Gianluca Gregori

Abstract:

A detailed knowledge of the physical phenomena underlying the transport of fast electrons generated in high-intensity laser-matter interactions is of fundamental importance for the fast ignition scheme for inertial confinement fusion. The fast electron currents largely exceed the Alfven limit, therefore a balancing return current is required to support the propagation of the fast electron beam in the target medium. An experimental study aimed at investigating the role of the return current on the dynamics of the fast electron beam was carried out with the Vulcan Petawatt beam. Two counter-propagating electron beams were generated by double-sided irradiation of a layered target containing a 5 micron thick Ti layer. Information on the energy coupling of the fast electron beam to the Ti layer was retrieved through X-ray measurements. In particular, high-resolution X-ray spectroscopy of the Ti emission lines was performed in the spectral range from 4.4 to 5.1 keV including the Lyα, the Heα and the Kα line. Spectra were acquired for double-sided irradiation with different timings between the two laser beams as well as for single-sided irradiation. The recorded spectra indicate a higher target temperature for a precise timing between the two beams in agreement with simulation results.

Abstract:

We review recent experimental results on the path to producing electron-positron pair plasmas using lasers. Relativistic pair-plasmas and jets are believed to exist in many astrophysical objects and are often invoked to explain energetic phenomena related to Gamma Ray Bursts and Black Holes. On earth, positrons from radioactive isotopes or accelerators are used extensively at low energies (sub-MeV) in areas related to surface science positron emission tomography and basic antimatter science. Experimental platforms capable of producing the high-temperature pair-plasma and high-flux jets required to simulate astrophysical positron conditions have so far been absent. In the past few years, we performed extensive experiments generating positrons with intense lasers where we found that relativistic electron and positron jets are produced by irradiating a solid gold target with an intense picosecond laser pulse. The positron temperatures in directions parallel and transverse to the beam both exceeded 0.5 MeV, and the density of electrons and positrons in these jets are of order 1016 cm-3 and 1013 cm-3, respectively. With the increasing performance of high-energy ultra-short laser pulses, we expect that a high-density, up to 1018 cm-3, relativistic pair-plasma is achievable, a novel regime of laboratory-produced hot dense matter. © 2011 Elsevier B.V.

Abstract:

Here we present results from ultra-intense experiments demonstrating the viability of polarization spectroscopy as a diagnostic of the electron return current and spatial anisotropy and distribution function of the fast electron beam. The measurements extend to ultra-relativistic intensities of 10 21 W cm-2, including laserplasma interaction regimes important for fast ignition studies, for example HiPER, and the development of secondary sources from next generation ultra-short pulse, ultra-intense laser facilities such as Astra-Gemini and ELI. As an in situ diagnostic, spectroscopic measurements are vital to understanding fast electron beams, enabling extrapolation of results to define fast ignition inertial confinement fusion and secondary source facilities. © 2011 Elsevier B.V.

Abstract:

Using numerical simulations, we investigate the equilibrium dynamics of a single-component fluid with Yukawa interaction potential. We show that, for a wide range of densities and temperatures, the dynamics of the system are in striking agreement with a simple model of generalized hydrodynamics. Since the Yukawa potential can describe the ion-ion interactions in a plasma, our results have significant applicability for both analyzing and interpreting the results of x-ray scattering data from high-power lasers and fourth-generation light sources. © 2011 American Physical Society.