E. Giuffreda

Polarization of plastic targets by laser ablation

Charge separation in plasmas produced on plastic targets by low laser irradiance, structure of the ion front, and the current of fast electrons expanding into the vacuum chamber ahead of ions are characterized. Of particular interest is the negative current flowing through the plastic targets to the grounded vacuum chamber during the period of laser-target interaction. The subsequent multi - peaked structure of positive target current is correlated with occurrence of double sheet layers. The late-time negative charging of targets provides evidence for production of very slow ions by ionization of neutrals ablated at the target crater by radiation from plasma produced by 23 ns excimer KrF laser. The experimental setting allowing the target current observation is discussed.

Analysis of selective laser cleaning of patina on bronze coins

The patina, is the result of a large number of chemical, electrochemical and physical processes which occur spontaneously during interaction of metal surfaces with the environment. In this work we want to analyze and remove the patina in artefacts, exposed to atmosphere for various decades. Here, experimental results about the laser cleaning of bronze coins by KrF (248 nm) and Nd:YAG (532 nm) lasers are reported. Both laser wavelengths were efficient to reduce the chlorine concentration on the surface of the coins more than 80 %, as demonstrated by Energy Dispersive X-Ray Fluorescence analyses.

High magnetic pulse by solenoids for intense ion beam transport

A high voltage pulser up to 50 kV, with a main pulse of 2 μs time duration, was realized using a fast high voltage capacitor of 150 nF (50 kV) connected to the ground by a homemade spark-gap. The circuit, contained in a metallic box to avoid irradiation, had short electric connections in order to reduce the inductances. The diagnostic was performed by two systems: a self-integrating Rogowski coil and an integrator connected to a current transformer, both suitable for pulses longer than 1 μs. To focus ion beams with energy reaching 40 keV, we made two solenoids having an inductance of 3.6 and 16 μH which were fed by the current generated by the pulser up to 12 kA, 1 μs at 50 kV of charging voltage of capacitor. The measurements determined a magnetic field at the solenoid center of 0.61 T for the more inductive solenoid and 1.14 T for the other one.

Mapping of acceleration field in FSA configuration of a LIS

The Front Surface Acceleration (FSA) obtained in Laser Ion Source (LIS) systems is one of the most interesting methods to produce accelerated protons and ions. We implemented a LIS to study the ion acceleration mechanisms. In this device, the plasma is generated by a KrF excimer laser operating at 248 nm, focused on an aluminum target mounted inside a vacuum chamber. The laser energy was varied from 28 to 56 mJ/pulse and focused onto the target by a 15 cm focal lens forming a spot of 0.05 cm in diameter. A high impedance resistive probe was used to map the electric potential inside the chamber, near the target. In order to avoid the effect of plasma particles investing the probe, a PVC shield was realized. Particles inevitably streaked the shield but their influence on the probe was negligible. We detected the time resolved profiles of the electric potential moving the probe from 4.7 cm to 6.2 cm with respect to the main target axis, while the height of the shield from the surface normal on the target symmetry center was about 3 cm. The corresponding electric field can be very important to elucidate the phenomenon responsible of the accelerating field formation. The behavior of the field depends on the distance x as 1/x1.85 with 28 mJ laser energy, 1/x1.77 with 49 mJ and 1/x1.74 with 56 mJ. The dependence of the field changes slightly for our three cases, the power degree decreases at increasing laser energy. It is possible to hypothesize that the electric field strength stems from the contribution of an electrostatic and an induced field. Considering exclusively the induced field at the center of the created plasma, a strength of some tenth kV/m could be reached, which could deliver ions up to 1 keV of energy. These values were justified by measurement performed with an electrostatic barrier.

An antibacterial coating obtained through implantation of titanium ions

Everyday life is exposed to the risks of contracting severe diseases due to the diffusion of severe pathogens. For this reason, efficient antimicrobial surfaces becomes a need of primary importance. In this work we report the first evidences of a new technique to synthesize an antibacterial coating on Ultra High Molecular Weight Polyethylene (UHMWPE)samples, based on a non-stoichiometric, visible light responsive, titanium oxide. The coating was obtained through laser ablation of a titanium target, then the resulting ions were accelerated and implanted on the samples. The samples where tested against a Staphylococcus aureus strain, in order to assay their antimicrobial efficacy. Results show that this treatment strongly discourages bacterial adhesion to the treated surfaces.