B. Paroli

Mapping the transverse coherence of the self amplified spontaneous emission of a free-electron laser with the heterodyne speckle method

The two-dimensional single shot transverse coherence of the Self-Amplified Spontaneous Emission of the SPARC_LAB Free-Electron Laser was measured through the statistical analysis of a speckle field produced by heterodyning the radiation beam with a huge number of reference waves, scattered by a suspension of particles. In this paper we report the measurements and the evaluation of the transverse coherence along the SPARC_LAB undulator modules. The measure method was demonstrated to be precise and robust, it does not require any a priori assumptions and can be implemented over a wide range of wavelengths, from the optical radiation to the x-rays.

Shape and size constraints on dust optical properties from the Dome C ice core, Antarctica

Mineral dust aerosol (dust) is widely recognized as a fundamental component of the climate system and is closely coupled with glacial-interglacial climate oscillations of the Quaternary period. However, the direct impact of dust on the energy balance of the Earth system remains poorly quantified, mainly because of uncertainties in dust radiative properties, which vary greatly over space and time. Here we provide the first direct measurements of the aerosol optical thickness of dust particles windblown to central East Antarctica (Dome C) during the last glacial maximum (LGM) and the Holocene. By applying the Single Particle Extinction and Scattering (SPES) technique and imposing preferential orientation to particles, we derive information on shape from samples of a few thousands of particles. These results highlight that clear shape variations occurring within a few years are hidden to routine measurement techniques. With this novel measurement method the optical properties of airborne dust can be directly measured from ice core samples, and can be used as input into climate model simulations. Based on simulations with an Earth System Model we suggest an effect of particle non-sphericity on dust aerosol optical depth (AOD) of about 30% compared to spheres, and differences in the order of ~10% when considering different combinations of particles shapes.

Asymmetric lateral coherence of betatron radiation emitted in laser-driven light sources

We show that the radiation emitted by betatron oscillations of a high-energy electron beam undergoing wake-field acceleration is endowed with peculiar coherence properties which deliver quantitative information about the electron trajectories. Such results are achieved by means of accurate numerical simulations and a simple geometrical model gives a clear physical interpretation.

Analogical optical modeling of the asymmetric lateral coherence of betatron radiation.

By exploiting analogical optical modeling of the radiation emitted by ultrarelativistic electrons undergoing betatron oscillations, we demonstrate peculiar properties of the spatial coherence through an interferometric method reminiscent of the classical Youngs double slit experiment. The expected effects due to the curved trajectory and the broadband emission are accurately reproduced. We show that by properly scaling the fundamental parameters for the wavelength, analogical optical modeling of betatron emission can be realized in many cases of broad interest. Applications to study the feasibility of future experiments and to the characterization of beam diagnostics tools are described.

A modified two-slit interferometer for characterizing the asymmetric lateral coherence of undulator radiation

Recently we have shown that the asymmetric lateral coherence of betatron radiation is characterized by peculiar properties that are evidenced with the analysis of the coherence factor of radiation. We extend such results to radiation emitted by ultra-relativistic proton beams in a 2-periods undulator. Results of a 2-dimensional simulation show that the real analysis of the asymmetric lateral coherence substantially improves the resolving power, thus transverse beam non-uniformities and the beam size can be measured at large distance.

Measurement of power spectral density of broad-spectrum visible light with heterodyne near field scattering and its scalability to betatron radiation.

We exploit the speckle field generated by scattering from a colloidal suspension to access both spatial and temporal coherence properties of broadband radiation. By applying the Wiener-Khinchine theorem to the retrieved temporal coherence function, information about the emission spectrum of the source is obtained in good agreement with the results of a grating spectrometer. Experiments have been performed with visible light. We prove more generally that our approach can be considered as a tool for modeling a variety of cases. Here we discuss how to apply such diagnostics to broad-spectrum betatron radiation produced in the laser-driven wakefield accelerator under development at SPARC LAB facility in Frascati.

Note: Nanosecond LED-based source for optical modeling of scintillators illuminated by partially coherent X-ray radiation.

We developed a broad-spectrum light source specifically designed to reproduce the temporal behavior of the optical pulses emitted by scintillators for X-ray detection. Nanosecond-to-millisecond pulses are generated through a fast circuit driving Light Emitting Diodes (LEDs) and are endowed with the peculiar time features of the most employed scintillators by means of a dedicated pulse shaping stage. We implement the light source for the optical modeling of the single-shot X-ray coherence measurements with near-field speckles generated by the scattering from colloidal suspensions (heterodyne near field speckle method). Moreover, we derive a rigorous scaling law that quantitatively relates visible and X-ray signal-to-noise ratios.

Two-dimensional mapping of the asymmetric lateral coherence of thermal light.

We report in this work the first experimental verification of the asymmetric lateral coherence which is a measurement of the spatio-temporal coherence by using a wide-band Young interference experiment with a fixed off-axis slit. We demonstrate the coherence properties through the measurement of the real part of the coherence factor of thermal light. We extend our recent results obtained for betatron and undulator radiations providing a robust experimental method for the two-dimensional mapping of the two-point correlation function of broadband radiation preserving the phase information. The proposed method can be used as a high-sensitivity alternative to traditional interferometry with quasi-monochromatic radiation.