Particle acceleration and radiation friction effects in the filamentation instability of pair plasmas

The evolution of the filamentation instability produced by two counter-streaming pair plasmas is studied with particle-in-cell (PIC) simulations in both one (1D) and two (2D) spatial dimensions. Radiation friction effects on particles are taken into account. After an exponential growth of both the magnetic field and the current density, a nonlinear quasi-stationary phase sets up characterized by filaments of opposite currents. During the nonlinear stage, a strong broadening of the particle energy spectrum occurs accompanied by the formation of a peak at twice their initial energy. A simple theory of the peak formation is presented. The presence of radiative losses does not change the dynamics of the instability but affects the structure of the particle spectra.Comment: 8 pages, 8 figures, submitted to MNRA

Laser ion acceleration using a solid target coupled with a low density layer

We investigate by particle-in-cell simulations in two and three dimensions the laser-plasma interaction and the proton acceleration in multilayer targets where a low density "near-critical" layer of a few micron thickness is added on the illuminated side of a thin, high density layer. This target design can be obtained by depositing a "foam" layer on a thin metallic foil. The presence of the near-critical plasma strongly increases both the conversion efficiency and the energy of electrons and leads to enhanced acceleration of proton from a rear side layer via the Target Normal Sheath Acceleration mechanism. The electrons of the foam are strongly accelerated in the forward direction and propagate on the rear side of the target building up a high electric field with a relatively flat longitudinal profile. In these conditions the maximum proton energy is up to three times higher than in the case of the bare solid target.Comment: 9 pages, 11 figures. Submitted to Physical Review

Simulation of the laser-plasma acceleration for the PLASMONX project with the PIC code ALaDyn

In this paper we will briefly introduce laser–plasma acceleration for electrons and present some numerical simulations. The simulations have been performed to find a suitable working point for one of the test experiments of the INFN–CNR PLASMONX project. FLAME (Frascati laser for acceleration and multidisciplinary experiments), a 300 TW Ti:Sa laser, is being installed and commissioned at Laboratori Nazionali di Frascati (LFN). The first pilot experiment SITE (self-injection test experiment) is planned for this year (2010). The simulations have been run using a fully self-consistent particle-in-cell code AlaDyn (Acceleration by LAser and DYNamics of charged particles) developed and maintained at the Department of Physics at the University of Bologna within the PLAMSONX project

Reconstructing Late Quaternary Paleovalley Systems of Italy Through mHVSR: A Tool for Seismic Hazard Assessment in Modern Coastal Lowlands

Effective site characterization in highly urbanized coastal lowlands requires accurate stratigraphic and geophysical investigations. In these regions, which typically host shallowly buried paleovalley systems formed in response to Quaternary glacio-eustatic fluctuations, the marked lithologic contrast between soft sediment paleovalley fills and the adjacent, stiff substrate has the potential to modify earthquake motions, and assessment of critical parameters, such as shear wave velocities (VS) and resonance frequencies (f), should be coupled with detailed stratigraphic architecture. To evaluate the potential of the microtremor horizontal-to-vertical spectral ratio (mHVSR) for paleovalley recognition and mapping, we performed mHVSR measurements along the Adriatic coastal plain of Italy, where two paleovalley systems (Pescara and Manfredonia) have been recently identified. In both areas, we detected rapid lateral variations in resonance frequencies and highlighted laterally continuous impedance contrasts. Relying on a robust stratigraphic framework, we carefully evaluated the relation between geological and geophysical data and identified the stratigraphic surfaces responsible for the observed resonances. We derived VS models for the sediment fill, reconstructing the geometry of the two buried paleovalleys. We address the importance of evaluating the geological context when designing microzonation studies, for a reliable interpretation of changes in resonance frequencies.When earthquakes occur, buildings shake differently based on several factors, including seismic wave velocity, natural resonance frequencies, and local geological characteristics. Beneath modern coastal lowlands, the presence of paleovalley systems can significantly modify the ground motion. Identification of these buried bodies is therefore essential to assess and reduce seismic hazard. Paleovalleys are shallow incisions formed under periods of fluvial erosion in response to Quaternary climate fluctuations, and subsequently filled with very soft clay. These bodies are found worldwide, and do not have any geomorphological evidence, making their recognition challenging. Geologists typically use expensive sediment core analysis to identify paleovalleys, but this method can only provide spotty information. Geophysical exploration techniques that rely on microtremors (small vibrations on the Earth) can complement mapping of these buried bodies. In this work, we tested this technique in Pescara and Mafredonia (Adriatic coastal plain, Italy), providing dense information about paleovalley geometries and geophysical parameters crucial for predicting how the ground will shake during an earthquake. This study also highlights the importance of integrating disciplines to improve our understanding of subsoil and to design future studies to mitigate seismic hazards.Paleovalley fills are key sediment bodies made up of soft clay, tens of m thick and few km wide, buried beneath coastal lowlands worldwideMicrotremor-based paleovalley profiles and stratigraphic cross-sections exhibit strong similarityMicrotremor can provide shear wave velocities and resonance frequencies of paleovalleys, key parameters for seismic hazard mitigatio

Polarization Dependence of Bulk Ion Acceleration from Ultrathin Foils Irradiated by High-Intensity Ultrashort Laser Pulses

The acceleration of ions from ultrathin (10-100 nm) carbon foils has been investigated using intense (∼ 6 x1020 Wcm-2), ultrashort (45 fs) laser pulses, highlighting a strong dependence of the ion beam parameters on the laser polarization, with circularly polarized (CP) pulses producing the highest energies for both protons and carbons (25-30 MeV/nucleon); carbon ion energies obtained employing CP pulses were signicantly higher (∼2.5 times) than for irradiations employing linearly polarized (LP) pulses. Particle-in-cell simulations indicate that Radiation Pressure Acceleration becomes the dominant mechanism for the thinnest targets and CP pulses

Evidence of resonant surface wave excitation in the relativistic regime through measurements of proton acceleration from grating targets

The interaction of laser pulses with thin grating targets, having a periodic groove at the irradiated surface, has been experimentally investigated. Ultrahigh contrast ($\sim 10^{12}$) pulses allowed to demonstrate an enhanced laser-target coupling for the first time in the relativistic regime of ultra-high intensity >10^{19} \mbox{W/cm}^{2}. A maximum increase by a factor of 2.5 of the cut-off energy of protons produced by Target Normal Sheath Acceleration has been observed with respect to plane targets, around the incidence angle expected for resonant excitation of surface waves. A significant enhancement is also observed for small angles of incidence, out of resonance.Comment: 5 pages, 5 figures, 2nd version implements final correction

Relativistic surface plasmon enhanced harmonic generation from gratings

The role of relativistic surface plasmons (SPs) in high order harmonic emission from laser-irradiated grating targets has been investigated by means of particle-in-cell simulations. SP excitation drives a strong enhancement of the intensity of harmonics, particularly in the direction close to the surface tangent. The SP-driven enhancement overlaps with the angular separation of harmonics generated by the grating, which is beneficial for applications requiring monochromatic extreme ultraviolet (XUV) pulses

Extreme Ultraviolet Beam Enhancement by Relativistic Surface Plasmons

The emission of high-order harmonics in the extreme ultraviolet range from the interaction of a short, intense laser pulse with a grating target is investigated experimentally. When resonantly exciting a surface plasmon, both the intensity and the highest order observed for the harmonic emission along the grating surface increase with respect to a flat target. Harmonics are obtained when a suitable density gradient is preformed at the target surface, demonstrating the possibility to manipulate the grating profile on a nanometric scale without preventing the surface plasmon excitation. In support of this, the harmonic emission is spatiotemporally correlated to the acceleration of multi-MeV electron bunches along the grating surface. Particle-in-cell simulations reproduce the experimental results and give insight on the mechanism of high harmonic generation in the presence of surface plasmons

Evaluation of tsunamigenic hazard through numerical modeling from seismic and non-seismic sources in the Crotone offshore (Calabria, Southern Italy)

Tsunamis in the Mediterranean Sea can be considered among the major sources of hazard, both for the extension of the area that can be involved by the water impact and for the closeness of potential sources to the coast, which reduces dramatically the alert and evacuation time. Moreover, landslides, as other non-seismic tsunami sources, are often characterized by a lack of precursors (such as seismic shaking), a reason for which the ensuing waves are sometimes called “surprise tsunamis”

Development of foam-based layered targets for laser-driven ion beam production

We report on the development of foam-based double-layer targets (DLTs) for laser-driven ion acceleration. Foam layers with a density of a few mg cm-3 and controlled thickness in the 8-36 μm range were grown on μm-thick Al foils by pulsed laser deposition (PLD). The DLTs were experimentally investigated by varying the pulse intensity, laser polarisation and target properties. Comparing DLTs with simple Al foils, we observed a systematic enhancement of the maximum and average energies and number of accelerated ions. Maximum energies up to 30 MeV for protons and 130 MeV for C6+ ions were detected. Dedicated three-dimensional particle-in-cell (3D-PIC) simulations were performed considering both uniform and cluster-assembled foams to interpret the effect of the foam nanostructure on the acceleration process