Publications


Axion particle production in a laser-induced dynamical spacetime

Physics Letters B Elsevier 777 (2017) 388-393

M Wadud, B King, R Bingham, G Gregori

We consider the dynamics of a charged particle (e.g., an electron) oscillating in a laser field in flat spacetime and describe it in terms of the variable mass metric. By applying Einstein’s equivalence principle, we show that, after representing the electron motion in a time-dependent manner, the variable mass metric takes the form of the Friedmann–Lemaître–Robertson–Walker metric. We quantize a pseudoscalar field in this spacetime and derive the production rate of electrically neutral, spinless particles. We show that this approach can provide an alternative experimental method to axion searches.


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.


Reply to ‘Thomson scattering in inhomogeneous plasmas: The Role of the Fluctuation-Dissipation Theorem’

Scientific Reports Nature Publishing Group 8 (2018) Article number 7947-

PM Kozlowski, G Gregori

In a comment on our article “Theory of Thomson scattering in inhomogeneous media”, V. V. Belyi asserts that there is an inconsistency in our method of applying gradient effects via the dielectric superposition principle, in violation of the fluctuation-dissipation theorem; and that his Klimontovich-Langevin formulation would be more appropriate to our application. While we agree that a generalization, along the lines of Belyi’s work, would be required for strongly coupled systems, for the weakly coupled systems which we considered, these corrections are not necessary and our approach is still appropriate.


Simultaneous 8.2 keV phase-contrast imaging and 24.6 keV X-ray diffraction from shock-compressed matter at the LCLS

Applied Physics Letters AIP Publishing 112 (2018) 221907

F Seiboth, LB Fletcher, D McGonegle, S Anzellini, LE Dresselhaus-Cooper, M Frost, E Galtier, S Goede, M Harmand, HJ Lee, A Levitan, K Miyanishi, B Nagler, I Nam, N Ozaki, M Rodel, A Schropp, C Spindloe, P Sun, J Wark, J Hastings, SH Glenzer, EE McBride

In this work, we demonstrate simultaneous phase-contrast imaging (PCI) and X-ray diffraction from shock compressed matter at the Matter in Extreme Conditions (MEC) endstation, Linac Coherent Light Source (LCLS). We utilize the chromaticity from compound refractive X-ray lenses to focus the 24.6 keV 3rd order undulator harmonic of the LCLS to a spot size of 5 μm on target to perform X-ray diffraction. Simultaneous PCI from the 8.2 keV fundamental X-ray beam is used to visualize and measure the transient properties of the shock wave over a 500 μm field of view. Furthermore, we demonstrate the ability to extend the reciprocal space by 5˚A−1, relative to the fundamental X-ray energy, by utilizing X-ray diffraction from the 3rd harmonic of the LCLS.


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.


Electron acceleration by wave turbulence in a magnetized plasma

Nature Physics Springer Nature 14 (2018) 475-479

A Rigby, F Cruz, B Albertazzi, R Bamford, A Bell, JE Cross, F Fraschetti, P Graham, Y Hara, PM Kozlowski, Y Kuramitsu, DQ Lamb, S Lebedev, F Miniati, T Morita, M Oliver, B Reville, Y Sakawa, S Sarkar, C Spindloe, R Trines, R Bingham, M Koenig, G Gregori

Astrophysical shocks are commonly revealed by the non-thermal emission of energetic electrons accelerated in situ1,2,3. Strong shocks are expected to accelerate particles to very high energies4,5,6; however, they require a source of particles with velocities fast enough to permit multiple shock crossings. While the resulting diffusive shock acceleration4 process can account for observations, the kinetic physics regulating the continuous injection of non-thermal particles is not well understood. Indeed, this injection problem is particularly acute for electrons, which rely on high-frequency plasma fluctuations to raise them above the thermal pool7,8. Here we show, using laboratory laser-produced shock experiments, that, in the presence of a strong magnetic field, significant electron pre-heating is achieved. We demonstrate that the key mechanism in producing these energetic electrons is through the generation of lower-hybrid turbulence via shock-reflected ions. Our experimental results are analogous to many astrophysical systems, including the interaction of a comet with the solar wind9, a setting where electron acceleration via lower-hybrid waves is possible.


Laboratory evidence of dynamo amplification of magnetic fields in a turbulent plasma

Nature Communications Springer Nature 9 (2018) 591

P Tzeferacos, A Rigby, A Bott, A Bell, R Bingham, A Casner, F Cattaneo, EM Churazov, J Emig, F Fiuza, CB Forest, J Foster, C Graziani, J Katz, M Koenig, CK Li, J Meinecke, R Petrasso, HS Park, BA Remington, JS Ross, D Ryu, D Ryutov, TG White, B Reville

Magnetic fields are ubiquitous in the Universe. Diffuse radiosynchrotron emission observations and Faraday rotation measurements have revealed magnetic field strengths ranging from a few nG and tens of µG in extragalactic disks, halos and clusters [1], up to hundreds of TG in magnetars, as inferred from their spin-down [2]. The energy density of these fields is typically comparable to the energy density of the fluid motions of the plasma in which they are embedded, making magnetic fields essential players in the dynamics of the luminous matter. The standard theoretical model for the origin of these strong magnetic fields is through the amplification of tiny seed fields via turbulent dynamo to the level consistent with current observations [3–7]. Here we demonstrate, using laser-produced colliding plasma flows, that turbulence is indeed capable of rapidly amplifying seed fields to near equipartition with the turbulent fluid motions. These results support the notion that turbulent dynamo is a viable mechanism responsible for the observed present-day magnetization.


Measurement of temperature and density using non-collective X-ray Thomson scattering in pulsed power produced warm dense plasmas

Scientific Reports Nature Publishing Group 8 (2018) 8432-

JC Valenzuela, C Krauland, D Mariscal, I Krashennikov, C Niemann, T Ma, P Mabey, G Gregori, P Wiewior, A Covington, FN Beg

We present the first experimental measurement of temperature and density of a warm dense plasma produced by a pulsed power driver at the Nevada Terawatt Facility (NTF). In the early phases of discharge, most of the mass remains in the core, and it has been challenging to diagnose with traditional methods, e.g. optical probing, because of the high density and low temperature. Accurate knowledge of the transport coefficients as well as the thermodynamic state of the plasma is important to precisely test or develop theoretical models. Here, we have used spectrally resolved non-collective X-ray Thomson scattering to characterize the dense core region. We used a graphite load driven by the Zebra current generator (0.6 MA in 200 ns rise time) and the Ti He-α line produced by irradiating a Ti target with the Leopard laser (30 J, 0.8 ns) as an X-ray probing source. Using this configuration, we obtained a signal-to-noise ratio ~2.5 for the scattered signal. By fitting the experimental data with predicted spectra, we measured T=2±1.9 eV, ρ=0.6±0.5 gr/cc, 70 ns into the current pulse. The complexity of the dense core is revealed by the electrons in the dense core that are found to be degenerate and weakly coupled, while the ions remain highly coupled.


ALICE: A non-LTE plasma atomic physics, kinetics and lineshape package

High Energy Density Physics Elsevier 26 (2018) 56-67

EG Hill, G Pérez-Callejo, S Rose

All three parts of an atomic physics, atomic kinetics and lineshape code, ALICE, are described. Examples of the code being used to model the emissivity and opacity of plasmas are discussed and interesting features of the code which build on the existing corpus of models are shown throughout.


Analytical modelling of the expansion of a solid obstacle interacting with a radiative shock

High Power Laser Science and Engineering Cambridge University Press 6 (2018) e30

T Michel, E Falize, B Albertazzi, G Rigon, Y Sakawa, G Gregori, E al.

In this paper, we present a model characterizing the interaction of a radiative shock (RS) with a solid material, as described in a recent paper (Koenig et al., Phys. Plasmas, 24, 082707 (2017)), the new model is then related to recent experiments performed on the GEKKO XII laser facility. The RS generated in a xenon gas cell propagates towards a solid obstacle that is ablated by radiation coming from the shock front and the radiative precursor, mimicking processes occurring in astrophysical phenomena. The model presented here calculates the dynamics of the obstacle expansion, which depends on several parameters, notably the geometry and the temperature of the shock. All parameters required for the model have been obtained from experiments. Good agreement between experimental data and the model is found when spherical geometry is taken into account. As a consequence, this model is a useful and easy tool to infer parameters from experimental data (such as the shock temperature), and also to design future experiments.


A sensitive EUV Schwarzschild microscope for plasma studies with sub-micrometer resolution

Review of Scientific Instruments AIP Publishing 89 (2018) 023703

U Zastrau, C Rödel, M Nakatsutsumi, T Feigl, K Appel, B Chen, T Döppner, T Fennel, T Fiedler, LB Fletcher, E Förster, E Gamboa, Gericke, S Göde, C Grote-Fortmann, V Hilbert, L Kazak, T Laarmann, HJ Lee, P Mabey, F Martinez, KH Meiwes-Broer, H Pauer, M Perske, A Przystaw

We present an extreme ultraviolet (EUV) microscope using a Schwarzschild objective which is optimized for single-shot sub-micrometer imaging of laser-plasma targets. The microscope has been designed and constructed for imaging the scattering from an EUV-heated solid-density hydrogen jet. Imaging of a cryogenic hydrogen target was demonstrated using single pulses of the free-electron laser in Hamburg (FLASH) free-electron laser at a wavelength of 13.5 nm. In a single exposure, we observe a hydrogen jet with ice fragments with a spatial resolution in the sub-micrometer range. In situ EUV imaging is expected to enable novel experimental capabilities for warm dense matter studies of micrometer-sized samples in laser-plasma experiments.


Laser-driven strong magnetostatic fields with applications to charged beam transport and magnetized high energy-density physics

Physics of Plasmas AIP Publishing 25 (2018) 056705

JJ Santos, M Bailly-Grandvaux, M Ehret, AV Arefiev, D Batani, FN Beg, A Calisti, S Ferri, R Florido, P Forestier-Colleoni, S Fujioka, MA Gigosos, L Giuffrida, L Gremillet, JJ Honrubia, S Kojima, P Korneev, KFF Law, J-R Marques, A Morace, C Mosse, O Peyrusse, S Rose, M Roth, F Suzuki-Vidal

Powerful laser-plasma processes are explored to generate discharge currents of a few 100 kA in coil targets, yielding magnetostatic fields (B-fields) in excess of 0.5 kT. The quasi-static currents are provided from hot electron ejection from the laser-irradiated surface. According to our model, which describes the evolution of the discharge current, the major control parameter is the laser irradiance Ilasλ 2 las. The space-time evolution of the B-fields is experimentally characterized by high-frequency bandwidth B-dot probes and by protondeflectometry measurements. The magnetic pulses, of ns-scale, are long enough to magnetize secondary targets through resistive diffusion. We applied it in experiments of laser-generated relativistic electron transport through solid dielectric targets, yielding an unprecedented 5-fold enhancement of the energy-density flux at 60 µm depth, compared to unmagnetized transport conditions. These studies pave the ground for magnetized high-energy density physics investigations, related to laser-generated secondary sources of radiation and/or high-energy particles and their transport, to high-gain fusion energy schemes and to laboratory astrophysics.


Guiding of relativistic electron beams in dense matter by laser-driven magnetostatic fields

Nature Communications Nature Publishing Group 9 (2018) 1-8

M Bailly-Grandvaux, JJ Santos, C Bellei, JE Cross, G Gregori

Intense lasers interacting with dense targets accelerate relativistic electron beams, which transport part of the laser energy into the target depth. However, the overall laser-to-target energy coupling efficiency is impaired by the large divergence of the electron beam, intrinsic to the laser–plasma interaction. Here we demonstrate that an efficient guiding of MeV electrons with about 30 MA current in solid matter is obtained by imposing a laser-driven longitudinal magnetostatic field of 600 T. In the magnetized conditions the transported energy density and the peak background electron temperature at the 60-μm-thick target's rear surface rise by about a factor of five, as unfolded from benchmarked simulations. Such an improvement of energy-density flux through dense matter paves the ground for advances in laser-driven intense sources of energetic particles and radiation, driving matter to extreme temperatures, reaching states relevant for planetary or stellar science as yet inaccessible at the laboratory scale and achieving high-gain laser-driven thermonuclear fusion.


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.


Channel optimization of high-intensity laser beams in millimeter-scale plasmas

Physical Review E American Physical Society 97 (2018) 043208

L Ceurvorst, A Savin, N Ratan, J Sadler, P Norreys, H Habara, KA Tanaka, S Zhang, Wei, S Ivancic, D Froula, W Theobald

Channeling experiments were performed at the OMEGA EP facility using relativistic intensity ( > 10 18 W / cm 2 ) kilojoule laser pulses through large density scale length ( ∼ 390 – 570 μ m ) laser-produced plasmas, demonstrating the effects of the pulse's focal location and intensity as well as the plasma's temperature on the resulting channel formation. The results show deeper channeling when focused into hot plasmas and at lower densities, as expected. However, contrary to previous large-scale particle-in-cell studies, the results also indicate deeper penetration by short (10 ps), intense pulses compared to their longer-duration equivalents. This new observation has many implications for future laser-plasma research in the relativistic regime.


Advantages to a diverging Raman amplifier

Communications Physics Nature Publishing Group 1 (2018) 19

J Sadler, LO Silva, RA Fonseca, K Glize, M Kasim, A Savin, R Aboushelbaya, M Mayr, B Spiers, RH-W Wang, R Bingham, RMGM Trines, P Norreys

The plasma Raman instability can efficiently compress a nanosecond long high power laser pulse to sub-picosecond duration. Although many authors envisaged a converging beam geometry for Raman amplification, here we propose the exact opposite geometry; the amplification should start at the intense focus of the seed. We generalise the coupled laser envelope equations to include this non-collimated case. The new geometry completely eradicates the usual trailing secondary peaks of the output pulse, which typically lower the efficiency by half. It also reduces, by orders of magnitude, the initial seed pulse energy required for efficient operation. As in the collimated case, the evolution is self-similar, although the temporal pulse envelope is different. A two-dimensional particle-in-cell simulation demonstrates efficient amplification of a diverging seed with only 0:3mJ energy. The pulse has no secondary peaks and almost constant intensity as it amplifies and diverges.


Evolution of the Design and Fabrication of Astrophysics Targets for Turbulent Dynamo (TDYNO) Experiments on OMEGA

FUSION SCIENCE AND TECHNOLOGY 73 (2018) 434-445

SA Muller, DN Kaczala, HM Abu-Shawareb, EL Alfonso, LC Carlson, M Mauldin, P Fitzsimmons, D Lamb, P Tzeferacos, L Chen, G Gregori, A Rigby, A Bott, TG White, D Froula, J Katz


Validating Continuum Lowering Models via Multi-Wavelength Measurements of Integrated X-ray Emission.

Sci Rep 8 (2018) 6276-6276

MF Kasim, JS Wark, SM Vinko

X-ray emission spectroscopy is a well-established technique used to study continuum lowering in dense plasmas. It relies on accurate atomic physics models to robustly reproduce high-resolution emission spectra, and depends on our ability to identify spectroscopic signatures such as emission lines or ionization edges of individual charge states within the plasma. Here we describe a method that forgoes these requirements, enabling the validation of different continuum lowering models based solely on the total intensity of plasma emission in systems driven by narrow-bandwidth x-ray pulses across a range of wavelengths. The method is tested on published Al spectroscopy data and applied to the new case of solid-density partially-ionized Fe plasmas, where extracting ionization edges directly is precluded by the significant overlap of emission from a wide range of charge states.


Clocking femtosecond collisional dynamics via resonant X-ray spectroscopy

Physical Review Letters American Physical Society 120 (2018) 055002

QY van den Berg, EV Fernandez-Tello, T Burian, J Chalupský, H-K Chung, O Ciricosta, GL Dakovski, V Hájková, P Hollebon, L Juha, J Krzywinski, RW Lee, MP Minitti, TR Preston, AG de la Varga, V Vozda, U Zastrau, J Wark, P Velarde, SM Vinko

Electron-ion collisional dynamics is of fundamental importance in determining plasma transport properties, non-equilibrium plasma evolution and electron damage in diffraction imaging applications using bright x-ray free-electron lasers (FELs). Here we describe the first experimental measurements of ultra-fast electron impact collisional ionization dynamics using resonant core-hole spectroscopy in a solid-density magnesium plasma, created and diagnosed with the Linac Coherent Light Source x-ray FEL. By resonantly pumping the 1s ! 2p transition in highly-charged ions within an optically-thin plasma we have measured how off-resonance charge states are populated via collisional processes on femtosecond times scales. We present a collisional cross section model that matches our results and demonstrates how the cross sections are enhanced by dense-plasma effects including continuum lowering. Non-LTE (local thermodynamic equilibrium) collisional radiative simulations show excellent agreement with the experimental results, and provide new insight on collisional ionization and three-body-recombination processes in the dense plasma regime.


Proton imaging of stochastic magnetic fields

Journal of Plasma Physics Cambridge University Press 83 (2017) 905830614

AFA Bott, C Graziani, P Tzeferacos, P White, DQ Lamb, G Gregori, A Schekochihin

Recent laser-plasma experiments [1, 2, 3, 4] report the existence of dynamically significant magnetic fields, whose statistical characterisation is essential for a complete understanding of the physical processes these experiments are attempting to investigate. In this paper, we show how a proton imaging diagnostic can be used to determine a range of relevant magnetic field statistics, including the magnetic-energy spectrum. To achieve this goal, we explore the properties of an analytic relation between a stochastic magnetic field and the image-flux distribution created upon imaging that field. This ‘Kugland image-flux relation’ was previously derived [5] under simplifying assumptions typically valid in actual proton-imaging set-ups. We conclude that, as in the case of regular electromagnetic fields, features of the beam’s final image-flux distribution often display a universal character determined by a single, field-scale dependent parameter – the contrast parameter µ ≡ ds/MlB – which quantifies the relative size of the correlation length lB of the stochastic field, proton displacements ds due to magnetic deflections, and the image magnification M. For stochastic magnetic fields, we establish the existence of four contrast regimes – linear, nonlinear injective, caustic and diffusive – under which proton-flux images relate to their parent fields in a qualitatively distinct manner. As a consequence, it is demonstrated that in the linear or nonlinear injective regimes, the path-integrated magnetic field experienced by the beam can be extracted uniquely, as can the magnetic-energy spectrum under a further statistical assumption of isotropy. This is no longer the case in the caustic or diffusive regimes. We also discuss complications to the contrast-regime characterisation arising for inhomogeneous, multi-scale stochastic fields, which can encompass many contrast regimes, as well as limitations currently placed by experimental capabilities on one’s ability to extract magnetic field statistics. The results presented in this paper are of consequence in providing a comprehensive description of proton images of stochastic magnetic fields, with applications for improved analysis of individual proton-flux images, or for optimising implementation of proton-imaging diagnostics on future laser-plasma experiments.

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