Publications


Axion particle production in a laser-induced dynamical spacetime

Physics Letters B Elsevier 777 (2017) 388-393

M Wadud, B King, G Gregori, R Bingham

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.


AWAKE readiness for the study of the seeded self-modulation of a 400GeV proton bunch

PLASMA PHYSICS AND CONTROLLED FUSION 60 (2017) ARTN 014046

P Muggli, E Adli, R Apsimon, F Asmus, R Baartman, A-M Bachmann, MB Marin, F Batsch, J Bauche, VKB Olsen, M Bernardini, B Biskup, EB Vinuela, A Boccardi, T Bogey, T Bohl, C Bracco, F Braunmuller, S Burger, G Burt, S Bustamante, B Buttenschoen, A Butterworth, A Caldwell, M Cascella, E Chevallay, M Chung, H Damerau, L Deacon, A Dexter, P Dirksen, S Doebert, J Farmer, V Fedosseev, T Feniet, G Fior, R Fiorito, R Fonseca, F Friebel, P Gander, S Gessner, I Gorgisyan, AA Gorn, O Grulke, E Gschwendtner, A Guerrero, J Hansen, C Hessler, W Hofle, J Holloway, M Huther, M Ibison, MR Islam, L Jensen, S Jolly, M Kasim, F Keeble, S-Y Kim, F Kraus, A Lasheen, T Lefevre, G LeGodec, Y Li, S Liu, N Lopes, KV Lotov, M Martyanov, S Mazzoni, DM Godoy, O Mete, VA Minakov, R Mompo, J Moody, MT Moreira, J Mitchell, C Mutin, P Norreys, E Oz, E Ozturk, W Pauw, A Pardons, C Pasquino, K Pepitone, A Petrenko, S Pitmann, G Plyushchev, A Pukhov, K Rieger, H Ruhl, J Schmidt, IA Shalimova, E Shaposhnikova, P Sherwood, L Silva, AP Sosedkin, R Speroni, RI Spitsyn, K Szczurek, J Thomas, PV Tuev, M Turner, V Verzilov, J Vieira, H Vincke, CP Welsch, B Williamson, M Wing, G Xia, H Zhang, AWAKE Collaboration


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


Observation of extremely strong shock waves in solids launched by petawatt laser heating

PHYSICS OF PLASMAS 24 (2017) ARTN 083115

KL Lancaster, APL Robinson, J Pasley, P Hakel, T Ma, K Highbarger, FN Beg, SN Chen, RL Daskalova, RR Freeman, JS Green, H Habara, P Jaanimagi, MH Key, J King, R Kodama, K Krushelnick, H Nakamura, M Nakatsutsumi, AJ MacKinnon, AG MacPhee, RB Stephens, L Van Woerkom, PA Norreys


Transition from collisional to collisionless regimes in interpenetrating plasma flows on the National Ignition Facility

Physical Review Letters American Physical Society 118 (2017) 185003

JS Ross, DP Higginson, D Ryutov, F Fiuza, R Hatarik, CM Huntington, DH Kalantar, A Link, BB Pollock, BA Remington, HG Rinderknecht, GF Swadling, DP Turnbull, S Weber, S Wilks, DH Froula, T Morita, MJ Rosenberg, Y Sakawa, H Takabe, RP Drake, C Kuranz, G Gregori, J Meinecke, MC Levy

A study of the transition from collisional to collisionless plasma flows has been carried out at the National Ignition Facility using high Mach number (M>4) counterstreaming plasmas. In these experiments, CD-CD and CD-CH planar foils separated by 6-10 mm are irradiated with laser energies of 250 kJ per foil, generating ∼1000  km/s plasma flows. Varying the foil separation distance scales the ion density and average bulk velocity and, therefore, the ion-ion Coulomb mean free path, at the interaction region at the midplane. The characteristics of the flow interaction have been inferred from the neutrons and protons generated by deuteron-deuteron interactions and by x-ray emission from the hot, interpenetrating, and interacting plasmas. A localized burst of neutrons and bright x-ray emission near the midpoint of the counterstreaming flows was observed, suggesting strong heating and the initial stages of shock formation. As the separation of the CD-CH foils increases we observe enhanced neutron production compared to particle-in-cell simulations that include Coulomb collisions, but do not include collective collisionless plasma instabilities. The observed plasma heating and enhanced neutron production is consistent with the initial stages of collisionless shock formation, mediated by the Weibel filamentation instability.


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.


Interaction of a highly radiative shock with a solid obstacle

Physics of Plasmas American Institute of Physics 24 (2017) 082707-

M Koenig, T Michel, C Michaut, R Yurchak, B Albertazzi, S Laffite, E Falize, L Van Box Som, Y Sakawa, T Sano, Y Hara, T Morita, Y Kuramitsu, P Barroso, A Pelka, R Kodama, G Gregori, P Tzeferacos, D Lamb, N Ozaki

In this paper, we present the recent results obtained regarding highly radiative shocks (RSs) generated in a low-density gas filled cell on the GEKKO XII laser facility. The RS was generated by using an ablator-pusher two-layer target (CH/Sn) and a propagation medium (Xe). High velocity RSs have been generated (100-140 km/s), while limiting as much as possible the preheating produced by the corona emission. Both self-emission and visible probe diagnostics highlighted a strong emission in the shock and an electron density in the downstream gas. The RS characteristics that depend on the initial conditions are described here as well as its precursor interaction with an aluminium foil used as an obstacle. The obtained results are discussed which show a strong extension of the radiative precursor (1 mm) leading to an expansion velocity of the obstacle up to 30 km/s compatible to a 20 eV temperature.


Measurements of the K-shell opacity of a solid-density magnesium plasma heated by an X-ray free electron laser

Physical Review Letters American Physical Society 119 (2017) 085001-

TR Preston, SM Vinko, O Ciricosta, P Hollebon, H-K Chung, T Preston, GL Dakovski, J Krzywinski, M Minitti, T Burian, J Chalupský, V Hájková, V Vozda, L Juha, U Zastrau, JS Wark, RW Lee

We present measurements of the spectrally-resolved X-rays emitted from solid-density magnesium targets of varying sub-μm thicknesses isochorically heated by an X-ray laser. The data exhibit a largely thickness-independent source function, allowing the extraction of a measure of the opacity to K-shell X-rays within well-defined regimes of electron density and temperature, extremely close to local thermodynamic equilibrium (LTE) conditions. The deduced opacities at the peak of the K-α transitions of the ions are consistent with those predicted by detailed atomic-kinetics calculations.


Observation of reverse saturable absorption of an X-ray laser

Physical Review Letters American Physical Society 119 (2017) 075002-

BI Cho, M Kim, B Barbrel, H-K Chung, K Engelhorn, S Chalupský, T Burian, O Ciricosta, GL Davovski, V Hájková, M Holmes, L Juha, J Krzywinski, RW Lee, CH Nam, D Rackstraw, S Toleikis, JJ Turner, SM Vinko, JS Wark, U Zastrau, PA Heimann

A nonlinear absorber in which the excited state absorption is larger than the ground state can undergo a process called reverse saturable absorption (RSA). It is a well-known phenomenon in laser physics in the optical regime, but is more difficult to generate in the x-ray regime, where fast non-radiative core electron transitions typically dominate the population kinetics during light matter interactions. Here, we report the first observation of decreasing x-ray transmission in a solid target pumped by intense x-ray free electron laser pulses. The measurement has been made below the K-absorption edge of aluminum, and the x-ray intensity ranges are 10^16~17 W/cm2. It has been confirmed by collisional radiative population kinetic calculations, underscoring the fast spectral modulation of the x-ray pulses and charge states relevant to the absorption and transmission of x-ray photons. The processes shown through detailed simulations are consistent with reverse saturable absorption, which would be the first observation of this phenomena in the x-ray regime. These light matter interactions provide a unique opportunity to investigate optical transport properties in extreme state of matters, as well as affording the potential to regulate ultrafast XFEL pulses.


Femtosecond measurement of shock wave driven twinning and lattice dynamics

Nature Nature Publishing Group 550 (2017) 496–499-

A Higginbotham, C Bolme, A Lazicki, HJ Lee, H-S Park, B Nagler, M Sliwa, RE Rudd, CE Wehrenberg, D McGonegle, BA Remington, M Suggit, D Swift, F Tavella, L Zepeda-Ruiz, J Wark

<p>Pressure-driven shock waves in solid materials can cause extreme damage and deformation. Understanding this deformation and the associated defects that are created in the material is crucial in the study of a wide range of phenomena, including planetary formation and asteroid impact sites, the formation of interstellar dust clouds, ballistic penetrators, spacecraft shielding and ductility in high-performance ceramics. At the lattice level, the basic mechanisms of plastic deformation are twinning (whereby crystallites with a mirror-image lattice form) and slip (whereby lattice dislocations are generated and move), but determining which of these mechanisms is active during deformation is challenging. Experiments that characterized lattice defects have typically examined the microstructure of samples after deformation, and so are complicated by post-shock annealing and reverberations. In addition, measurements have been limited to relatively modest pressures (less than 100 gigapascals). In situ X-ray diffraction experiments can provide insights into the dynamic behaviour of materials, but have only recently been applied to plasticity during shock compression and have yet to provide detailed insight into competing deformation mechanisms. Here we present X-ray diffraction experiments with femtosecond resolution that capture in situ, lattice-level information on the microstructural processes that drive shock-wave-driven deformation. To demonstrate this method we shock-compress the body-centred-cubic material tantalum-an important material for high-energy-density physics owing to its high shock impedance and high X-ray opacity. Tantalum is also a material for which previous shock compression simulations and experiments have provided conflicting information about the dominant deformation mechanism. Our experiments reveal twinning and related lattice rotation occurring on the timescale of tens of picoseconds. In addition, despite the common association between twinning and strong shocks, we find a transition from twinning to dislocation-slip-dominated plasticity at high pressure (more than 150 gigapascals), a regime that recovery experiments cannot accurately access. The techniques demonstrated here will be useful for studying shock waves and other high-strain-rate phenomena, as well as a broad range of processes induced by plasticity.</p>


Attosecond-scale absorption at extreme intensities

PHYSICS OF PLASMAS 24 (2017) ARTN 113103

AF Savin, AJ Ross, M Serzans, RMGM Trines, L Ceurvorst, N Ratan, B Spiers, R Bingham, APL Robinson, PA Norreys


Time evolution and asymmetry of a laser produced blast wave

PHYSICS OF PLASMAS 24 (2017) ARTN 103124

ER Tubman, RHH Scott, HW Doyle, J Meinecke, H Ahmed, RAB Alraddadi, R Bolis, JE Cross, R Crowston, D Doria, D Lamb, B Reville, APL Robinson, P Tzeferacos, M Borghesi, G Gregori, NC Woolsey


Simultaneous diagnosis of radial profiles and mix in NIF ignition-scale implosions via X-ray spectroscopy

Physics of Plasmas AIP Publishing 24 (2017) 112703

O Ciricosta, H Scott, P Durey, BA Hammel, R Epstein, T Preston, SP Regan, NC Woolsey, S Vinko, J Wark

In a NIF implosion hydrodynamic instabilities may cause cold material from the imploding shell to be injected into the hot-spot (hot-spot mix), enhancing the radiative and conductive losses, which in turn may lead to a quenching of the ignition process. The bound-bound features of the spectrum emitted by high-Z ablator dopants that get mixed into the hot-spot have been previously used to infer the total amount of mixed mass; however, the typical errorbars are larger than the maximum tolerable mix. We present here an improved 2D model for mix spectroscopy which can be used to retrieve information on both the amount of mixed mass and on the full imploded plasma profile. By performing radiation transfer, and simultaneously fitting all of the features exhibited by the spectra, we are able to constrain self-consistently the effect of the opacity of the external layers of the target on the emission, thus improving the accuracy of the inferred mixed mass. The model's predictive capabilities are first validated by fitting simulated spectra arising from fully characterized hydrodynamic simulations, then the model is applied to previously published experimental results, providing values of mix mass in agreement with previous estimates. We show that the new self consistent procedure leads to better constrained estimates of mix, and also provides insight on the sensitivity of the hot-spot spectroscopy to the spatial properties of the imploded capsule, such as the in- ight aspect ratio of the cold fuel surrounding the hotspot.


Brilliant X-rays using a two-stage plasma insertion device

Scientific Reports Springer Nature 7 (2017) 3985

JA Holloway, P Norreys, R Bartolini, AGR Thomas, R Bingham, J Nydell, RMGM Trines, R Walker, M Wing

Particle accelerators have made an enormous impact in all fields of natural sciences, from elementary particle physics, to the imaging of proteins and the development of new pharmaceuticals. Modern light sources have advanced many fields by providing extraordinarily bright, short X-ray pulses. Here we present a novel numerical study, demonstrating that existing third generation light sources can significantly enhance the brightness and photon energy of their X-ray pulses by undulating their beams within plasma wakefields. This study shows that a three order of magnitude increase in X-ray brightness and over an order of magnitude increase in X-ray photon energy is achieved by passing a 3 GeV electron beam through a two-stage plasma insertion device. The production mechanism micro-bunches the electron beam and ensures the pulses are radially polarised on creation. We also demonstrate that the micro-bunched electron beam is itself an effective wakefield driver that can potentially accelerate a witness electron beam up to 6 GeV.


Short-wavelength free-electron laser sources and science: a review

Reports on Progress in Physics IOP Science 80 (2017) 115901

EA Seddon, JA Clarke, C Masciovecchio, DJ Dunning, F Parmigiani, CJ Milne, D Rugg, JCH Spence, NR Thompson, SM Vinko, K Ueda, J Wark, W Wurth

This review is focused on free-electron lasers (FELs) in the hard to soft x-ray regime. The aim is to provide newcomers to the area with insights into: the basic physics of FELs, the qualities of the radiation they produce, the challenges of transmitting that radiation to end users and the diversity of current scientific applications. Initial consideration is given to FEL theory in order to provide the foundation for discussion of FEL output properties and the technical challenges of short-wavelength FELs. This is followed by an overview of existing x-ray FEL facilities, future facilities and FEL frontiers. To provide a context for information in the above sections, a detailed comparison of the photon pulse characteristics of FEL sources with those of other sources of high brightness x-rays is made. A brief summary of FEL beamline design and photon diagnostics then precedes an overview of FEL scientific applications. Recent highlights are covered in sections on structural biology, atomic and molecular physics, photochemistry, non-linear spectroscopy, shock physics, solid density plasmas. A short industrial perspective is also included to emphasise potential in this area.


X-Ray diffraction measurements of plasticity in shock-compressed vanadium in the region of 10-70 GPa

Journal of Applied Physics American Institute of Physics 122 (2017) 025117-

JM Foster, AJ Comley, SD Rothman, P Avraam, GS Case, A Higginbotham, EKR Floyd, D McGonegle, ET Gumbrell, JJD Luis, NT Park, LJ Peacock, CP Poulter, M Suggit, JS Wark

We report experiments in which powder-diffraction data were recorded from polycrystalline vanadium foils, shock-compressed to pressures in the range 10 – 70 GPa. Anisotropic strain in the compressed material is inferred from the asymmetry of Debye-Scherrer diffraction images, and used to infer residual strain and yield strength (residual von Mises stress) of the vanadium sample material. We find residual anisotropic strain corresponding to yield strength in the range 1.2 GPa – 1.8 GPa for shock pressures below 30 GPa, but significantly less anisotropy of strain in the range of shock pressures above this. This is in contrast to our simulations of the experimental data using a multi-scale crystal plasticity strength model, where significant yield strength persists up to the highest pressures we access in the experiment. Possible mechanisms that could contribute to the dynamic response of vanadium that we observe for shock pressures ≥ 30 GPa are discussed.


Magneto-optic probe measurements in low density-supersonic jets

Journal of Instrumentation IOP Publishing 12 (2017) P12001

M Oliver, T White, P Mabey, M Kuhn-Kauffeldt, L Dohl, R Bingham, R Clarke, P Graham, R Heathcote, M Koenig, Y Kuramitsu, DQ Lamb, J Meinecke, T Michel, F Miniati, M Notley, B Reville, S Sarkar, Y Sakawa, A Schekochihin, P Tzeferacos, N Woolsey, G Gregori

A magneto-optic probe was used to make time-resolved measurements of the magnetic field in both a single supersonic jet and in a collision between two supersonic turbulent jets, with an electron density ⇡ 1018 cm3 and electron temperature ⇡ 4 eV. The magneto-optic data indicated the magnetic field reaches B ⇡ 200 G. The measured values are compared against those obtained with a magnetic induction probe. Good agreement of the time-dependent magnetic field measured using the two techniques is found.


Identifying deformation mechanisms in molecular dynamics simulations of laser shocked matter

Journal of Computational Physics Elsevier 350 (2017) 16-24

G Gregori, D Eakins, TG White, A Tikku, MF Alves Silva, A Higginbotham

In this paper we demonstrate a new post-processing technique that allows straightforward identification of deformation mechanisms in molecular dynamics simulations. We utilise reciprocal space methods by calculating a per-atom structure factor (PASF) to visualise changes in volume, orientation and structure, thus allowing unambiguous discrimination between key deformation/relaxation mechanisms such as uniaxial strain, twinning and structural phase transformations. The full 3-D PASF is reduced to a 2-D representation by taking only those points which lie on the surface of an ellipsoid passing through the nearest reciprocal lattice points. Projecting this 2-D representation onto the set of spherical harmonics allows for a numerical characterisation of the system state that easily captures various plastic deformation mechanisms that have been historically difficult to identify. The technique is used to successfully classify high temperature twinning rotations in shock compressed tantalum and to identify the α to ω phase transition in group-IV hcp metals.


X-ray line coincidence photopumping in a solar flare

Monthly Notices of the Royal Astronomical Society Oxford University Press 474 (2017) 3782–3786-

K Poppenhaeger, FP Keenan, M Mathioudakis, S Rose, J Flowerdew, D Hynes, DJ Christian, J Nilsen, WR Johnson

Line coincidence photopumping is a process where the electrons of an atomic or molecular species are radiatively excited through the absorption of line emission from another species at a coincident wavelength. There are many instances of line coincidence photopumping in astrophysical sources at optical and ultraviolet wavelengths, with the most famous example being Bowen fluorescence (pumping of O III 303.80 Å by He II), but none to our knowledge in X-rays. However, here we report on a scheme where a He-like line of Ne IX at 11.000 Å is photopumped by He-like Na X at 11.003 Å, which predicts significant intensity enhancement in the Ne IX 82.76 Å transition under physical conditions found in solar flare plasmas. A comparison of our theoretical models with published X-ray observations of a solar flare obtained during a rocket flight provides evidence for line enhancement, with the measured degree of enhancement being consistent with that expected from theory, a truly surprising result. Observations of this enhancement during flares on stars other than the Sun would provide a powerful new diagnostic tool for determining the sizes of flare loops in these distant, spatially unresolved, astronomical sources.


Modelling K shell spectra from short pulse heated buried microdot targets

HIGH ENERGY DENSITY PHYSICS 23 (2017) 178-183

DJ Hoarty, N Sircombe, P Beiersdorfer, CRD Brown, MP Hill, LMR Hobbs, SF James, J Morton, E Hill, M Jeffery, JWO Harris, R Shepherd, E Marley, E Magee, J Emig, J Nilsen, HK Chung, RW Lee, SJ Rose

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