Francesco Giammanco

MEASUREMENT OF THE CALCIUM 1P1-1D2 TRANSITION RATE IN A LASER-COOLED ATOMIC-BEAM

The efficiency of laser cooling in a thermal beam of calcium has been investigated under different cooling schemes. Special attention has been paid to analysis of the loss factor represented by the decay of atoms excited by the cooling laser to the 1P1 state into the metastable 1D2 level. The cooling scheme at a fixed laser frequency makes possible a good estimation of the branching ratio R between the decay probability from the 1P1 level to the ground 1S0 state and to the metastable 1D2 state. The measured value of (1.0 ± 0.15) × 105 seems high enough to produce only a small reduction in the cooling efficiency.

Toward the optimization of double-pulse LIBS underwater: effects of experimental parameters on the reproducibility and dynamics of laser-induced cavitation bubble

Double-pulse laser-induced breakdown spectroscopy (LIBS) was recently proposed for the analysis of underwater samples, since it overcomes the drawbacks of rapid plasma quenching and of large continuum emission, typical of single-pulse ablation. Despite the attractiveness of the method, this approach suffers nevertheless from a poor spectroscopic reproducibility, which is partially due to the scarce reproducibility of the cavitation bubble induced by the first laser pulse, since pressure and dimensions of the bubble strongly affect plasma emission. In this work, we investigated the reproducibility and the dynamics of the cavitation bubble induced on a solid target in water, and how they depend on pulse duration, energy, and wavelength, as well as on target composition. Results are discussed in terms of the effects on the laser ablation process produced by the crater formation and by the interaction of the laser pulse with floating particles and gas bubbles. This work, preliminary to the optimization of the spectroscopic signal, provides an insight of the phenomena occurring during laser ablation in water, together with useful information for the choice of the laser source to be used in the apparatus.

Dendrimer-Capped Nanoparticles Prepared by Picosecond Laser Ablation in Liquid Environment

Fifth generation ethylendiamine-core poly(amidoamine) (PAMAM G5) is presented as an efficient capping agent for the preparation of metal and semiconductor nanoparticles by ps laser ablation in water. In particular, we describe results obtained with the fundamental, second and third harmonic of a ps Nd:YAG laser and the influence of laser wavelength and pulse energy on gold particle production and subsequent photofragmentation. In this framework, the role of the dendrimer and, in particular, its interactions with gold clusters and cations are accounted.

Preparation of small size palladium nanoparticles by picosecond laser ablation and control of metal concentration in the colloid

We assessed a method for the preparation of small, highly stable and unprotected Pd nanoparticles by picosecond laser ablation in 2-propanol. The nanoparticles can be extracted from 2-propanol by centrifugation and redispersed in water, where a strongly negative ζ-potential assures long term stability. The proposed procedure permits reduction of particle size down to 1.6nm and optimization of the Pd(0):Pd(II) ratio which, in the best cases, was of the order of 6:1. The increase of this ratio with ablation times has been correlated to the high temperature conversion of PdO to metallic Pd by a simple theoretical model. A study of the relationship between colloid absorption at 400nm and Pd concentration permitted the role of PdO in the determination of the UV-vis spectra to be clarified and the limits of the Mie theory for the evaluation of colloid concentration to be established. The absorption at 400nm can be used as a fast method to estimate the Pd content in the colloids, provided that a calibration of the ablation process is preliminarily performed.

Versatile second-harmonic interferometer with high temporal resolution and high sensitivity based on a continuous-wave Nd:YAG laser

A second-harmonic interferometer based on a CW Nd:YAG laser is presented. The versatile device measures the line-integrated dispersion at the fundamental and second-harmonic wavelengths. A temporal resolution of 1.25 micros with sensitivity of 1.3 mrad and a temporal resolution of 10 micros with sensitivity of 0.3 mrad are demonstrated for a laser power of 0.6 W. For a laser power of only 150 mW a temporal resolution of 10 micros with sensitivity of 1 mrad is reported.

Multiphoton absorption of solutions of polydiacetylene polyDCHD-HS measured using ps Z-scan at 1064 and 1500 nm

Abstract The nonlinear absorption of benzene and toluene solutions of polydiacetylene polyDCHD-HS was measured at λ=1064 and 1500 nm by using Z-scan and picosecond pulses with a trimmed Airy beam configuration. In the data analysis, we took into account both the saturation of the open aperture Z-scan traces occurring for high values of nonlinear absorption and the possible occurrence of cross-talk effects between nonlinear refraction and multiphoton absorption. The polymer exhibits three-photon absorption at both 1064 and 1500 nm. The molecular three-photon absorption coefficient at 1064 nm was σ 3 =1.8×10 −38 cm 6 / W 2 and σ 3 =2.3×10 −38 cm 6 / W 2 in toluene and benzene, respectively, while at 1500 nm it was σ 3 =1.5×10 −39 cm 6 / W 2 in toluene. On this basis, the optical limiting behavior of polyDCHD-HS in the near infrared range is also shown.

Laser ionization and time-resolved ion collection in cesium vapor

Abstract Cesium atoms are ionized by resonant two-photon pulsed laser absorption. The ion collection is investigated when the application of the collecting electric field is time delayed with respect to the laser pulse. It is shown that the time evolution of the atomic ion density is governed by a conversion of atomic ions to molecular ones. Owing to the large molecular recombination coefficient the ion density decreases exponentially. The conversion rate constants for collisions with cesium or argon atoms have been determined.

Perspectives for the high field approach in fusion research and advances within the Ignitor Program

The Ignitor Program maintains the objective of approaching D–T ignition conditions by incorporating systematical advances made with relevant high field magnet technology and with experiments on high density well confined plasmas in the present machine design. An additional objective is that of charting the development of the high field line of experiments that goes from the Alcator machine to the ignitor device. The rationale for this class of experiments, aimed at producing poloidal fields with the highest possible values (compatible with proven safety factors of known plasma instabilities) is given. On the basis of the favourable properties of high density plasmas produced systematically by this line of machines, the envisioned future for the line, based on novel high field superconducting magnets, includes the possibility of investigating more advanced fusion burn conditions than those of the D–T plasmas for which Ignitor is designed. Considering that a detailed machine design has been carried out (Coppi et al 2013 Nucl. Fusion 53 104013), the advances made in different areas of the physics and technology that are relevant to the Ignitor project are reported. These are included within the following sections of the present paper: main components issues, assembly and welding procedures; robotics criteria; non-linear feedback control; simulations with three-dimensional structures and disruption studies; ICRH and dedicated diagnostics systems; anomalous transport processes including self-organization for fusion burning regimes and the zero-dimensional model; tridimensional structures of the thermonuclear instability and control provisions; superconducting components of the present machine; envisioned experiments with high field superconducting magnets.

Influence of the photon orbital angular momentum on electric dipole transitions: negative experimental evidence

We describe an experiment of atomic spectroscopy devoted to ascertaining whether the orbital angular momentum (OAM) of photons has the same property of interacting with atoms or molecules as occurs for the spin angular momentum (SAM). In our experiment, rubidium vapors are excited by means of laser radiation with different combinations of OAM and SAM, particularly selected to inhibit or enhance the fluorescence according to the selection rules for the electric dipole transitions between the fundamental state and the first excited doublet. Our results clearly show that an electric-dipole-type transition is insensitive to the OAM value, and provide an original validation of a problem long debated in theoretical works.

New developments, plasma physics regimes and issues for the Ignitor experiment

The scientific goal of the Ignitor experiment is to approach, for the first time, the ignition conditions of a magnetically confined D–T plasma. The IGNIR collaboration between Italy and Russia is centred on the construction of the core of the Ignitor machine in Italy and its installation and operation within the Triniti site (Troitsk). A parallel initiative has developed that integrates this programme, involving the study of plasmas in which high-energy populations are present, with ongoing research in high-energy astrophysics, with a theory effort involving the National Institute for High Mathematics, and with INFN and the University of Pisa for the development of relevant nuclear and optical diagnostics. The construction of the main components of the machine core has been fully funded by the Italian Government. Therefore, considerable attention has been devoted towards identifying the industrial groups having the facilities necessary to build these components. An important step for the Ignitor programme is the adoption of the superconducting MgB2 material for the largest poloidal field coils (P14) that is compatible with the He-gas cooling system designed for the entire machine. The progress made in the construction of these coils is described. An important advance has been made in the reconfiguration of the cooling channels of the toroidal magnet that can double the machine duty cycle. A facility has been constructed to test the most important components of the ICRH system at full scale, and the main results of the tests carried out are presented. The main physics issues that the Ignitor experiment is expected to face are analysed considering the most recent developments in both experimental observations and theory for weakly collisional plasma regimes. Of special interest is the I-regime that has been investigated in depth only recently and combines advanced confinement properties with a high degree of plasma purity. This is a promising alternative to the high-density L-regime that had been observed by the Alcator experiment and whose features motivated the Ignitor project. The provisions that are incorporated in the machine design, and in that of the plasma chamber in particular, in order to withstand or prevent the development of macroscopic instabilities with deleterious amplitudes are presented together with relevant analyses.