Publications


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.


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.


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.


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


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.


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.


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.


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.


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


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.


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

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

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

FP Keenan, K Poppenhaeger, 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.


Attosecond-scale absorption at extreme intensities

Physics of Plasmas AIP Publishing 24 (2017) 113103

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

A novel non-ponderomotive absorption mechanism, originally presented by Baeva et al. [Phys. Plasmas 18, 056702 (2011)] in one dimension, is extended into higher dimensions for the first time. This absorption mechanism, the Zero Vector Potential (ZVP), is expected to dominate the interactions of ultra-intense laser pulses with critically over-dense plasmas such as those that are expected with the Extreme Light Infrastructure laser systems. It is shown that the mathematical form of the ZVP mechanism and its key scaling relations found by Baeva et al. in 1D are identically reproduced in higher dimensions. The two dimensional particle-in-cell simulations are then used to validate both the qualitative and quantitative predictions of the theory.


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, S Vinko, NC Woolsey, 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.


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

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

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


Femtosecond measurement of shock wave driven twinning and lattice dynamics

Nature Nature Publishing Group 550 (2017) 496-499

CE Wehrenberg, D McGonegle, C Bolme, A Higginbotham, A Lazicki, HJ Lee, B Nagler, H-S Park, BA Remington, RE Rudd, M Sliwa, 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>


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


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


Interaction of a highly radiative shock with a solid obstacle

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

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

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.

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