Phase transition lowering in dynamically compressed silicon

Nature Physics Springer Nature 15 (2018) 89–94-

EE McBride, A Krygier, A Ehnes, E Galtier, M Harmand, Z Konôpková, HJ Lee, HP Liermann, B Nagler, A Pelka, M Rödel, A Schropp, RF Smith, C Spindloe, D Swift, F Tavella, S Toleikis, T Tschentscher, J Wark, A Higginbotham

Silicon, being one of the most abundant elements in nature, attracts wide-ranging scientific and technological interest. Specifically, in its elemental form, crystals of remarkable purity can be produced. One may assume that this would lead to silicon being well understood, and indeed, this is the case for many ambient properties, as well as for higher-pressure behaviour under quasi-static loading. However, despite many decades of study, a detailed understanding of the response of silicon to rapid compression—such as that experienced under shock impact—remains elusive. Here, we combine a novel free-electron laser-based X-ray diffraction geometry with laser-driven compression to elucidate the importance of shear generated during shock compression on the occurrence of phase transitions. We observe lowering of the hydrostatic phase boundary in elemental silicon, an ideal model system for investigating high-strength materials, analogous to planetary constituents. Moreover, we unambiguously determine the onset of melting above 14 GPa, previously ascribed to a solid–solid phase transition, undetectable in the now conventional shocked diffraction geometry; transitions to the liquid state are expected to be ubiquitous in all systems at sufficiently high pressures and temperatures.

The use of geometric effects in diagnosing ion density in ICF-related dot spectroscopy experiments

High Energy Density Physics Elsevier 30 (2019) 45-51

J Wark, G Perez-Callejo, S Rose, M Schneider, D Liedahl

We describe a method to calculate the ion density of High Energy Density (HED) cylindrical plasmas used in Dot Spectroscopy experiments. This method requires only spectroscopic measurements of the Heα region obtained from two views (Face-on and Side-on). We make use of the fact that the geometry of the plasma affects the observed flux of optically thick lines. The ion density can be derived from the aspect ratio (height-to-radius) of the cylinder and the optical depth of the Heα-y line (1s2p 3P1 → 1s 2 1S0). The aspect ratio and the optical depth of the y line are obtained from the spectra using ratios measured from the two directions of emission of the optically thick Heα-w line (1s2p 1P1 → 1s 2 1S0) and the ratio of the optically thick to thin lines. The method can be applied to mid-Z elements at ion densities of 1019 − 1020 cm−3 and temperatures of a the order of keV, which is a relevant regime for Inertial Confinement Fusion (ICF) experiments.

A proposal to measure iron opacity at conditions close to the solar convective zone-radiative zone boundary

High Energy Density Physics Elsevier BV (2019)

DJ Hoarty, J Morton, M Jeffery, LK Pattison, A Wardlow, SPD Mangles, SJ Rose, C Iglesias, K Opachich, RF Heeter, TS Perry

Ultrafast laser-matter interaction with nanostructured targets


RS Marjoribanks, L Lecherbourg, J Sipe, G Kulcsar, A Heron, J-C Adam, A Miscampbell, G Thomas, R Royle, O Humphries, R Ko, S Le Moal, A Tan, J Li, T Preston, Q van den Berg, M Kasim, B Nagler, E Galtier, E Cunningham, J Wark, S Vinko

Setup for meV-resolution inelastic X-ray scattering measurements and X-ray diffraction at the Matter in Extreme Conditions endstation at the Linac Coherent Light Source

Review of Scientific Instruments AIP Publishing 89 (2018) 10F104

EE McBride, TG White, A Descamps, LB Fletcher, K Appel, F Condamine, CB Curry, S Funk, E Galtier, M Gauthier, S Goede, JB Kim, HJ Lee, BK Ofori-Okai, M Oliver, A Rigby, C Schoenwaelder, P Sun, T Tschentscher, B Witte, U Zastrau, G Gregori, B Nagler, J Hastings, G Monaco

We describe a setup for performing inelastic X-ray scattering and X-ray diffraction measurements at the Matter in Extreme Conditions (MEC) endstation of the Linac Coherent Light Source. This technique is capable of performing high-, meV-resolution measurements of dynamic ion features in both crystalline and non-crystalline materials. A four-bounce silicon (533) monochromator was used in conjunction with three silicon (533) diced crystal analyzers to provide an energy resolution of ∼50 meV over a range of ∼500 meV in single shot measurements. In addition to the instrument resolution function, we demonstrate the measurement of longitudinal acoustic phonon modes in polycrystalline diamond. Furthermore, this setup may be combined with the high intensity laser drivers available at MEC to create warm dense matter and subsequently measure ion acoustic modes.

Femtosecond x-ray diffraction studies of the reversal of the microstructural effects of plastic deformation during shock release of tantalum

Physical Review Letters American Physical Society 120 (2018) 265502

M Sliwa, D McGonegle, C Wehrenberg, CA Bolme, PG Heighway, A Higginbotham, A Lazicki, HJ Lee, B Nagler, HS Park, RE Rudd, MJ Suggit, D Swift, F Tavella, L Zepeda-Ruiz, BA Remington, J Wark

We have used femtosecond x-ray diffraction to study laser-shocked fiber-textured polycrystalline tantalum targets as the 37–253 GPa shock waves break out from the free surface. We extract the time and depth-dependent strain profiles within the Ta target as the rarefaction wave travels back into the bulk of the sample. In agreement with molecular dynamics simulations, the lattice rotation and the twins that are formed under shock compression are observed to be almost fully eliminated by the rarefaction process.

Comments on A new theory for X-ray diffraction

Acta Crystallographica Section A: Foundations and Advances International Union of Crystallography 74 (2018) A74

J Fraser, J Wark

In an article entitled A new theory for X-ray diffraction [Fewster (2014). Acta Cryst. A70, 257–282], hereafter referred to as NTXRD, it is claimed that when X-rays are scattered from a small crystallite, whatever its size and shape, the diffraction pattern will contain enhanced scattering at angles of exactly 2B, whatever the orientation of the crystal. It is claimed that in this way scattering from a powder, with randomly oriented crystals, gives rise to Bragg scattering even if the Bragg condition is never satisfied by an individual crystallite. The claims of the theory put forward in NTXRD are examined and they are found to be in error. Whilst for a certain restricted set of shapes of crystals it is possible to obtain some diffraction close to (but not exactly at) the Bragg angle as the crystallite is oriented away from the Bragg condition, this is generally not the case. Furthermore, contrary to the claims made within NTXRD, the recognition of the origin of the type of effects described is not new, and has been known since the earliest days of X-ray diffraction.

Observation of Laser Power Amplification in a Self-Injecting Laser Wakefield Accelerator

Physical Review Letters 120 (2018)

MJV Streeter, S Kneip, MS Bloom, RA Bendoyro, O Chekhlov, AE Dangor, A Döpp, CJ Hooker, J Holloway, J Jiang, NC Lopes, H Nakamura, PA Norreys, CAJ Palmer, PP Rajeev, J Schreiber, DR Symes, M Wing, SPD Mangles, Z Najmudin

© 2018 American Physical Society. We report on the depletion and power amplification of the driving laser pulse in a strongly driven laser wakefield accelerator. Simultaneous measurement of the transmitted pulse energy and temporal shape indicate an increase in peak power from 187±11 TW to a maximum of 318±12 TW after 13 mm of propagation in a plasma density of 0.9×1018 cm-3. The power amplification is correlated with the injection and acceleration of electrons in the nonlinear wakefield. This process is modeled by including a localized redshift and subsequent group delay dispersion at the laser pulse front.

Turbulent hydrodynamics experiments in high energy density plasmas: scientific case and preliminary results of the TurboHEDP project


A Casner, G Rigon, B Albertazzi, T Michel, T Pikuz, A Faenov, P Mabey, N Ozaki, Y Sakawa, T Sano, J Ballet, P Tzeferacos, D Lamb, E Falize, G Gregori, M Koenig

Single-shot frequency-resolved optical gating for retrieving the pulse shape of high energy picosecond pulses

Review of Scientific Instruments AIP Publishing 89 (2018) 103509

R Aboushelbaya, A Savin, L Ceurvorst, J Sadler, PA Norreys, AS Davies, DH Froula, A Boyle, M Galimberti, P Oliveira, B Parry, Y Katzir, K Glize

Accurate characterization of laser pulses used in experiments is a crucial step to the analysis of their results. In this paper, a novel single-shot frequency-resolved optical gating (FROG) device is described, one that incorporates a dispersive element which allows it to fully characterize pulses up to 25 ps in duration with a 65 fs per pixel temporal resolution. A newly developed phase retrieval routine based on memetic algorithms is implemented and shown to circumvent the stagnation problem that often occurs with traditional FROG analysis programs when they encounter a local minimum.

Erratum: "Setup for meV-resolution inelastic X-ray scattering measurements and X-ray diffraction at the Matter in Extreme Conditions endstation at the Linac Coherent Light Source" [Rev. Sci. Instrum. 89, 10F104 (2018)].

The Review of scientific instruments 89 (2018) 129901-129901

EE McBride, TG White, A Descamps, LB Fletcher, K Appel, F Condamine, CB Curry, F Dallari, S Funk, E Galtier, EJ Gamboa, M Gauthier, S Goede, JB Kim, HJ Lee, BK Ofori-Okai, M Oliver, A Rigby, C Schoenwaelder, P Sun, T Tschentscher, BBL Witte, U Zastrau, G Gregori, B Nagler, J Hastings, SH Glenzer, G Monaco

Analytical estimates of proton acceleration in laser-produced turbulent plasmas

Journal of Plasma Physics Cambridge University Press 84 (2018) 905840608

K Beyer, B Reville, A Bott, H-S Park, S Sarkar, G Gregori

With the advent of high power lasers, new opportunities have opened up for simulating astrophysical processes in the laboratory. We show that second-order Fermi acceleration can be directly investigated at the National Ignition Facility, Livermore. This requires measuring the momentum-space diffusion of 3 MeV protons produced within a turbulent plasma generated by a laser. Treating Fermi acceleration as a biased diffusion process, we show analytically that a measurable broadening of the initial proton distribution is then expected for particles exiting the plasma.

Femtosecond diffraction studies of solid and liquid phase changes in shock-compressed bismuth

Scientific Reports Springer Nature Publishing Group 8 (2018) 16927-

Gorman, AL Coleman, R Briggs, RS McWilliams, D McGonegle, CA Bolme, AE Gleason, E Galtier, HJ Lee, E Granados, M Śliwa, C Sanloup, S Rothman, DE Fratanduono, RF Smith, GW Collins, JH Eggert, J Wark, MI McMahon

Bismuth has long been a prototypical system for investigating phase transformations and melting at high pressure. Despite decades of experimental study, however, the lattice-level response of Bi to rapid (shock) compression and the relationship between structures occurring dynamically and those observed during slow (static) compression, are still not clearly understood. We have determined the structural response of shock-compressed Bi to 68 GPa using femtosecond X-ray diffraction, thereby revealing the phase transition sequence and equation-of-state in unprecedented detail for the first time. We show that shocked-Bi exhibits a marked departure from equilibrium behavior - the incommensurate Bi-III phase is not observed, but rather a new metastable phase, and the Bi-V phase is formed at significantly lower pressures compared to static compression studies. We also directly measure structural changes in a shocked liquid for the first time. These observations reveal new behaviour in the solid and liquid phases of a shocked material and give important insights into the validity of comparing static and dynamic datasets.

Ultrafast imaging of laser driven shock waves using betatron x-rays from a laser wakefield accelerator

Scientific Reports Nature 8 (2018) 11010

JC Wood, DJ Chapman, K Poder, NC Lopes, M Rutherford, TG White, F Albert, KT Behm, N Booth, JSJ Bryant, PS Foster, S Glenzer, E Hill, K Krushelnick, Z Najmudin, BB Pollock, S Rose, W Schumaker, RHH Scott, M Sherlock, AGR Thomas, Z Zhao, D Eakins, SPD Mangles

Betatron radiation from laser wakefield accelerators is an ultrashort pulsed source of hard, synchrotron-like x-ray radiation. It emanates from a centimetre scale plasma accelerator producing GeV level electron beams. In recent years betatron radiation has been developed as a unique source capable of producing high resolution x-ray images in compact geometries. However, until now, the short pulse nature of this radiation has not been exploited. This report details the first experiment to utilize betatron radiation to image a rapidly evolving phenomenon by using it to radiograph a laser driven shock wave in a silicon target. The spatial resolution of the image is comparable to what has been achieved in similar experiments at conventional synchrotron light sources. The intrinsic temporal resolution of betatron radiation is below 100 fs, indicating that significantly faster processes could be probed in future without compromising spatial resolution. Quantitative measurements of the shock velocity and material density were made from the radiographs recorded during shock compression and were consistent with the established shock response of silicon, as determined with traditional velocimetry approaches. This suggests that future compact betatron imaging beamlines could be useful in the imaging and diagnosis of high-energy-density physics experiments.

Developing an Experimental Basis for Understanding Transport in NIF Hohlraum Plasmas.

Physical review letters 121 (2018) 095002-095002

MA Barrios, JD Moody, LJ Suter, M Sherlock, H Chen, W Farmer, J Jaquez, O Jones, RL Kauffman, JD Kilkenny, J Kroll, OL Landen, DA Liedahl, SA Maclaren, NB Meezan, A Nikroo, MB Schneider, DB Thorn, K Widmann, G Pérez-Callejo

We report on the first multilocation electron temperature (T_{e}) and flow measurements in an ignition hohlraum at the National Ignition Facility using the novel technique of mid-Z spectroscopic tracer "dots." The measurements define a low resolution "map" of hohlraum plasma conditions and provide a basis for the first multilocation tests of particle and energy transport physics in a laser-driven x-ray cavity. The data set is consistent with classical heat flow near the capsule but reduced heat flow near the laser entrance hole. We evaluate the role of kinetic effects, self-generated magnetic fields, and instabilities in causing spatially dependent heat transport in the hohlraum.

Experimental platform for the investigation of magnetized-reverse-shock dynamics in the context of POLAR

High Power Laser Science and Engineering Cambridge University Press 6 (2018) e43

G Gregori, B Albertazzi, E Falize, E Falize, A Pelka, F Brack, F Kroll, R Yurchak, E Brambrink, P Mabey, N Ozaki, S Pikuz, L Van Box Som, JM Bonnet-Bidaud, JE Cross, E Filippov, R Kodama, M Mouchet, T Morita, Y Sakawa, RP Drake, CC Kuranz, C Li, P Tzeferacos, D Lamb

The influence of a strong external magnetic field on the collimation of a high Mach number plasma flow and its collision with a solid obstacle is investigated experimentally and numerically. The laser irradiation (I ∼ 2 × 1014 W · cm−2 ) of a multilayer target generates a shock wave that produces a rear side plasma expanding flow. Immersed in a homogeneous 10 T external magnetic field, this plasma flow propagates in vacuum and impacts an obstacle located a few mm from the main target. A reverse shock is then formed with typical velocities of the order of 15–20 ± 5 km/s. The experimental results are compared with 2D radiative magnetohydrodynamic simulations using the FLASH code. This platform allows investigating the dynamics of reverse shock, mimicking the processes occurring in a cataclysmic variable of polar type.

Production of photoionized plasmas in the laboratory with x-ray line radiation

Physical Review E American Physical Society 97 (2018) 063203

S White, R Irwin, R Warwick, G Gribakin, G Sarri, FP Keenan, D Riley, S Rose, EG Hill, GJ Ferland, B Han, F Wang, G Zhao

In this paper we report the experimental implementation of a theoretically proposed technique for creating a photoionized plasma in the laboratory using x-ray line radiation. Using a Sn laser plasma to irradiate an Ar gas target, the photoionization parameter, ξ = 4πF/Ne, reached values of order 50 erg cm s−1, where F is the radiation flux in erg cm−2 s−1. The significance of this is that this technique allows us to mimic effective spectral radiation temperatures in excess of 1 keV. We show that our plasma starts to be collisionally dominated before the peak of the x-ray drive. However, the technique is extendable to higher-energy laser systems to create plasmas with parameters relevant to benchmarking codes used to model astrophysical objects.

Soft X-ray backlighter source driven by a short-pulse laser for pump-probe characterization of warm dense matter

Review of Scientific Instruments AIP Publishing 89 (2018) 10F122

C McGuffey, M Dozieres, J Kim, A Savin, J Park, J Emig, C Brabetz, L Carlson, RF Heeter, HS McLean, J Moody, MB Schneider, Wei, FN Beg

Here we propose a pump-probe X-ray absorption spectroscopy temperature measurement technique appropriate for matter having temperature in the range of 10 to a few 100 eV and density up to solid density. Atomic modeling simulations indicate that for various low- to mid-Z materials in this range the energy and optical depth of bound-bound and bound-free absorption features are sensitive to temperature. We discuss sample thickness and tamp layer considerations. A series of experimental investigations was carried out using a range of laser parameters with pulse duration ≤5 ps and various pure and alloyed materials to identify backlighter sources suitable for the technique.

Implementation of a Faraday rotation diagnostic at the OMEGA laser facility

High Power Laser Science and Engineering Cambridge University Press 6 (2018) e49-

A Rigby, A Bott, T White, P Tzeferacos, DQ Lamb, DH Froula, G Gregori

Magnetic field measurements in turbulent plasmas are often difficult to perform. Here we show that for ⩾ kG magnetic fields, a time-resolved Faraday rotation measurement can be made at the OMEGA laser facility. This diagnostic has been implemented using the Thomson scattering probe beam and the resultant path-integrated magnetic field has been compared with that of proton radiography. Accurate measurement of magnetic fields is essential for satisfying the scientific goals of many current laser–plasma experiments.

Axion-driven cosmic magnetogenesis prior to the QCD crossover

Physical Review Letters American Physical Society 121 (2018) 021301

F Miniati, G Gregori, B Reville, S Sarkar

We propose a mechanism for the generation of a magnetic field in the early Universe during the QCD crossover assuming that dark matter is made of axions. Thermoelectric fields arise at pressure gradients in the primordial plasma due to the difference in charge, energy density, and equation of state between the quark and lepton components. The axion field is coupled to the EM field, so when its spatial gradient is misaligned with the thermoelectric field, an electric current is driven. Because of the finite resistivity of the plasma, an electric field appears that is generally rotational. For a QCD axion mass consistent with observational constraints and a conventional efficiency for turbulent dynamo amplification—driven by the same pressure gradients responsible for the thermoelectric fields—a magnetic field is generated on subhorizon scales. After significant Alfvénic unwinding, it reaches a present-day strength of B ∼ 10 − 13     G on a characteristic scale L B ∼ 20     pc . The resulting combination of B L 1 / 2 B is significantly stronger than in any astrophysical scenario, providing a clear test for the cosmological origin of the field through γ -ray observations of distant blazars. The amplitude of the pressure gradients may be inferred from the detection of concomitant gravitational waves, while several experiments are underway to confirm or rule out the existence of axions.