Publications by Gianluca Gregori
Nature communications 7 (2016) 10970-
The shock-induced transition from graphite to diamond has been of great scientific and technological interest since the discovery of microscopic diamonds in remnants of explosively driven graphite. Furthermore, shock synthesis of diamond and lonsdaleite, a speculative hexagonal carbon polymorph with unique hardness, is expected to happen during violent meteor impacts. Here, we show unprecedented in situ X-ray diffraction measurements of diamond formation on nanosecond timescales by shock compression of pyrolytic as well as polycrystalline graphite to pressures from 19 GPa up to 228 GPa. While we observe the transition to diamond starting at 50 GPa for both pyrolytic and polycrystalline graphite, we also record the direct formation of lonsdaleite above 170 GPa for pyrolytic samples only. Our experiment provides new insights into the processes of the shock-induced transition from graphite to diamond and uniquely resolves the dynamics that explain the main natural occurrence of the lonsdaleite crystal structure being close to meteor impact sites.
PHYSICAL REVIEW E 93 (2016) ARTN 033201
NATURE PHOTONICS 9 (2015) 274-279
Developed turbulence and nonlinear amplification of magnetic fields in laboratory and astrophysical plasmas.
Proceedings of the National Academy of Sciences of the United States of America 112 (2015) 8211-8215
The visible matter in the universe is turbulent and magnetized. Turbulence in galaxy clusters is produced by mergers and by jets of the central galaxies and believed responsible for the amplification of magnetic fields. We report on experiments looking at the collision of two laser-produced plasma clouds, mimicking, in the laboratory, a cluster merger event. By measuring the spectrum of the density fluctuations, we infer developed, Kolmogorov-like turbulence. From spectral line broadening, we estimate a level of turbulence consistent with turbulent heating balancing radiative cooling, as it likely does in galaxy clusters. We show that the magnetic field is amplified by turbulent motions, reaching a nonlinear regime that is a precursor to turbulent dynamo. Thus, our experiment provides a promising platform for understanding the structure of turbulence and the amplification of magnetic fields in the universe.
HIGH ENERGY DENSITY PHYSICS 17 (2015) 24-31
Investigation of the solid-liquid phase transition of carbon at 150 GPa with spectrally resolved X-ray scattering
High Energy Density Physics 14 (2015) 38-43
JOURNAL OF INSTRUMENTATION 10 (2015) ARTN P04010
PHYSICS OF PLASMAS 22 (2015) ARTN 056311
HIGH ENERGY DENSITY PHYSICS 14 (2015) 1-5
Nature communications 6 (2015) 6839-
A key component for the description of charged particle systems is the screening of the Coulomb interaction between charge carriers. First investigated in the 1920s by Debye and Hückel for electrolytes, charge screening is important for determining the structural and transport properties of matter as diverse as astrophysical and laboratory plasmas, nuclear matter such as quark-gluon plasmas, electrons in solids, planetary cores and charged macromolecules. For systems with negligible dynamics, screening is still mostly described using a Debye-Hückel-type approach. Here, we report the novel observation of a significant departure from the Debye-Hückel-type model in high-energy-density matter by probing laser-driven, shock-compressed plastic with high-energy X-rays. We use spectrally resolved X-ray scattering in a geometry that enables direct investigation of the screening cloud, and demonstrate that the observed elastic scattering amplitude is only well described within a more general approach.
Laboratory measurements of resistivity in warm dense plasmas relevant to the microphysics of brown dwarfs.
Nature communications 6 (2015) 8742-
Since the observation of the first brown dwarf in 1995, numerous studies have led to a better understanding of the structures of these objects. Here we present a method for studying material resistivity in warm dense plasmas in the laboratory, which we relate to the microphysics of brown dwarfs through viscosity and electron collisions. Here we use X-ray polarimetry to determine the resistivity of a sulphur-doped plastic target heated to Brown Dwarf conditions by an ultra-intense laser. The resistivity is determined by matching the plasma physics model to the atomic physics calculations of the measured large, positive, polarization. The inferred resistivity is larger than predicted using standard resistivity models, suggesting that these commonly used models will not adequately describe the resistivity of warm dense plasma related to the viscosity of brown dwarfs.
Evidence of locally enhanced target heating due to instabilities of counter-streaming fast electron beams
PHYSICS OF PLASMAS 22 (2015) ARTN 020701
The generation and amplification of intergalactic magnetic fields in analogue laboratory experiments with high power lasers
PHYSICS REPORTS-REVIEW SECTION OF PHYSICS LETTERS 601 (2015) 1-34
PHYSICS OF PLASMAS 22 (2015) ARTN 056307
NEW JOURNAL OF PHYSICS 17 (2015) ARTN 083051
Observation of magnetic field generation via the Weibel instability in interpenetrating plasma flows
NATURE PHYSICS 11 (2015) 173-176
JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS 48 (2015) ARTN 224004
Scientific reports 4 (2014) 5214-
Here, we report results of an experiment creating a transient, highly correlated carbon state using a combination of optical and x-ray lasers. Scattered x-rays reveal a highly ordered state with an electrostatic energy significantly exceeding the thermal energy of the ions. Strong Coulomb forces are predicted to induce nucleation into a crystalline ion structure within a few picoseconds. However, we observe no evidence of such phase transition after several tens of picoseconds but strong indications for an over-correlated fluid state. The experiment suggests a much slower nucleation and points to an intermediate glassy state where the ions are frozen close to their original positions in the fluid.
PHYSICS OF PLASMAS 21 (2014) ARTN 056302
NATURE PHYSICS 10 (2014) 520-524