High Energy Density Physics 7 (2011) 225-229
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.
Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment (2011)
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-
Time-resolved plasma temperature measurements in a laser-triggered hydrogen-filled capillary discharge waveguide
Plasma Sources Science and Technology 20 (2011)
Temporally resolved, spatially integrated measurements of the temperature of the plasma channel formed by a hydrogen-filled discharge capillary waveguide are presented. Plasma temperatures of 4-7 eV are measured for peak discharge currents between 80 and 150 A. It is demonstrated that laser-triggering the capillary discharge enables capillary discharges with a peak current as low as 23 A to be driven, reducing the plasma temperature to approximately 3 eV. This plasma temperature meets the requirements of a recently proposed soft x-ray recombination laser. © 2011 IOP Publishing Ltd.
EPL 94 (2011)
We develop a new theoretical approach that demonstrates the abilities of elastic X-ray scattering to yield thermodynamic, structural, and mixing properties of dense matter with multiple ion species. The novel decomposition of the electron structure factor in multi-component systems provides the basis to study dense mixtures as found in giant gas planets or during inertial confinement fusion. We show that the scattering signal differs significantly between single species, microscopic mixtures, and phase-separated fluids. Thus, these different phases can be distinguished experimentally via elastic X-ray scattering. © 2011 Europhysics Letters Association.
In-situ determination of dispersion and resolving power in simultaneous multiple-angle XUV spectroscopy
JOURNAL OF INSTRUMENTATION 6 (2011) ARTN P10001
Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 653 (2011) 137-139
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.
Journal of Applied Physics 109 (2011)
We report on nonequilibrium molecular dynamics simulations of single crystals of copper experiencing rapid shear strain. A model system, with periodic boundary conditions, which includes a single dislocation dipole is subjected to a total shear strain of close to 10 on time-scales ranging from the instantaneous to 50 ps. When the system is strained on a time-scale short compared with a phonon period, the initial total applied shear is purely elastic, and the eventual temperature rise in the system due to the subsequent plastic work can be determined from the initial elastic strain energy. The rate at which this plastic work occurs, and heat is generated, depends on the dislocation velocity, which itself is a function of shear stress. A determination of the stress-dependence of the dislocation velocity allows us to construct a simple analytic model for the temperature rise in the system as a function of strain rate, and this model is found to be in good agreement with the simulations. For the effective dislocation density within the simulations, 7.8 10 11 cm - 2, we find that applying the total shear strain on time-scales of a few tens of picoseconds greatly reduces the final temperature. We discuss these results in the context of the growing interest in producing high pressure, solid-state matter, by quasi-isentropic (rather than shock) compression. © 2011 American Institute of Physics.
38th EPS Conference on Plasma Physics 2011, EPS 2011 - Europhysics Conference Abstracts 35 2 (2011) 1484-1487
A detailed knowledge of the physical phenomena underlying 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. The fast electron currents largely exceed the Alfven limit, therefore a balancing return current is required to support the propagation of the fast electron beam in the target medium. An experimental study aimed at investigating the role of the return current on the dynamics of the fast electron beam was carried out with the Vulcan Petawatt beam. Two counter-propagating electron beams were generated by double-sided irradiation of a layered target containing a 5 micron thick Ti layer. Information on the energy coupling of the fast electron beam to the Ti layer was retrieved through X-ray measurements. In particular, high-resolution X-ray spectroscopy of the Ti emission lines was performed in the spectral range from 4.4 to 5.1 keV including the Lyα, the Heα and the Kα line. Spectra were acquired for double-sided irradiation with different timings between the two laser beams as well as for single-sided irradiation. The recorded spectra indicate a higher target temperature for a precise timing between the two beams in agreement with simulation results.
High Energy Density Physics 7 (2011) 105-109
Polarisation sensitive emission spectroscopy measurements are reported for a petawatt laser-solid target interaction at intensities up to 5 × 1020 W cm-2. These measurements were single-shot and used pairs of highly-orientated graphite spectrometers to resolve the sulphur Ly-α doublet. The sulphur Ly-α1 component shows a large positive polarisation indicative of a low energy electron beam in the plasma, the Ly-α2 component acts as a cross-spectrometer calibration. The measurements show a significant anisotropic or beam-like component to a cold return current. © 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
PHYSICAL REVIEW SPECIAL TOPICS-ACCELERATORS AND BEAMS 14 (2011) ARTN 164801
Proceedings of SPIE - The International Society for Optical Engineering 8080 (2011)
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.
High Energy Density Physics 7 (2011) 111-116
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.
Optics InfoBase Conference Papers (2011)
We investigate quasi-phase-matching of high harmonic generation over a range of harmonic orders using trains of up to 8 uniformly-spaced counter-propagating pulses, produced using an array of birefringent crystals. © 2012 OSA.
High Energy Density Physics 7 (2011) 40-42
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.
Canadian Journal of Physics 89 (2011) 647-651
The production of X-rays from electron transitions into K-shell vacancies (Kα,β) emission) is a well-known process in atomic physics and has been extensively studied as a plasma diagnostic in low-and mid-Z materials. However, X-ray spectra from near neutral high-Z ions are very complex, and their interpretation requires the use of state-of-the-art atomic calculations. In this experiment, the Titan laser system at Lawrence Livermore National Laboratory was used to deliver an approximately 350 J laser pulse, with a 10 ps duration and a wavelength of 1054 nm, to a gold (Au) target. A transparent bent quartz crystal spectrometer with a hard X-ray energy window, ranging from 17 to 102 keV, was used to measure the emission spectrum. Kα1,α2 and Kβ1,γ1 transitions were observed over a range of target sizes. Additionally, a series of shots were conducted with a pre-ionizing long pulse (3 ns, 1-10 J, 527 nm) on the backside of the target. FLYCHK, an atomic non-LTE code, designed to provide ionization and population distributions, was used to model the experiment. K α/Kβ ratios were found to be in good agreement with the predicted value for room temperature Au targets. © 2011 Published by NRC Research Press.
In-situ determination of dispersion and resolving power in simultaneous multiple-angle XUV spectroscopy
Journal of Instrumentation 6 (2011)
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.
PHYSICAL REVIEW B 81 (2010) ARTN 092102
High Energy Density Physics 6 (2010) 305-310
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.
New Journal of Physics 12 (2010)
The role of plasma channels as waveguides for laser-wakefield accelerators is discussed in terms of the results of experiments performed with the Astra-Gemini laser, numerical simulations using the code WAKE, and the theory of self-focusing and self-guiding of intense laser beams. It is found that at a given electron density, electron beams can be accelerated using lower laser powers in a waveguide structure than in a gas-jet or cell. The transition between relativistically self-guided and channel-assisted guiding is seen in the simulations and in the behaviour of the production of electron beams. We also show that by improving the quality of the driving laser beam the threshold laser energy required to produce electron beams can be reduced by a factor of almost 2. The use of an aperture allows the production of a quasi-monoenergetic electron beam of energy 520 MeV with an input laser power of only 30 TW. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.