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, MJ Rosenberg, T Morita, 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.

Magnetic field production via the Weibel instability in interpenetrating plasma flows

Physics of Plasmas American Institute of Physics 24 (2017) 041410-

CM Huntington, MJ-E Manuel, JS Ross, SC Wilks, F Fiuza, HG Rinderknecht, H-S Park, G Gregori, DP Higginson, J Park, BB Pollock, BA Remington, DD Ryutov, C Ruyer, Y Sakawa, H Sio, A Spitkovsky, GF Swadling, H Takabe, AB Zylstra

Many astrophysical systems are effectively “collisionless,” that is, the mean free path for collisions between particles is much longer than the size of the system. The absence of particle collisions does not preclude shock formation, however, as shocks can be the result of plasma instabilities that generate and amplify electromagnetic fields. The magnetic fields required for shock formation may either be initially present, for example, in supernova remnants or young galaxies, or they may be self-generated in systems such as gamma-ray bursts (GRBs). In the case of GRB outflows, the Weibel instability is a candidate mechanism for the generation of sufficiently strong magnetic fields to produce shocks. In experiments on the OMEGA Laser, we have demonstrated a quasi-collisionless system that is optimized for the study of the non-linear phase of Weibel instability growth. Using a proton probe to directly image electromagnetic fields, we measure Weibel-generated magnetic fields that grow in opposing, initially unmagnetized plasma flows. The collisionality of the system is determined from coherent Thomson scattering measurements, and the data are compared to similar measurements of a fully collisionless system. The strong, persistent Weibel growth observed here serves as a diagnostic for exploring large-scale magnetic field amplification and the microphysics present in the collisional-collisionless transition.

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.

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, T Preston, H-K Chung, GL Dakovski, J Krzywinski, M Minitti, T Burian, J Chalupský, V Hájková, L Juha, V Vozda, U Zastrau, RW Lee, JS Wark

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, Cho, M Kim, H-K Chung, B Barbrel, K Engelhorn, T Burian, S Chalupský, O Ciricosta, GL Davovski, V Hájková, M Holmes, L Juha, J Krzywinski, RW Lee, CH Nam, DS 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-

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>

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, 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.

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

Scientific Reports Springer Nature 7 (2017) 3985

JA Holloway, P Norreys, AGR Thomas, R Bartolini, 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, 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.

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, GS Case, P Avraam, SD Rothman, A Higginbotham, EKR Floyd, ET Gumbrell, JJD Luis, D McGonegle, 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.

Nonlinear parametric resonance of relativistic electrons with a linearly polarized laser pulse in a plasma channel

Physics of Plasmas American Institute of Physics 24 (2017) 043105-

TW Huang, CT Zhou, APL Robinson, B Qiao, AV Arefiev, P Norreys, XT He, SC Ruan

The direct laser-acceleration mechanism, nonlinear parametric resonance, of relativistic electrons in a linearly polarized laser-produced plasma channel is examined by a self-consistent model including the relativistic laser dispersion in plasmas. Nonlinear parametric resonance can be excited, and the oscillation amplitude of electrons grows exponentially when the betatron frequency of electron motion varies roughly twice the natural frequency of the oscillator. It is shown analytically that the region of parametric resonance is defined by the self-similar parameter ne/nca0. The width of this region decreases with ne/nca0, but the energy gain and oscillation amplitude increases. In this regime, the electron transverse momentum grows faster than that in the linear classical resonance regime.

Machine learning applied to proton radiography of high-energy-density plasmas

Physical Review E American Physical Society 95 (2017) 043305-

NFY Chen, MF Kasim, L Ceurvorst, N Ratan, J Sadler, MC Levy, R Trines, R Bingham, P Norreys

Proton radiography is a technique extensively used to resolve magnetic field structures in high-energy-density plasmas, revealing a whole variety of interesting phenomena such as magnetic reconnection and collisionless shocks found in astrophysical systems. Existing methods of analyzing proton radiographs give mostly qualitative results or specific quantitative parameters, such as magnetic field strength, and recent work showed that the line-integrated transverse magnetic field can be reconstructed in specific regimes where many simplifying assumptions were needed. Using artificial neural networks, we demonstrate for the first time 3D reconstruction of magnetic fields in the nonlinear regime, an improvement over existing methods, which reconstruct only in 2D and in the linear regime. A proof of concept is presented here, with mean reconstruction errors of less than 5% even after introducing noise. We demonstrate that over the long term, this approach is more computationally efficient compared to other techniques. We also highlight the need for proton tomography because (i) certain field structures cannot be reconstructed from a single radiograph and (ii) errors can be further reduced when reconstruction is performed on radiographs generated by proton beams fired in different directions.

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.

Modelling K shell spectra from short pulse heated buried microdot targets


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

Optimization of plasma amplifiers

Physical Review E American Physical Society (2017)

JD Sadler, RMGM Trines, M Tabak, D Haberberger, DH Froula, AS Davies, S Bucht, LO Silva, EP Alves, F Fiuza, L Ceurvorst, N Ratan, MF Kasim, R Bingham, P Norreys

Plasma amplifiers offer a route to side-step limitations on chirped pulse amplification and generate laser pulses at the power frontier. They compress long pulses by transferring energy to a shorter pulse via the Raman or Brillouin instabilities.We present an extensive kinetic numerical study of the three-dimensional parameter space for the Raman case. Further particle-in-cell simulations find the optimal seed pulse parameters for experimentally relevant constraints. The high-efficiency self-similar behavior is observed only for seeds shorter than the linear Raman growth time. A test case similar to an upcoming experiment at the Laboratory for Laser Energetics is found to maintain good transverse coherence and high-energy efficiency. Effective compression of a 10 kJ, nanosecond-long driver pulse is also demonstrated in a 15-cm-long amplifier.

Robustness of raman plasma amplifiers and their potential for attosecond pulse generation

High Energy Density Physics Elsevier 23 (2017) 212–216-

JD Sadler, M Sliwa, T Miller, MF Kasim, N Ratan, L Ceurvorst, A Savin, R Aboushelbaya, P Norreys, D Haberberger, AS Davies, S Bucht, DH Froula, J Vieira, RA Fonseca, LO Silva, R Bingham, K Glize, RMGM Trines

Raman back-scatter from an under-dense plasma can be used to compress laser pulses, as shown by several previous experiments in the optical regime. A short seed pulse counter-propagates with a longer pump pulse and energy is transferred to the shorter pulse via stimulated Raman scattering. The robustness of the scheme to non-ideal plasma density conditions is demonstrated through particle-in-cell simulations. The scale invariance of the scheme ensures that compression of XUV pulses from a free electron laser is also possible, as demonstrated by further simulations. The output is as short as 300 as, with energy typical of fourth generation sources.

High flux, beamed neutron sources employing deuteron-rich ion beams from D 2 O-ice layered targets

Plasma Physics and Controlled Fusion Institute of Physics 59 (2017) 064004-

A Alejo, AG Krygier, H Ahmed, JT Morrison, RJ Clarke, J Fuchs, A Green, JS Green, D Jung, A Kleinschmidt, Z Najmudin, H Nakamura, P Norreys, M Notley, M Oliver, M Roth, L Vassura, M Zepf, M Borghesi, RR Freeman, S Kar

A forwardly-peaked bright neutron source was produced using a laser-driven, deuteron-rich ion beam in a pitcher-catcher scenario. A proton-free ion source was produced via target normal sheath acceleration from Au foils having a thin layer of D2O ice at the rear side, irradiated by sub-petawatt laser pulses (∼200 J, ∼750 fs) at peak intensity . The neutrons were preferentially produced in a beam of ∼70 FWHM cone along the ion beam forward direction, with maximum energy up to ∼40 MeV and a peak flux along the axis for neutron energy above 2.5 MeV. The experimental data is in good agreement with the simulations carried out for the d(d,n)3He reaction using the deuteron beam produced by the ice-layered target.