arXiv subjects

G. Gregori

Publications and source records attributed to G. Gregori.

At least 37 records · Page 2Linked to original sources

arXiv2020

Generating ultra-dense pair beams using 400 GeV/c protons

A previously unexplored experimental scheme is presented for generating low-divergence, ultra-dense, relativistic, electron-positron beams using 400 GeV/c protons available at facilities such as HiRadMat and AWAKE at CERN. Preliminary Monte-Carlo and Particle-in-cell simulations demonstrate the possibility of generating beams containing $10^{13}-10^{14}$ electron-positron pairs at sufficiently high densities to drive collisionless beam-plasma instabilities, which are expected to play an important role in magnetic field generation and the related radiation signatures of relativistic astrophysical phenomena. The pair beams are quasi-neutral, with size exceeding several skin-depths in all dimensions, allowing for the first time the examination of the effect of competition between transverse and longitudinal instability modes on the growth of magnetic fields. Furthermore, the presented scheme allows for the possibility of controlling the relative density of hadrons to electron-positron pairs in the beam, making it possible to explore the parameter spaces for different astrophysical environments.

physics.plasm-ph↗

arXiv2020

Observations of Pressure Anisotropy Effects within Semi-Collisional Magnetized-Plasma Bubbles

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

physics.plasm-ph↗

arXiv2020

Building high accuracy emulators for scientific simulations with deep neural architecture search

Computer simulations are invaluable tools for scientific discovery. However, accurate simulations are often slow to execute, which limits their applicability to extensive parameter exploration, large-scale data analysis, and uncertainty quantification. A promising route to accelerate simulations by building fast emulators with machine learning requires large training datasets, which can be prohibitively expensive to obtain with slow simulations. Here we present a method based on neural architecture search to build accurate emulators even with a limited number of training data. The method successfully accelerates simulations by up to 2 billion times in 10 scientific cases including astrophysics, climate science, biogeochemistry, high energy density physics, fusion energy, and seismology, using the same super-architecture, algorithm, and hyperparameters. Our approach also inherently provides emulator uncertainty estimation, adding further confidence in their use. We anticipate this work will accelerate research involving expensive simulations, allow more extensive parameters exploration, and enable new, previously unfeasible computational discovery.

stat.ML↗

arXiv2020

Inefficient magnetic-field amplification in supersonic laser-plasma turbulence

We report a laser-plasma experiment that was carried out at the LMJ-PETAL facility and realized the first magnetized, turbulent, supersonic plasma with a large magnetic Reynolds number ($\mathrm{Rm} \approx 45$) in the laboratory. Initial seed magnetic fields were amplified, but only moderately so, and did not become dynamically significant. A notable absence of magnetic energy at scales smaller than the outer scale of the turbulent cascade was also observed. Our results support the notion that moderately supersonic, low-magnetic-Prandtl-number plasma turbulence is inefficient at amplifying magnetic fields.

physics.plasm-ph↗

arXiv2020

Time-resolved fast turbulent dynamo in a laser plasma

Understanding magnetic-field generation and amplification in turbulent plasma is essential to account for observations of magnetic fields in the universe. A theoretical framework attributing the origin and sustainment of these fields to the so-called fluctuation dynamo was recently validated by experiments on laser facilities in low-magnetic-Prandtl-number plasmas ($\mathrm{Pm} < 1$). However, the same framework proposes that the fluctuation dynamo should operate differently when $\mathrm{Pm} \gtrsim 1$, the regime relevant to many astrophysical environments such as the intracluster medium of galaxy clusters. This paper reports a new experiment that creates a laboratory $\mathrm{Pm} \gtrsim 1$ plasma dynamo for the first time. We provide a time-resolved characterization of the plasma's evolution, measuring temperatures, densities, flow velocities and magnetic fields, which allows us to explore various stages of the fluctuation dynamo's operation. The magnetic energy in structures with characteristic scales close to the driving scale of the stochastic motions is found to increase by almost three orders of magnitude from its initial value and saturate dynamically. It is shown that the growth of these fields occurs exponentially at a rate that is much greater than the turnover rate of the driving-scale stochastic motions. Our results point to the possibility that plasma turbulence produced by strong shear can generate fields more efficiently at the driving scale than anticipated by idealized MHD simulations of the nonhelical fluctuation dynamo; this finding could help explain the large-scale fields inferred from observations of astrophysical systems.

physics.plasm-ph↗

arXiv2020

Axion detection through resonant photon-photon collisions

We investigate the prospect of an alternative laboratory-based search for the coupling of axions and axion-like particles to photons. Here, the collision of two laser beams resonantly produces axions, and a signal photon is detected after magnetic reconversion, as in light-shining-through-walls (LSW) experiments. Conventional searches, such as LSW or anomalous birefrigence measurements, are most sensitive to axion masses for which substantial coherence can be achieved; this is usually well below optical energies. We find that using currently available high-power laser facilities, the bounds that can be achieved by our approach outperform traditional LSW at axion masses between $0.5-6$ eV, set by the optical laser frequencies and collision angle. These bounds can be further improved through coherent scattering off laser substructures, probing axion-photon couplings down to $g_{aγγ}\sim 10^{-8} {\text{GeV}^{-1}}$, comparable with existing CAST bounds. Assuming a day long measurement per angular step, the QCD axion band can be reached.

hep-ph↗

arXiv2020

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

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.

physics.plasm-ph↗

arXiv2019

Modified Friedmann equations via conformal Bohm -- De Broglie gravity

We use an alternative interpretation of quantum mechanics, based on the Bohmian trajectory approach, and show that the quantum effects can be included in the classical equation of motion via a conformal transformation on the background metric. We apply this method to the Robertson-Walker metric to derive a modified version of Friedmann's equations for a Universe consisting of scalar, spin-zero, massive particles. These modified equations include additional terms that result from the non-local nature of matter and appear as an acceleration in the expansion of the Universe. We see that the same effect may also be present in the case of an inhomogeneous expansion.

gr-qc↗

arXiv2019

Retrieving fields from proton radiography without source profiles

Proton radiography is a technique in high energy density science to diagnose magnetic and/or electric fields in a plasma by firing a proton beam and detecting its modulated intensity profile on a screen. Current approaches to retrieve the integrated field from the modulated intensity profile require the unmodulated beam intensity profile before the interaction, which is rarely available experimentally due to shot-to-shot variability. In this paper, we present a statistical method to retrieve the integrated field without needing to know the exact source profile. We apply our method to experimental data, showing the robustness of our approach. Our proposed technique allows not only for the retrieval of the path-integrated fields, but also of the statistical properties of the fields.

physics.plasm-ph↗

arXiv2019

QED-driven laser absorption

Absorption covers the physical processes which convert intense photon flux into energetic particles when a high-power laser illuminates optically-thick matter. It underpins important petawatt-scale applications today, e.g., medical-quality proton beam production. However, development of ultra-high-field applications has been hindered since no study so far has described absorption throughout the entire transition from the classical to the quantum electrodynamical (QED) regime of plasma physics. Here we present a model of absorption that holds over an unprecedented six orders-of-magnitude in optical intensity and lays the groundwork for QED applications of laser-driven particle beams. We demonstrate 58% efficient γ-ray production at $1.8\times 10^{25}~\mathrm{W~ cm^{-2}}$ and the creation of an anti-matter source achieving $4\times 10^{24}\ \mathrm{positrons}\ \mathrm{cm^{-3}}$, $10^{6}~\times$ denser than of any known photonic scheme. These results will find applications in scaled laboratory probes of black hole and pulsar winds, γ-ray radiography for materials science and homeland security, and fundamental nuclear physics.

physics.plasm-ph↗

arXiv2019

Axion-like-particle decay in strong electromagnetic backgrounds

The decay of a massive pseudoscalar, scalar and U(1) boson into an electron-positron pair in the presence of strong electromagnetic backgrounds is calculated. Of particular interest is the constant-crossed-field limit, relevant for experiments that aim to measure high-energy axion-like-particle conversion into electron-positron pairs in a magnetic field. The total probability depends on the quantum nonlinearity parameter - a product of field and lightfront momentum invariants. Depending on the seed particle mass, different decay regimes are identified. In the below-threshold case, we find the probability depends on a non-perturbative tunnelling exponent depending on the quantum parameter and the particle mass. In the above-threshold case, we find that when the quantum parameter is varied linearly, the probability oscillates nonlinearly around the spontaneous decay probability. A strong-field limit is identified in which the threshold is found to disappear. In modelling the fall-off of a quasi-constant-crossed magnetic field, we calculate probabilities beyond the constant limit and investigate when the decay probability can be regarded as locally constant.

hep-ph↗

arXiv2018

Inverse Problem Instabilities in Large-Scale Plasma Modelling

Our understanding of physical systems generally depends on our ability to match complex computational modelling with 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 machine learning can be used to address inverse problem instabilities, and apply it to two popular experimental diagnostics in plasma physics. We find that the extraction of information from measurements simply on the basis of agreement with simulations 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.

physics.plasm-ph↗

arXiv2018

Setup for meV-resolution inelastic X-ray scattering measurements at the Matter in Extreme Conditions Endstation at the LCLS

We describe a setup for performing inelastic X-ray scattering measurements at the Matter in Extreme Conditions (MEC) endstation of the Linac Coherent Light Source (LCLS). This technique is capable of performing high-, meV-resolution measurements of dynamic ion features in both crystalline and non-crystalline materials. A four-bounce silicon (533) monochromator was used in conjunction with three silicon (533) diced crystal analyzers to provide an energy resolution of ~50 meV over a range of ~500 meV in single shot measurements. In addition to the instrument resolution function, we demonstrate the measurement of longitudinal acoustic phonon modes in polycrystalline diamond. Furthermore, this setup may be combined with the high intensity laser drivers available at MEC to create warm dense matter, and subsequently measure ion acoustic modes.

physics.ins-det↗

arXiv2017

Proton imaging of stochastic magnetic fields

Recent laser-plasma experiments report the existence of dynamically significant magnetic fields, whose statistical characterisation is essential for understanding 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. We conclude that 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 - which quantifies the relative size of the correlation length of the stochastic field, proton displacements due to magnetic deflections, and the image magnification. 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, as well as limitations currently placed by experimental capabilities on extracting magnetic field statistics. The results presented in this paper provide a comprehensive description of proton images of stochastic magnetic fields, with applications for improved analysis of given proton-flux images.

physics.plasm-ph↗

arXiv2017

Axion-like particle production in a laser-induced dynamical spacetime

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 massive complex scalar or pseudo-scalar field in this spacetime and derive the production rate of electrically neutral, spinless particle-antiparticle pairs. We show that this approach can provide an alternative experimental method to axion searches.

hep-ph↗

arXiv2017

Perovskite Substrates Boost the Thermopower of Cobaltate Thin Films at High Temperatures

Transition metal oxides are promising candidates for thermoelectric applications, because they are stable at high temperature and because strong electronic correlations can generate large Seebeck coefficients, but their thermoelectric power factors are limited by the low electrical conductivity. We report transport measurements on Ca3Co4O9 films on various perovskite substrates and show that reversible incorporation of oxygen into SrTiO3 and LaAlO3 substrates activates a parallel conduction channel for p-type carriers, greatly enhancing the thermoelectric performance of the film-substrate system at temperatures above 450 °C. Thin-film structures that take advantage of both electronic correlations and the high oxygen mobility of transition metal oxides thus open up new perspectives for thermopower generation at high temperature.

cond-mat.str-el↗

arXiv2017

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

Magnetic fields are ubiquitous in the Universe. Extragalactic disks, halos and clusters have consistently been shown, via diffuse radio-synchrotron emission and Faraday rotation measurements, to exhibit magnetic field strengths ranging from a few nG to tens of $μ$G. 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. 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 of the Universe.

physics.plasm-ph↗

arXiv2017

Numerical modeling of laser-driven experiments aiming to demonstrate magnetic field amplification via turbulent dynamo

The universe is permeated by magnetic fields, with strengths ranging from a femtogauss in the voids between the filaments of galaxy clusters to several teragauss in black holes and neutron stars. The standard model behind cosmological magnetic fields is the nonlinear amplification of seed fields via turbulent dynamo to the values observed. We have conceived experiments that aim to demonstrate and study the turbulent dynamo mechanism in the laboratory. Here we describe the design of these experiments through simulation campaigns using FLASH, a highly capable radiation magnetohydrodynamics code that we have developed, and large-scale three-dimensional simulations on the Mira supercomputer at Argonne National Laboratory. The simulation results indicate that the experimental platform may be capable of reaching a turbulent plasma state and study dynamo amplification. We validate and compare our numerical results with a small subset of experimental data using synthetic diagnostics.

physics.plasm-ph↗