Publications by Gianluca Gregori

Observations of pressure anisotropy effects within semi-collisional magnetized plasma bubbles

Nature Communications Nature 12 (2021) 334

E Tubman, A Joglekar, A Bott, M Borghesi, B Coleman, G Cooper, C Danson, P Durey, J Foster, P Graham, G Gregori, E Gumbrell, M Hill, T Hodge, S Kar, R Kingham, M Read, C Ridgers, J Skidmore, C Spindloe, A Thomas, P Treadwell, S Wilson, L Willingale, N Woolsey

Magnetized plasma interactions are ubiquitous in astrophysical and laboratory plasmas. Various physical effects have been shown to be important within colliding plasma flows influenced by opposing magnetic fields, however, experimental verification of the mechanisms within the interaction region has remained elusive. Here we discuss a laser-plasma experiment whereby experimental results verify that Biermann battery generated magnetic fields are advected by Nernst flows and anisotropic pressure effects dominate these flows in a reconnection region. These fields are mapped using time-resolved proton probing in multiple directions. Various experimental, modelling and analytical techniques demonstrate the importance of anisotropic pressure in semi-collisional, high-β plasmas, causing a reduction in the magnitude of the reconnecting fields when compared to resistive processes. Anisotropic pressure dynamics are crucial in collisionless plasmas, but are often neglected in collisional plasmas. We show pressure anisotropy to be essential in maintaining the interaction layer, redistributing magnetic fields even for semi-collisional, high energy density physics (HEDP) regimes.

High-resolution inelastic x-ray scattering at the high energy density scientific instrument at the European X-Ray Free-Electron Laser (vol 92, 013101, 2021)


L Wollenweber, TR Preston, A Descamps, V Cerantola, A Comley, JH Eggert, LB Fletcher, G Geloni, DO Gericke, SH Glenzer, S Goede, J Hastings, OS Humphries, A Jenei, O Karnbach, Z Konopkova, R Loetzsch, B Marx-Glowna, EE McBride, D McGonegle, G Monaco, BK Ofori-Okai, CAJ Palmer, C Plueckthun, R Redmer, C Strohm, I Thorpe, T Tschentscher, I Uschmann, JS Wark, TG White, K Appel, G Gregori, U Zastrau

X-ray radiography based on the phase-contrast imaging with using LiF detector

Journal of Physics: Conference Series IOP Publishing 1787 (2021)

S Makarov, T Pikuz, A Buzmakov, A Chernyaev, P Mabey, T Vinci, G Rigon, B Albertazzi, A Casner, V Bouffetier, R Kodama, K Katagiri, N Kamimura, Y Umeda, N Ozaki, E Falize, O Poujade, T Togashi, M Yabashi, Y Inubushi, K Miyanishi, K Sueda, M Manuel, G Gregori, M Koenig

An x-ray radiography technique based upon phase contrast imaging using a lithium fluoride detector has been demonstrated for goals of high energy density physics experiments. Based on the simulation of propagation an x-ray free-electron laser beam through a test-object, the visibility of phase-contrast image depending on an object-detector distance was investigated. Additionally, the metrological capabilities of a lithium fluoride crystal as a detector were demonstrated.

High-resolution inelastic x-ray scattering at the high energy density scientific instrument at the European X-Ray Free-Electron Laser

Review of Scientific Instruments American Institute of Physics 92 (2021) 013101

V Cerantola, A Comley, J Eggert, L Fletcher, G Geloni, D Gericke, S Glenzer, S Göde, J Hastings, O Humphries, A Jenei, O Karnbach, Z Konopkova, R Loetzsch, B Marx-Glowna, E McBride, D McGonegle, B Ofori-Okai, C Palmer, C Plückthun, R Redmer, C Strohm, T Tschentscher, K Appel, G Gregori

We introduce a setup to measure high-resolution inelastic x-ray scattering at the High Energy Density scientific instrument at the European X-Ray Free-Electron Laser (XFEL). The setup uses the Si (533) reflection in a channel-cut monochromator and three spherical diced analyzer crystals in near-backscattering geometry to reach a high spectral resolution. An energy resolution of 44 meV is demonstrated for the experimental setup, close to the theoretically achievable minimum resolution. The analyzer crystals and detector are mounted on a curved-rail system, allowing quick and reliable changes in scattering angle without breaking vacuum. The entire setup is designed for operation at 10 Hz, the same repetition rate as the high-power lasers available at the instrument and the fundamental repetition rate of the European XFEL. Among other measurements, it is envisioned that this setup will allow studies of the dynamics of highly transient laser generated states of matter.

An approach for the measurement of the bulk temperature of single crystal diamond using an X-ray free electron laser

Scientific Reports Nature Research 10 (2020) 14564

J Wark, A Descamps, BK Ofori-Okai, K Appel

We present a method to determine the bulk temperature of a single crystal diamond sample at an X-Ray free electron laser using inelastic X-ray scattering. The experiment was performed at the high energy density instrument at the European XFEL GmbH, Germany. The technique, based on inelastic X-ray scattering and the principle of detailed balance, was demonstrated to give accurate temperature measurements, within 8% for both room temperature diamond and heated diamond to 500 K. Here, the temperature was increased in a controlled way using a resistive heater to test theoretical predictions of the scaling of the signal with temperature. The method was tested by validating the energy of the phonon modes with previous measurements made at room temperature using inelastic X-ray scattering and neutron scattering techniques. This technique could be used to determine the bulk temperature in transient systems with a temporal resolution of 50 fs and for which accurate measurements of thermodynamic properties are vital to build accurate equation of state and transport models.

Electron acceleration in laboratory-produced turbulent collisionless shocks

Nature Physics Springer Nature 16 (2020) 916-920

GF Swadling, A Grassi, HG Rinderknecht, DP Higginson, DD Ryutov, C Bruulsema, RP Drake, S Funk, S Glenzer, G Gregori, CK Li, BB Pollock, BA Remington, JS Ross, W Rozmus, Y Sakawa, A Spitkovsky, S Wilks, H-S Park

Astrophysical collisionless shocks are among the most powerful particle accelerators in the Universe. Generated by violent interactions of supersonic plasma flows with the interstellar medium, supernova remnant shocks are observed to amplify magnetic fields and accelerate electrons and protons to highly relativistic speeds. In the well-established model of diffusive shock acceleration, relativistic particles are accelerated by repeated shock crossings. However, this requires a separate mechanism that pre-accelerates particles to enable shock crossing. This is known as the ‘injection problem’, which is particularly relevant for electrons, and remains one of the most important puzzles in shock acceleration. In most astrophysical shocks, the details of the shock structure cannot be directly resolved, making it challenging to identify the injection mechanism. Here we report results from laser-driven plasma flow experiments, and related simulations, that probe the formation of turbulent collisionless shocks in conditions relevant to young supernova remnants. We show that electrons can be effectively accelerated in a first-order Fermi process by small-scale turbulence produced within the shock transition to relativistic non-thermal energies, helping overcome the injection problem. Our observations provide new insight into electron injection at shocks and open the way for controlled laboratory studies of the physics underlying cosmic accelerators.

Laboratory Study of Bilateral Supernova Remnants and Continuous MHD Shocks


P Mabey, B Albertazzi, G Rigon, J-R Marques, C Palmer, J Topp-Mugglestone, P Perez-Martin, F Kroll, F-E Brack, T Cowan, U Schramm, K Falk, G Gregori, E Falize, M Koenig

© 2020. The American Astronomical Society. All rights reserved. Many supernova remnants (SNRs), such as G296.5+10.0, exhibit an axisymmetric or barrel shape. Such morphologies have previously been linked to the direction of the Galactic magnetic field, although this remains uncertain. These SNRs generate magnetohydrodynamic shocks in the interstellar medium, modifying its physical and chemical properties. The ability to study these shocks through observations is difficult due to the small spatial scales involved. In order to answer these questions, we perform a scaled laboratory experiment in which a laser-generated blast wave expands under the influence of a uniform magnetic field. The blast wave exhibits a spheroidal shape, whose major axis is aligned with the magnetic field, in addition to a more continuous shock front. The implications of our results are discussed in the context of astrophysical systems.

Axion detection through resonant photon-photon collisions

Physical Review D American Physical Society (APS) 101 (2020) 95018

K Beyer, G Marocco, R Bingham, G Gregori

Transport of high-energy charged particles through spatially-intermittent turbulent magnetic fields

Astrophysical Journal American Astronomical Society 892 (2020) 114

LE Chen, AFA Bott, P Tzeferacos, A Rigby, A Bell, R Bingham, C Graziani, J Katz, R Petrasso, G Gregori, F Miniati

Identifying the sources of the highest energy cosmic rays requires understanding how they are deflected by the stochastic, spatially intermittent intergalactic magnetic field. Here we report measurements of energetic charged-particle propagation through a laser-produced magnetized plasma with these properties. We characterize the diffusive transport of the particles experimentally. The results show that the transport is diffusive and that, for the regime of interest for the highest-energy cosmic rays, the diffusion coefficient is unaffected by the spatial intermittency of the magnetic field.

Role of collisionality and radiative cooling in supersonic plasma jet collisions of different materials

Physical Review E American Physical Society 101 (2020) 023205

Collins, Valenzuela, Speliotopoulos, Aybar, Conti, Beg, Tzeferacos, Khiar, G Gregori

Currently there is considerable interest in creating scalable laboratory plasmas to study the mechanisms behind the formation and evolution of astrophysical phenomena such as Herbig-Haro objects and supernova remnants. Laboratory-scaled experiments can provide a well diagnosed and repeatable supplement to direct observations of these extraterrestrial objects if they meet similarity criteria demonstrating that the same physics govern both systems. Here, we present a study on the role of collision and cooling rates on shock formation using colliding jets from opposed conical wire arrays on a compact pulsed-power driver. These diverse conditions were achieved by changing the wire material feeding the jets, since the ion-ion mean free path (λmfp-ii) and radiative cooling rates (Prad) increase with atomic number. Low Z carbon flows produced smooth, temporally stable shocks. Weakly collisional, moderately cooled aluminum flows produced strong shocks that developed signs of thermal condensation instabilities and turbulence. Weakly collisional, strongly cooled copper flows collided to form thin shocks that developed inconsistently and fragmented. Effectively collisionless, strongly cooled tungsten flows interpenetrated, producing long axial density perturbations.

Experimental characterization of the interaction zone between counter propagating Taylor Sedov blast waves

Physics of Plasmas AIP Publishing 27 (2020) 022111

B Albertazzi, P Mabey, T Michel, G Rigon, Marques, S Pikuz, S Ryazantsev, E Falize, L Van Box Som, J Meinecke, N Ozaki, A Ciardi, G Gregori, M Koenig

Astronomical observations reveal that the interaction between shock waves and/or blast waves with astrophysical objects (molecular clouds, stars, jets winds etc.) is a common process which leads to a more intricate structure of the Interstellar medium (ISM). In particular, when two isolated massive stars are relatively close and explode, the resulting Supernovae Remnants (SNR) can interact. The impact zone presents fascinating complex hydrodynamic physics which depends on the age of the SNRs, their relative evolution stage and the distance between the two stars. In this letter, we investigate experimentally the interaction region (IR) formed when two blast waves (BW) collide during their Taylor-Sedov expansion phase. The two BWs are produced by the laser irradiation (1 ns, ∼ 500 J) of 300 µm diameter carbon rods and propagate in different gases (Ar and N) at different pressures. The physical parameters, such as density and temperature of the IR are measured for the first time using a set of optical diagnostics (interferometry, schlieren, time-resolved optical spectroscopy etc.). This allows us to determine precisely the thermodynamic conditions of the IR. A compression ratio of r ∼ 1.75 is found and a 17-20 % increase of temperature is measured compared to the shell of a single blast wave. Moreover, we observe the generation of vorticity, inducing strong electron density gradients, in the IR at long times after the interaction. This could in principle generate magnetic fields through the Biermann Battery effect.

Axion-like-particle decay in strong electromagnetic backgrounds

Journal of High Energy Physics Springer 2019 (2019) 162

B King, BM Dillon, K Beyer, G Gregori

Supersonic plasma turbulence in the laboratory

Nature Communications Nature Research 10 (2019) 1758

TG White, MT Oliver, P Mabey, AFA Bott, AA Schekochihin, G Gregori

Laboratory study of stationary accretion shock relevant to astrophysical systems

Scientific Reports Springer Nature 9 (2019) 8157

P Mabey, B Albertazzi, E Falize, T Michel, G Rigon, L Van Box Som, A Pelka, F-E Brack, F Kroll, E Filippov, G Gregori, Y Kuramitsu, DQ Lamb, C Li, N Ozaki, S Pikuz, Y Sakawa, P Tzeferacos, M Koenig

Accretion processes play a crucial role in a wide variety of astrophysical systems. Of particular interest are magnetic cataclysmic variables, where, plasma flow is directed along the star's magnetic field lines onto its poles. A stationary shock is formed, several hundred kilometres above the stellar surface; a distance far too small to be resolved with today's telescopes. Here, we report the results of an analogous laboratory experiment which recreates this astrophysical system. The dynamics of the laboratory system are strongly influenced by the interplay of material, thermal, magnetic and radiative effects, allowing a steady shock to form at a constant distance from a stationary obstacle. Our results demonstrate that a significant amount of plasma is ejected in the lateral direction; a phenomenon that is under-estimated in typical magnetohydrodynamic simulations and often neglected in astrophysical models. This changes the properties of the post-shock region considerably and has important implications for many astrophysical studies.

Inverse problem instabilities in large-scale modelling of matter in extreme conditions

Physics of Plasmas AIP Publishing 26 (2019) 112706

MF Kasim, TP Galligan, J Topp-Mugglestone, G Gregori, S Vinko

Our understanding of physical systems often depends on our ability to match complex computational modeling with the measured experimental outcomes. However, simulations with large parameter spaces suffer from inverse problem instabilities, where similar simulated outputs can map back to very different sets of input parameters. While of fundamental importance, such instabilities are seldom resolved due to the intractably large number of simulations required to comprehensively explore parameter space. Here, we show how Bayesian inference can be used to address inverse problem instabilities in the interpretation of x-ray emission spectroscopy and inelastic x-ray scattering diagnostics. We find that the extraction of information from measurements on the basis of agreement with simulations alone is unreliable and leads to a significant underestimation of uncertainties. We describe how to statistically quantify the effect of unstable inverse models and describe an approach to experimental design that mitigates its impact.

Reply to: Reconsidering X-ray plasmons

NATURE PHOTONICS 13 (2019) 751-753

L Fletcher, H Lee, T Doppner, E Galtier, B Nagler, P Heimann, C Fortmann, S LePape, T Ma, M Millot, A Pak, D Turnbull, D Chapman, D Gericke, J Vorberger, G Gregori, B Barbrel, R Falcone, C-C Kao, H Nuhn, J Welch, U Zastrau, P Neumayer, J Hastings, S Glenzer

Field reconstruction from proton radiography of intense laser driven magnetic reconnection

Physics of Plasmas AIP Publishing 26 (2019)

CAJ Palmer, PT Campbell, Y Ma, L Antonelli, AFA Bott, G Gregori, J Halliday, Y Katzir, P Kordell, K Krushelnick, SV Lebedev, E Montgomery, M Notley, DC Carroll, CP Ridgers, A Schekochihin, MJV Streeter, AGR Thomas, ER Tubman, N Woolsey, L Willingale

Magnetic reconnection is a process that contributes significantly to plasma dynamics and energy transfer in a wide range of plasma and magnetic field regimes, including inertial confinement fusion experiments, stellar coronae, and compact, highly magnetized objects like neutron stars. Laboratory experiments in different regimes can help refine, expand, and test the applicability of theoretical models to describe reconnection. Laser-plasma experiments exploring magnetic reconnection at a moderate intensity (IL ∼1014 W cm-2) have been performed previously, where the Biermann battery effect self-generates magnetic fields and the field dynamics studied using proton radiography. At high laser intensities (ILλL2>1018 Wcm-2μm2), relativistic surface currents and the time-varying electric sheath fields generate the azimuthal magnetic fields. Numerical modeling of these intensities has shown the conditions that within the magnetic field region can reach the threshold where the magnetic energy can exceed the rest mass energy such that σcold = B2/(μ0nemec2) > 1 [A. E. Raymond et al., Phys. Rev. E 98, 043207 (2018)]. Presented here is the analysis of the proton radiography of a high-intensity (∼1018 W cm-2) laser driven magnetic reconnection geometry. The path integrated magnetic fields are recovered using a "field-reconstruction algorithm" to quantify the field strengths, geometry, and evolution.

Thomson scattering cross section in a magnetized, high-density plasma

Physical Review E American Physical Society 99 (2019) 063204

AFA Bott, G Gregori

We calculate the Thomson scattering cross section in a nonrelativistic, magnetized, high-density plasma—in a regime where collective excitations can be described by magnetohydrodynamics. We show that, in addition to cyclotron resonances and an elastic peak, the cross section exhibits two pairs of peaks associated with slow and fast magnetosonic waves; by contrast, the cross section arising in pure hydrodynamics possesses just a single pair of Brillouin peaks. Both the position and the width of these magnetosonic-wave peaks depend on the ambient magnetic field and temperature, as well as transport and thermodynamic coefficients, and so can therefore serve as a diagnostic tool for plasma properties that are otherwise challenging to measure.

Maser radiation from collisionless shocks: application to astrophysical jets

High Power Laser Science and Engineering Cambridge University Press 7 (2019) e17

DC Speirs, K Ronald, ADR Phelps, A Rigby, JE Cross, PM Kozlowski, F Miniati, M Oliver, S Sarkar, P Tzeferacos, G Gregori, E al.

This paper describes a model of electron energization and cyclotron-maser emission applicable to astrophysical magnetized collisionless shocks. It is motivated by the work of Begelman, Ergun and Rees [Astrophys. J. 625, 51 (2005)] who argued that the cyclotron-maser instability occurs in localized magnetized collisionless shocks such as those expected in blazar jets. We report on recent research carried out to investigate electron acceleration at collisionless shocks and maser radiation associated with the accelerated electrons. We describe how electrons accelerated by lower-hybrid waves at collisionless shocks generate cyclotron-maser radiation when the accelerated electrons move into regions of stronger magnetic fields. The electrons are accelerated along the magnetic field and magnetically compressed leading to the formation of an electron velocity distribution having a horseshoe shape due to conservation of the electron magnetic moment. Under certain conditions the horseshoe electron velocity distribution function is unstable to the cyclotron-maser instability [Bingham and Cairns, Phys. Plasmas 7, 3089 (2000); Melrose, Rev. Mod. Plasma Phys. 1, 5 (2017)].

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

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

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