasma, and comment upon the role that collisions may play in determining the charge state distributions as a function of the neon ion number density. © 2011 Elsevier B.V.


Decay of cystalline order and equilibration during the solid-to-plasma transition induced by 20-fs microfocused 92-eV free-electron-laser pulses.

Phys Rev Lett 106 (2011) 164801-

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

We have studied a solid-to-plasma transition by irradiating Al foils with the FLASH free electron laser at intensities up to 10(16)  W/cm(2). Intense XUV self-emission shows spectral features that are consistent with emission from regions of high density, which go beyond single inner-shell photoionization of solids. Characteristic features of intrashell transitions allowed us to identify Auger heating of the electrons in the conduction band occurring immediately after the absorption of the XUV laser energy as the dominant mechanism. A simple model of a multicharge state inverse Auger effect is proposed to explain the target emission when the conduction band at solid density becomes more atomiclike as energy is transferred from the electrons to the ions. This allows one to determine, independent of plasma simulations, the electron temperature and density just after the decay of crystalline order and to characterize the early time evolution.


Measuring fast electron distribution functions at intensities up to 1021 W cm-2

Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment (2011)

N Booth, RJ Clarke, D Doria, LA Gizzi, G Gregori, P Hakel, P Koester, L Labate, T Levato, B Li, M Makita, RC Mancini, J Pasley, PP Rajeev, D Riley, APL Robinson, E Wagenaars, JN Waugh, NC Woolsey


Density fluctuations in the Yukawa one-component plasma: An accurate model for the dynamical structure factor

Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 84 (2011)

JP Mithen, J Daligault, BJB Crowley, G Gregori

Using numerical simulations, we investigate the equilibrium dynamics of a single-component fluid with Yukawa interaction potential. We show that, for a wide range of densities and temperatures, the dynamics of the system are in striking agreement with a simple model of generalized hydrodynamics. Since the Yukawa potential can describe the ion-ion interactions in a plasma, our results have significant applicability for both analyzing and interpreting the results of x-ray scattering data from high-power lasers and fourth-generation light sources. © 2011 American Physical Society.


Simulation of free-electron lasers seeded with broadband radiation

Physical Review Special Topics - Accelerators and Beams 14 (2011)

SI Bajlekov, WM Fawley, CB Schroeder, R Bartolini, SM Hooker

The longitudinal coherence of free-electron laser (FEL) radiation can be enhanced by seeding the FEL with high harmonics of an optical laser pulse. The radiation produced by high-harmonic generation (HHG), however, has a fast-varying temporal profile that can violate the slowly varying envelope approximation and limited frequency window that is employed in conventional free-electron laser simulation codes. Here we investigate the implications of violating this approximation on the accuracy of simulations. On the basis of both analytical considerations and 1D numerical studies, it is concluded that, for most realistic scenarios, conventional FEL codes are capable of accurately simulating the FEL process even when the seed radiation violates the slowly varying envelope approximation. We additionally discuss the significance of filtering the harmonic content of broadband HHG seeds. © 2011 American Physical Society.


Reply to "Comment on 'Free-free opacity in warm-dense aluminum'"

High Energy Density Physics 7 (2011) 40-42

SM Vinko, G Gregori, JS Wark

We reply to the comment by Iglesias [HEDP, XXX] regarding our implementation of a solid-state pseudopotential in a model for the calculation of the free--free opacity in warm-dense aluminum [HEDP 5(2009), 124-131]. Some further details are given describing the method used to determine the adjustable parameter in the pseudopotential and several important limitations are discussed. © 2010 Elsevier B.V.


Towards laboratory produced relativistic electron-positron pair plasmas

High Energy Density Physics 7 (2011) 225-229

H Chen, DD Meyerhofer, SC Wilks, RC Cauble, FJ Dollar, K Falk, G Gregori, AU Hazi, EI Moses, CD Murphy, JF Myatt, J Park, JF Seely, RL Shepherd, A Spitkovsky, C Stoeckl, CI Szabó, R Tommasini, C Zulick, P Beiersdörfer

We review recent experimental results on the path to producing electron-positron pair plasmas using lasers. Relativistic pair-plasmas and jets are believed to exist in many astrophysical objects and are often invoked to explain energetic phenomena related to Gamma Ray Bursts and Black Holes. On earth, positrons from radioactive isotopes or accelerators are used extensively at low energies (sub-MeV) in areas related to surface science positron emission tomography and basic antimatter science. Experimental platforms capable of producing the high-temperature pair-plasma and high-flux jets required to simulate astrophysical positron conditions have so far been absent. In the past few years, we performed extensive experiments generating positrons with intense lasers where we found that relativistic electron and positron jets are produced by irradiating a solid gold target with an intense picosecond laser pulse. The positron temperatures in directions parallel and transverse to the beam both exceeded 0.5 MeV, and the density of electrons and positrons in these jets are of order 1016 cm-3 and 1013 cm-3, respectively. With the increasing performance of high-energy ultra-short laser pulses, we expect that a high-density, up to 1018 cm-3, relativistic pair-plasma is achievable, a novel regime of laboratory-produced hot dense matter. © 2011 Elsevier B.V.


High Harmonic Optical Generator (Polarization Beating 1/2)

(2011) UK Patent Application GB1117355.6

LZ Liu, K O'Keeffe, SM Hooker


Measuring fast electron distribution functions at intensities up to 10 21 W cm-2

Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 653 (2011) 137-139

NA Booth, RJ Clarke, D Doria, LA Gizzi, G Gregori, P Hakel, P Koester, LL Labate, T Levato, B Li, M Makita, RC Mancini, JR Pasley, PP Rajeev, DR Riley, APL Robinson, E Wagenaars, JN Waugh, NC Woolsey

Here we present results from ultra-intense experiments demonstrating the viability of polarization spectroscopy as a diagnostic of the electron return current and spatial anisotropy and distribution function of the fast electron beam. The measurements extend to ultra-relativistic intensities of 10 21 W cm-2, including laserplasma interaction regimes important for fast ignition studies, for example HiPER, and the development of secondary sources from next generation ultra-short pulse, ultra-intense laser facilities such as Astra-Gemini and ELI. As an in situ diagnostic, spectroscopic measurements are vital to understanding fast electron beams, enabling extrapolation of results to define fast ignition inertial confinement fusion and secondary source facilities. © 2011 Elsevier B.V.


High resolution X-ray spectroscopy in fast electron transport studies

Proceedings of SPIE - The International Society for Optical Engineering 8080 (2011)

P Koester, NA Booth, CA Cecchetti, H Chen, RG Evans, G Gregori, LL Labate, T Levato, B Li, M Makita, JP Mithen, CD Murphy, MM Notley, RP Pattathil, DR Riley, NC Woolsey, LA Gizzia

A detailed knowledge of the physical phenomena underlying the generation and the transport of fast electrons generated in high-intensity laser-matter interactions is of fundamental importance for the fast ignition scheme for inertial confinement fusion. Here we report on an experiment carried out with the VULCAN Petawatt beam and aimed at investigating the role of collisional return currents in the dynamics of the fast electron beam. To that scope, in the experiment counter-propagating electron beams were generated by double-sided irradiation of layered target foils containing a Ti layer. The experimental results were obtained for different time delays between the two laser beams as well as for single-sided irradiation of the target foils. The main diagnostics consisted of two bent mica crystal spectrometers placed at either side of the target foil. High-resolution X-ray spectra of the Ti emission lines in the range from the Lyα to the Kα line were recorded. In addition, 2D X-ray images with spectral resolution were obtained by means of a novel diagnostic technique, the energy-encoded pin-hole camera, based on the use of a pin-hole array equipped with a CCD detector working in single-photon regime. The spectroscopic measurements suggest a higher target temperature for well-aligned laser beams and a precise timing between the two beams. The experimental results are presented and compared to simulation results. © 2011 SPIE.


In-situ determination of dispersion and resolving power in simultaneous multiple-angle XUV spectroscopy

Journal of Instrumentation 6 (2011)

U Zastrau, V Hilbert, CRD Brown, TD Döppner, SL Dziarzhytski, E Förster, H Glenzer, S Göde, G Gregori, M Harmand, DC Hochhaus, T Laarmann, J Lee, KH Meiwes-Broer, PB Neumayer, A Przystawik, P Radcliffe, MF Schulz, S Skruszewicz, F Tavella, J Tiggesbäumker, S Toleikis, TG White

We report on the simultaneous determination of non-linear dispersion functions and resolving power of three flat-field XUV grating spectrometers. A moderate-intense short-pulse infrared laser is focused onto technical aluminum which is commonly present as part of the experimental setup. In the XUV wavelength range of 10-19 nm, the spectrometers are calibrated using Al-Mg plasma emission lines. This cross-calibration is performed in-situ in the very same setup as the actual main experiment. The results are in excellent agreement with ray-tracing simulations. We show that our method allows for precise relative and absolute calibration of three different XUV spectrometers. © 2011 IOP Publishing Ltd and SISSA.


In situ x-ray diffraction measurements of the c/a ratio in the high-pressure epsilon phase of shock-compressed polycrystalline iron

PRB American Physical Society 83 (2011) 144114-

JA Hawreliak, B El-Dasher, H Lorenzana, G Kimminau, A Higginbotham, B Nagler, SM Vinko, WJ Murphy, T Whitcher, JS Wark, S Rothman, N Park


Extent of validity of the hydrodynamic description of ions in dense plasmas.

Phys Rev E Stat Nonlin Soft Matter Phys 83 (2011) 015401-

JP Mithen, J Daligault, G Gregori

We show that the hydrodynamic description can be applied to modeling the ionic response in dense plasmas for a wide range of length scales that are experimentally accessible. Using numerical simulations for the Yukawa model, we find that the maximum wave number k(max) at which the hydrodynamic description applies is independent of the coupling strength, given by k(max)λ(s)≃0.43, where λ(s) is the ionic screening length. Our results show that the hydrodynamic description can be used for interpreting x-ray scattering data from fourth generation light sources and high power lasers. In addition, our investigation sheds new light on how the domain of validity of the hydrodynamic description depends on both the microscopic properties and the thermodynamic state of fluids in general.


Laser-wakefield acceleration of electron beams in a low density plasma channel

Physical Review Special Topics - Accelerators and Beams 13 (2010)

TPA Ibbotson, N Bourgeois, TP Rowlands-Rees, LS Caballero, SI Bajlekov, PA Walker, S Kneip, SPD Mangles, SR Nagel, CAJ Palmer, N Delerue, G Doucas, DC Urner, OV Chekhlov, RJ Clarke, EJ Divall, KG Ertel, PS Foster, SJ Hawkes, CJ Hooker, BT Parry, PP Rajeev, MJV Streeter, SM Hooker

The generation of quasimonoenergetic electron beams, with energies greater than 500 MeV, in a laser-plasma accelerator driven by 2.5 J, 80 fs laser pulses guided in a low density plasma channel, is investigated. The laser energy required to achieve electron injection is found to depend strongly on the quality of the input laser focal spot. Simulations show that, although the matched spot size of the plasma channel is greater than the self-focused spot size, the channel assists relativistic self-focusing and enables electron injection to occur at lower plasma densities and laser powers than would be possible without a waveguide. © 2010 The American Physical Society.


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

TH Robinson, KO O'Keeffe, M Zepf, B Dromey, SM Hooker

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.


Ultrafast melting of carbon induced by intense proton beams

Physical Review Letters 105 (2010)

A Pelka, G Gregori, DO Gericke, J Vorberger, SH Glenzer, MM Günther, K Harres, RI Heathcote, AL Kritcher, NL Kugland, B Li, M Makita, JP Mithen, DJ Neely, C Niemann, A Otten, DR Riley, G Schaumann, MS Schollmeier, AN Tauschwitz, M Roth

Laser-produced proton beams have been used to achieve ultrafast volumetric heating of carbon samples at solid density. The isochoric melting of carbon was probed by a scattering of x rays from a secondary laser-produced plasma. From the scattering signal, we have deduced the fraction of the material that was melted by the inhomogeneous heating. The results are compared to different theoretical approaches for the equation of state which suggests modifications from standard models. © 2010 The American Physical Society.


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

Physical Review Letters 105 (2010)

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

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.


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.


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.


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.