L. Velardi

Production and acceleration of ion beams by laser ablation

In this work, we present a new pulsed laser ablation technique to obtain energetic ion beams. The accelerator we made is a compact device able to extract and accelerate the ionic components of plasma up to 160 keV per charge state. It is composed by a generating chamber containing an expansion chamber used like first electrode. Next, a second electrode connected to ground and a third electrode connected to negative voltage are used. The third electrode is used also as Faraday cup. By the analysis of the ion signals we studied the plume parameters such as TOF accelerated signals, charge state, and divergence.

Time-of-flight spectra for mapping of charge density of ions produced by laser

AbstractAspace-resolvedchargedensityofionsisderivedfromatime-resolvedcurrentofionsemittedfromlaser-producedplasmaandexpandedintothevacuumalongcollision-freeandfield-freepaths.Thisderivationisbasedonasimilarityrelationshipforioncurrentswith “frozen”charges observedatdifferentdistancesfromthetarget. This relationship makes itpossibletodetermine a map of ion charge density at selected times after the laser plasma interaction from signals of time-of-flightdetectors positioned at a certain distance from the target around a target-surface normal. In this work, we present mapsof the charge density of ions emitted from Cu and polyethylene plasmas. The mapping demonstrates that bursts of ionsare emitted at various ejection angles f n with respect to the target-surface normal. There are two basic directions f 1 and f 2 , one belonging to the fastest ions, i.e., protons and carbon ions, and the other one to the slowest ions being apart of each plasma plume.Keywords: Angular distribution of slow and fast ions; Ion expansion; Laser ion sources; Map of ion charge density;Modeling

Measurement of electromagnetic pulses generated during interactions of high power lasers with solid targets

A target irradiated with a high power laser pulse, blows off a large amount of charge and as a consequence the target itself becomes a generator of electromagnetic pulses (EMP) owing to high return current flowing to the ground through the target holder. The first measurement of the magnetic field induced by the neutralizing current reaching a value of a few kA was performed with the use of an inductive target probe at the PALS Laser Facility (Cikhardt et al. Rev. Sci. Instrum. 85 (2014) 103507). A full description of EMP generation should contain information on the spatial distribution and temporal variation of the electromagnetic field inside and outside of the interaction chamber. For this reason, we consider the interaction chamber as a resonant cavity in which different modes of EMP oscillate for hundreds of nanoseconds, until the EMP is transmitted outside through the glass windows and EM waves are attenuated. Since the experimental determination of the electromagnetic field distribution is limited by the number of employed antennas, a mapping of the electromagnetic field has to be integrated with numerical simulations. Thus, this work reports on a detailed numerical mapping of the electromagnetic field inside the interaction chamber at the PALS Laser Facility (covering a frequency spectrum from 100 MHz to 3 GHz) using the commercial code COMSOL Multiphysics 5.2. Moreover we carried out a comparison of the EMP generated in the parallelepiped-like interaction chamber used in the Vulcan Petawatt Laser Facility at the Rutherford Appleton Laboratory, against that produced in the spherical interaction chamber of PALS.

Exposure to static magnetic field stimulates quorum sensing circuit in luminescent Vibrio strains of the Harveyi clade.

In this study, the evidence of electron-dense magnetic inclusions with polyhedral shape in the cytoplasm of Harveyi clade Vibrio strain PS1, a bioluminescent bacterium living in symbiosis with marine organisms, led us to investigate the behavior of this bacterium under exposure to static magnetic fields ranging between 20 and 2000 Gauss. When compared to sham-exposed, the light emission of magnetic field-exposed bacteria growing on solid medium at 18°C ±0.1°C was increased up to two-fold as a function of dose and growth phase. Stimulation of bioluminescence by magnetic field was more pronounced during the post-exponential growth and stationary phase, and was lost when bacteria were grown in the presence of the iron chelator deferoxamine, which caused disassembly of the magnetic inclusions suggesting their involvement in magnetic response. As in luminescent Vibrio spp. bioluminescence is regulated by quorum sensing, possible effects of magnetic field exposure on quorum sensing were investigated. Measurement of mRNA levels by reverse transcriptase real time-PCR demonstrated that luxR regulatory gene and luxCDABE operon coding for luciferase and fatty acid reductase complex were significantly up-regulated in magnetic field-exposed bacteria. In contrast, genes coding for a type III secretion system, whose expression was negatively affected by LuxR, were down-regulated. Up-regulation of luxR paralleled with down-regulation of small RNAs that mediate destabilization of luxR mRNA in quorum sensing signaling pathways. The results of experiments with the well-studied Vibrio campbellii strain BB120 (originally classified as Vibrio harveyi) and derivative mutants unable to synthesize autoinducers suggest that the effects of magnetic fields on quorum sensing may be mediated by AI-2, the interspecies quorum sensing signal molecule.

Modification of polymer characteristics by laser and ion beam

In this work, two different techniques to modify polymeric surfaces are compared: laser irradiation and ion implantation. The treated polymers were samples of ultra high molecular weight polyethylene. The irradiation treatment was performed by utilizing two different laser sources operating in the UV and IR range by applying many laser shots in an air atmosphere. Ion implantation was performed using a new laser ion source accelerator with an accelerating voltage of 40 kV. Contact angle, roughness and Fourier transform infra red measurements were carried out before and after the treatments in order to compare surface characteristics. An increase of the wettability and roughness was observed when using UV laser treatment, while an increase of the hardness was obtained by ion implantation.

Bacterial bioluminescence and Gumbel statistics: From quorum sensing to correlation

We show that, in particular experimental conditions, the time course of the radiant fluxes, measured from a bioluminescent emission of a Vibrio harveyi related strain, collapse after suitable rescaling onto the Gumbel distribution of extreme value theory. We argue that the activation times of the strain luminous emission follow the universal behavior described by this statistical law, in spite of the fact that no extremal process is known to occur.

Ion emission from laser ablation of Cu and Cu98/Be2 alloy targets

The properties of pulse laser ablation of Cu and Cu98/Be2 materials are studied, and the differences in the emission of Cu ions are emphasized. The iodine high-power laser system PALS in Prague and a KrF laser were used to perform the experiments at the fundamental harmonics λ 0 = 1.315 μm and λ 0 = 248 nm delivering energy up to 500 J and 600 mJ, respectively. Pure Cu and Cu98/Be2 alloy targets of 50, 500 and 1000 μm thickness were ablated to measure the influence of the Be admixture on the emission of Cu ions. The alloy Cu98/Be2 was chosen due to the well-defined amount of a beryllium admixture in the plasma in contrast to the incidental amount of carbon, oxygen and hydrogen impurities chemisorbed on target surfaces. It was approved that the emission of Cu ions driven by the KrF laser exhibits a higher gain from the Cu98/Be2 plasma in contrast to the Cu plasma. The ion emission induced by laser intensities near the threshold of fast ion generation is significantly affected by the emission of ionized impurities chemisorbed on a target surface and by repetitive outbursts of fast ions if generated. Under these conditions, the influence of the 2% Be admixture on the emission of Cu ions plays only a minor role.

Generation of Si nanocrystals by ion implantation utilizing a LIS device

A newly developed ‘implantation machine’ was used to accelerate ions. Recently, the ability of composite materials containing silicon nanocrystals in SiO2 to emit in the visible region has become very attractive due to their potential applications. It has been studied that nanocrystals can be fabricated by a variety of methods, including even the ion implantation treatment followed by high-temperature annealing. We developed a laser ablation-induced plasma as ion source, which consists in an excimer laser of 248 nm and 20 ns. The success of the developed device was reached inserting a removable expansion chamber that allowed an initial free expansion of the plasma before the ion extraction reducing the probability of arcs during the acceleration. The target support is a stem mounted on an insulating flange and kept to positive high voltage. In this work, we present the preliminary experimental results of 40 keV energy implantation of Si ions into SiO2 films and the GAXRD analysis before and after 1000 ° C annealing.

Radiofrequency transmission line for bioluminescent Vibrio sp. irradiation

We present the study and the analyses of a transmission line for radiofrequency (RF) irradiation of bacteria belonging to Vibrio harveyi-related strain PS1, a bioluminescent bacterium living in symbiosis with many marine organisms. The bioluminescence represents a new biologic indicator which is useful for studying the behaviour of living samples in the presence of RF waves due to the modern communication systems. A suitable transmission line, used as an irradiating cell and tested up to the maximum frequency used by the global system for mobile communications and universal mobile telecommunications system transmissions, was characterized. In this experiment, the RF voltage applied to the transmission line was 1 V. Due to short dimensions of the line and the applied high frequencies, standing waves were produced in addition to progressing waves and the electric field strength varies particularly along the longitudinal direction. The magnetic field map was not strongly linked to the electric one due to the...

Characterization of laser-produced plasma of metal targets

This work has the purpose to characterize the plasma produced by solid target ablation. The experiment was performed by using a KrF laser with an irradiance of 108 W/cm2 and different solid targets, i.e., Si, Ge, Pd, and Pb. A very suitable Faraday cup has been utilized to study the angular distribution of the plasma plume as well as to perform measurements of the ion charge carried by the plasma during its free expansion into the vacuum. Particular attention was done to estimate the charge losses ascribed to the recombination processes during the plasma expansion. Different charge angular distributions have been found for the four laser-induced plasmas which suggest the dependence of the plasma characteristics with the different physics processes involved during the laser-target and laser–plasma interaction. These processes are responsible also of the duration of the recombination processes which results to be different according to the different plasma.