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

Coefficient of thermal expansion of nanostructured tungsten based coatings assessed by thermally induced substrate curvature method

The in plane coefficient of thermal expansion (CTE) and the residual stress of nanostructured W based coatings are extensively investigated. The CTE and the residual stresses are derived by means of an optimized ad-hoc developed experimental setup based on the detection of the substrate curvature by a laser system. The nanostructured coatings are deposited by Pulsed Laser Deposition. Thanks to its versatility, nanocrystalline W metallic coatings, ultra-nano-crystalline pure W and W-Tantalum coatings and amorphous-like W coatings are obtained. The correlation between the nanostructure, the residual stress and the CTE of the coatings are thus elucidated. We find that all the samples show a compressive state of stress that decreases as the structure goes from columnar nanocrystalline to amorphous-like. The CTE of all the coatings is higher than the one of the corresponding bulk W form. In particular, as the grain size shrinks, the CTE increases from 5.1 10$^{-6}$ K$^{-1}$ for nanocrystalline W to 6.6 10$^{-6}$ K$^{-1}$ in the ultra-nano-crystalline region. When dealing with amorphous W, the further increase of the CTE is attributed to a higher porosity degree of the samples. The CTE trend is also investigated as function of materials stiffness. In this case, as W coatings become softer, the easier they thermally expand.Comment: The research leading to these results has also received funding from the European Research Council Consolidator Grant ENSURE (ERC-2014-CoG No. 647554

Thermomechanical properties of amorphous metallic tungsten-oxygen and tungsten-oxide coatings

In this work, we investigate the correlation between morphology, composition, and the mechanical properties of metallic amorphous tungsten-oxygen and amorphous tungsten-oxide films deposited by Pulsed Laser Deposition. This correlation is investigated by the combined use of Brillouin Spectroscopy and the substrate curvature method. The stiffness of the films is strongly affected by both the oxygen content and the mass density. The elastic moduli show a decreasing trend as the mass density decreases and the oxygen-tungsten ratio increases. A plateaux region is detected in correspondence of the transition between metallic and oxide films. The compressive residual stresses, moderate stiffness and high local ductility that characterize compact amorphous tungsten-oxide films make them promising for applications involving thermal or mechanical loads. The coefficient of thermal expansion is quite high (i.e. 8.9 $\cdot$ 10$^{-6}$ K$^{-1}$), being strictly correlated to the amorphous structure and stoichiometry of the films. Under thermal treatments they show a quite low relaxation temperature (i.e. 450 K). They crystallize into the $\gamma$ monoclinic phase of WO$_3$ starting from 670 K, inducing an increase by about 70\% of material stiffness.Comment: The research leading to these results has also received funding from the European Research Council Consolidator Grant ENSURE (ERC-2014-CoG No. 647554). The views and opinions expressed herein do not necessarily reflect those of the European Commissio

Wavelet entropy and fractional Brownian motion time series

We study the functional link between the Hurst parameter and the Normalized Total Wavelet Entropy when analyzing fractional Brownian motion (fBm) time series--these series are synthetically generated. Both quantifiers are mainly used to identify fractional Brownian motion processes (Fractals 12 (2004) 223). The aim of this work is understand the differences in the information obtained from them, if any.Comment: 10 pages, 2 figures, submitted to Physica A for considering its publicatio

Electron heating in subpicosecond laser interaction with overdense and near-critical plasmas

n this work we investigate electron heating induced by intense laser interaction with micrometric flat solid foils in the context of laser-driven ion acceleration. We propose a simple law to predict the electron temperature in a wider range of laser parameters with respect to commonly used existing models. An extensive two-dimensional (2D) and 3D numerical campaign shows that electron heating is due to the combined actions of j×B and Brunel effect. Electron temperature can be well described with a simple function of pulse intensity and angle of incidence, with parameters dependent on pulse polarization. We then combine our model for the electron temperature with an existing model for laser-ion acceleration, using recent experimental results as a benchmark. We also discuss an exploratory attempt to model electron temperature for multilayered foam-attached targets, which have been proven recently to be an attractive target concept for laser-driven ion acceleration

Numerical investigation of non-linear inverse Compton scattering in double-layer targets

Non-linear inverse Compton scattering (NICS) is of significance in laser-plasma physics and for application-relevant laser-driven photon sources. Given this interest, we investigated this synchrotron-like photon emission in a promising configuration achieved when an ultra-intense laser pulse interacts with a double-layer target (DLT). Numerical simulations with two-dimensional particle-in-cell codes and analytical estimates are used for this purpose. The properties of NICS are shown to be governed by the processes characterizing laser interaction with the near-critical and solid layers composing the DLT. In particular, electron acceleration, laser focusing in the low-density layer, and pulse reflection on the solid layer determine the radiated power, the emitted spectrum, and the angular properties of emitted photons. Analytical estimates, supported by simulations, show that quantum effects are relevant at laser intensities as small as similar to 1 0 21 W/cm(2) Target and laser parameters affect the NICS competition with bremsstrahlung and the conversion efficiency and average energy of emitted photons. Therefore, DLT properties could be exploited to tune and enhance photon emission in experiments and future applications

LDL receptor expression on T lymphocytes in old patients with Down syndrome

BACKGROUND: In Down syndrome patients several metabolic abnormalities have been reported, some involving the lipid metabolism. The level of LDL in plasma is the major determinant of the risk of vascular disease. There appear to be no studies on the LDL receptor in Down syndrome patients. METHODS: Flow cytometric methods for measuring the LDL receptor in peripheral blood mononuclear cells (PBMC) can identify patients with hypercholesterolemia. We applied this method in 19 old patients with Down syndrome and 23 healthy controls. RESULTS: Down syndrome patients had high levels of triglycerides and low levels of HDL, and high levels of CRP. We also found a down-regulation of LDL receptor expression. CONCLUSIONS: Down syndrome patients show no increase in the frequency of cardiovascular disease. The low incidence in cardiovascular disease despite the low level of HDL, high levels of CRP and reduction of LDL receptor expression lead to the conclusion that either these are not risk factors in these patients or that other risks factors – not yet identified – are considerably lower

PMUTs Arrays for Structural Health Monitoring of Bolted-Joints

Micro-electro-mechanical systems (MEMS) have enabled new techniques for the miniaturization of sensors suitable for Structural Health Monitoring (SHM) applications. In this study, MEMS-based sensors, specifically Piezoelectric Micromachined Ultrasonic Transducers (PMUT), are used to evaluate and monitor the pre-tensioning of a bolted joint structural system. For bolted joints to function properly, it is essential to maintain a suitable level of pre-tensioning. In this work, an array of PMUTs attached to the head and to the end of a bolt, serve as transmitter and receiver, respectively, in a pitch-catch Ultrasonic Testing (UT) scenario. The primary objective is to detect the Change in Time of Flight (CTOF) of the acoustic wave generated by the PMUT array and propagating along the bolt’s axis between a non-loaded bolt and a bolt in service. To model the pre-tensioning of bolted joints and the transmission of the acoustic wave to and from a group of PMUTs through the bolt, a set of numerical models is created. The CTOF is found to be linearly related to the amount of pre-tensioning. The numerical model is validated through comparisons with the results of a preliminary experimental campaign

Laser-driven production with advanced targets of Copper-64 for medical applications

Radionuclides are of paramount importance in nuclear medicine both for clinical uses and radiopharmaceutical production. Among the others, nuclides suitable for theranostics like Copper-64 are particularly attractive since they can play both a diagnostic and therapeutic role. In the last years, the growing demand for these nuclides stimulated the research of new solutions, along with cyclotrons already in use, for their production. In this respect, a promising alternative is laser-driven proton accelerators based on the interaction of superintense laser pulses with target materials. Because of their potential compactness and flexibility, they are under investigation for several applications ranging from materials science to nuclear medicine. Moreover, the use of advanced Double-Layer targets (DLTs) was identified as a viable route to increase the number and energy of the accelerated protons to satisfy the requirements of demanding applications. In this contribution, we numerically investigate the use of DLT-based laser-driven sources for Copper-64 production. We show that activities relevant to pre-clinical studies can be achieved with an existing 150 TW laser and DLTs. Moreover, we extend the discussion by considering a broad range of laser systems by exploiting a theoretical model. Our results can guide the choice of laser and target parameters for future experimental investigations