Gianluca Gregori

Abstract:

We present a proposal for testing the prediction of non-equilibrium quantum field theory below the Schwinger limit. The proposed experiments should be able to detect a measurable number of gamma rays resulting from the annihilation of pairs in the focal spot of two opposing high intensity laser beams. We discuss the dependence of the expected number of gamma rays with the laser parameters and compare with the estimated background level of gamma hits for realistic laser conditions.

Abstract:

We investigate ultrafast (fs) electron dynamics in a liquid hydrogen sample, isochorically and volumetrically heated to a moderately coupled plasma state. Thomson scattering measurements using 91.8 eV photons from the free-electron laser in Hamburg (FLASH at DESY) show that the hydrogen plasma has been driven to a nonthermal state with an electron temperature of 13 eV and an ion temperature below 0.1 eV, while the free-electron density is 2.8x10{20} cm{-3}. For dense plasmas, our experimental data strongly support a nonequilibrium kinetics model that uses impact ionization cross sections based on classical free-electron collisions.

Abstract:

An experiment designed to launch laser-ablation-driven shock waves (10 to 70 Mbar) in a planar liquid-deuterium target on the OMEGA Laser System and to diagnose the shocked conditions using inelastic x-ray scattering is described. The electron temperature (Te) is inferred from the Doppler-broadened Compton-downshifted peak of the noncollective (αs = 1kλD > 1) x-ray scattering for Te > T Fermi. The electron density (ne) is inferred from the downshifted plasmon peak of the collective (αscatter > 1) x-ray scattering. A cylindrical layer of liquid deuterium is formed in a cryogenic cell with 8-μm-thick polyimide windows. The polyimide ablator is irradiated with peak intensities in the range of 1013 to 10 15 W/cm2 and shock waves are launched. Predictions from a 1-D hydrodynamics code show the shocked deuterium has a thickness of ∼0.1 mm with spatially uniform conditions. For the drive intensities under consideration, electron density up to ∼5 × 1023 cm -3 and electron temperature in the range of 10 to 25 eV are predicted. A laser-irradiated saran foil produces Cl Ly αemission. The spectrally resolved x-ray scattering is recorded at 90° for the noncollective scattering and at 40° for the collective scattering with a highly oriented pyrolytic graphite (HOPG) crystal spectrometer and an x-ray framing camera. © 2010 IOP Publishing Ltd.

Abstract:

Measurement of the dynamic properties of hydrogen and helium under extreme pressures is a key to understanding the physics of planetary interiors. The inelastic scattering signal from statically compressed hydrogen inside diamond anvil cells at 2.8 GPa and 6.4 GPa was measured at the Diamond Light Source synchrotron facility in the UK. The first direct measurement of the local field correction to the Coulomb interactions in degenerate plasmas was obtained from spectral shifts in the scattering data and compared to predictions by the Utsumi-Ichimaru theory for degenerate electron liquids. © 2010 IOP Publishing Ltd.

Abstract:

We report on soft X-ray scattering experiments on cryogenic hydrogen and simple metal samples. As a source of intense, ultrashort soft X-ray pulses we have used free-electron laser radiation at 92 eV photon energy from FLASH at DESY, Hamburg. X-ray pulses with energies up to 150 μJ and durations 15-50 fs provide interaction with the sample leading simultaneously to plasma formation and scattering. Experiments exploiting both of these interactions have been carried out, using the same experimental setup. Firstly, recording of soft X-ray inelastic scattering from near-solid density hydrogen plasmas at few electron volt temperatures confirms the feasibility of this diagnostics technique. Secondly, the soft X-ray excitation of few electron volt solid-density plasmas in bulk metal samples could be studied by recording soft X-ray line and continuum emission integrated over emission times from fs to ns. © 2009 Elsevier B.V.