Gianluca Gregori

Abstract:

Creating non-equilibrium states of matter with highly unequal electron and lattice temperatures (T(ele)≠T(ion)) allows unsurpassed insight into the dynamic coupling between electrons and ions through time-resolved energy relaxation measurements. Recent studies on low-temperature laser-heated graphite suggest a complex energy exchange when compared to other materials. To avoid problems related to surface preparation, crystal quality and poor understanding of the energy deposition and transport mechanisms, we apply a different energy deposition mechanism, via laser-accelerated protons, to isochorically and non-radiatively heat macroscopic graphite samples up to temperatures close to the melting threshold. Using time-resolved x ray diffraction, we show clear evidence of a very small electron-ion energy transfer, yielding approximately three times longer relaxation times than previously reported. This is indicative of the existence of an energy transfer bottleneck in non-equilibrium warm dense matter.

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.