Ruggero Barni

We discuss an innovative thin film deposition method, Plasma Assisted Supersonic Jet Deposition, which combines the chemistry richness of a reactive cold plasma environment and the assembly control of the film growth allowed by a supersonic jet directed at the substrate. Optical Emission Spectroscopy was used to characterize the plasma state and the supersonic jet, together with its interaction with the substrate. We obtained several results in the deposition of silicon oxide thin films from Hexamethyldisiloxane, with different degrees of organic groups retention. In particular we exploited the features of emission spectra to measure the plasma dissociation and oxidation degree of the organic groups, as a function of the jet parameters. If controlled growth is achieved, such films are nanostructured materials suitable for applications like catalysis, photo catalysis, energy conversion and storage, besides their traditional uses as a barrier or protective coatings.

Plasma actuators have been proposed as a tool to produce hydrodynamical effects in the boundary layer of aerodynamical flows. We have analyzed some properties of these systems using suitable plasma diagnostics based on the emissivity characteristics of such plasmas. The direction and the velocity of propagation of the ionizing wave spreading on the dielectric surface were measured (in the 100–200 km/s range), and it was demonstrated that it behaves like a cathode-directed streamer. The averaged electron temperature (4–5 eV) and the reduced field strength (E/N ≈ 6 × 10¹⁹ V·m²) of the ionizing wave switching the discharges on were measured, too.

We present results obtained from the numerical simulation of the gas-phase chemical kinetics in atmospheric pressure air non-equilibrium plasmas. In particular, we addressed the effect of the pulsed operation mode of a planar dielectric barrier discharge. As conjectured, the large difference in the time scales involved in the fast dissociation of molecules in plasmas and their subsequent reactions to produce stable chemical species makes the presence of a continuously repeated plasma production stage unnecessary and a waste of electrical power and efficiency. The results on NOx remediation, ozone production, water vapor and ammonia dissociation are discussed. A few comparisons with experimental findings in a dielectric barrier discharge reactor already used for applications are also briefly addressed. Our results clearly indicate a pattern for the optimization of the discharge using a carefully designed repetition rate and duty cycle.

We presents results obtained from the numerical simulation of the gas-phase chemical kinetics in atmospheric pressure air non-equilibrium plasmas. In particular we have addressed the effect of pulsed operation mode of a planar dielectric barrier discharge. As it was conjectured, the large difference in the time scales involved in the fast dissociation of molecules in plasmas and their subsequent reactions to produce stable chemical species, makes the presence of a continuously repeated plasma production stage unnecessary and a waste of electrical power and efficiency. Results on NOx remediation, ozone production, water vapor and ammonia dissociation are discussed. A few comparisons with experimental findings in a dielectric barrier discharge reactor already used for applications is briefly addressed too. Our results clearly indicate a pattern for the optimization of the discharge using a carefully designed repetition rate and duty-cycle.

An experimental study of the plasma gas phase in low pressure radiofrequency discharges of oxygen is presented. The plasma phase has been studied by means of mass spectroscopy of the neutral and of the charged species, directly sampling the plasma gas phase. We also measured the ion energy distributions. We have studied the influence of the operating conditions on the plasma gas-phase composition. Ion density and energy have been measured as a function of discharge parameters. In particular, we have identified operating conditions in low pressure discharges allowing the extraction of ions from the plasma state with energies exceeding 50 eV. This makes plasma processing with energetic oxygen ions feasible in our device.

A steady regime dominated by intermittent blob and hole structures is identified in the plasma state of a simple magnetized torus by achieving a quasi-stationary equilibrium using an open magnetic field line configuration. The open helical field line configuration is characterized by a connection length, L c = 2 a B φ B z ≳ 1750 cm, and pitch ratio, r B = B z B φ ≲ 0.01 . This is realized by superposing a vertical magnetic fied, B z , to the toroidal field, B φ , and the regime is achieved for B z ≤ 0.4 mT. The combined effect of plasma rotation, arising from a substantial radial electric field, together with an open field line, results in vertically elongated plasma profile and an asymmetric sheared poloidal flow. The analysis shows the existence of density fluctuations exhibiting universal statistical properties, dominated by non-Gaussian blob events in the edge region and holes in the core plasma, separated by a region ascribed as blob birth zone corresponding to a velocity shear layer. Two-dimensional conditional averaging analyses of fluctuations indicate that blobs form in the sheared layer, when the leading edge of an elongated coherent structure breaks off by differential stretching exerted by the background fluctuating field. Convection of this isolated blob out of the contour corresponding to the maximum radial electric field in the low field side, leads to its ejection while holes move along the same contour driven back into the main plasma. The corresponding potential structure shows counter-rotating E × B velocity field within oppositely charged structures, where the embedded electric field is consistent with the observed structure propagation. A comparison with cross-correlation analysis yields a similar conclusion except for a slight overestimation of the structure size and lifetime.

Optical emission spectroscopy was used to characterize the gas phase in a gliding arc tornado reactor used for plasma applications. Results concerning the transition between spark and fully developed arc regimes are presented, by comparing with the electrical characteristics of the discharge. An extended analysis of spectra of air and helium discharges mixed with water and hexamethyldisiloxane vapors was performed, with the identification of active radical species involved in the surface functionalization of polymeric nanoparticles.Graphic

We have analyzed the electrical characteristics of a plug probe as a function of the orientation relative to the local magnetic field. Our aim was to demonstrate that the probe characteristic, besides a better evaluation of the plasma potential, retains the capability to reliably estimate the electron temperature and density. This feature could be exploited with a multi-pin probe to measure simultaneously the fluctuations of all plasma parameters.