M. Passoni

Ion acceleration by superintense laser-plasma interaction

Ion acceleration driven by superintense laser pulses is attracting an impressive and steadily increasing effort. Motivations can be found in the potential for a number of foreseen applications and in the perspective to investigate novel regimes as far as available laser intensities will be increasing. Experiments have demonstrated in a wide range of laser and target parameters the generation of multi-MeV proton and ion beams with unique properties such as ultrashort duration, high brilliance and low emittance. In this paper we give an overview of the state-of-the art of ion acceleration by laser pulses as well as an outlook on its future development and perspectives. We describe the main features observed in the experiments, the observed scaling with laser and plasma parameters and the main models used both to interpret experimental data and to suggest new research directions.

Target normal sheath acceleration: theory, comparison with experiments and future perspectives

Ions can be effectively accelerated during the interaction of an ultra-intense ultra-short laser pulse irradiating a thin solid target via the so-called target normal sheath acceleration (TNSA) mechanism. One of the pivotal questions at this stage of the research is how to predict the properties of the accelerated ions, both from a fundamental point of view and in the light of foreseen applications. In this context, it is desirable to have a simple but reliable description to be used to extrapolate current results to future regimes, which will be made available in the near future, thanks to developments in laser technology. In this paper, the possible approaches for an analytical description of TNSA are discussed, and a theoretical TNSA model is developed. This model is then used to investigate the maximum ion energy as a function of laser parameters. Detailed comparisons with available experimental data and scaling laws are presented. In particular, the relative role played by both the laser pulse energy and irradiance in determining the ion features is investigated.

Thermoelectric properties of Bi–Te films with controlled structure and morphology

A study of the thermoelectric transport properties of Bi–Te thin films with different structures and morphologies is here presented. Films were grown by pulsed laser deposition (PLD), which permits to control the composition, phase and crystallinity of the deposited material, and the morphology at the micrometer/nanometer scale. The carrier density and mobility at room temperature and the in plane electrical resistivity and Seebeck coefficient in the temperature range 300–400 K have been measured both for films characterized by a compact morphology and by the presence of different phases (Bi2Te3, BiTe, and Bi4Te3) and for Bi2Te3 films with different morphologies at the micrometer/nanometer scale (from a compact structure to a less connected assembly of randomly oriented crystalline grains). The correlation among thermoelectric and structural properties has been investigated, showing the potential of PLD to produce n-type Bi–Te thin films with desired properties for peculiar applications. Films with a laye...

Evidence of resonant surface-wave excitation in the relativistic regime through measurements of proton acceleration from grating targets.

The interaction of laser pulses with thin grating targets, having a periodic groove at the irradiated surface, is experimentally investigated. Ultrahigh contrast (~10(12)) pulses allow us to demonstrate an enhanced laser-target coupling for the first time in the relativistic regime of ultrahigh intensity >10(19) W/cm(2). A maximum increase by a factor of 2.5 of the cutoff energy of protons produced by target normal sheath acceleration is observed with respect to plane targets, around the incidence angle expected for the resonant excitation of surface waves. A significant enhancement is also observed for small angles of incidence, out of resonance.

Relativistic electromagnetic solitons in a warm quasineutral electron–ion plasma

The one-dimensional model for the interaction of electromagnetic (EM) waves of relativistic amplitude with a multicomponent hot plasma developed in a previous paper [M. Lontano et al., Phys. Plasmas 9, 2562 (2002)] is applied to the case of an electron–ion plasma. It is assumed that the plasma responds to the presence of large amplitude EM fields by retaining its quasineutrality, that is |Ne−ZNi|/Ne≪1, where Ne and Ni are the electron and ion density, respectively, and Z is the ion charge state. Contrary to what happens with drifting solitons, it is found that standing solitons admit relativistic and ultrarelativistic amplitudes, depending on the plasma temperature. Moreover, it is shown that even in “cold” plasmas the finite temperature directly determines the intensity and the shape of the localized solutions. Large amplitude solitons are found also in the case of different electron and ion temperatures. In addition, the “penetration depth” of an EM wave in a relativistic plasma is discussed, and scalin...

A kinetic model for the one-dimensional electromagnetic solitons in an isothermal plasma

Two nonlinear second order differential equations for the amplitude of the vector potential and for the electrostatic potential are derived, starting from the full Maxwell equations where the field sources are calculated by integrating in the momentum space the particle distribution function, which is an exact solution of the relativistic Vlasov equation. The resulting equations are exact in describing a hot one-dimensional plasma sustaining a relativistically intense, circularly polarized electromagnetic radiation. The case of standing soliton-like structures in an electron–positron plasma is then investigated. It is demonstrated that at ultrarelativistic temperatures extremely large amplitude solitons can be formed in a strongly overdense plasma.

Nanostructured and amorphous-like tungsten films grown by pulsed laser deposition

An experimental investigation of nanostructured, micrometer-thick, tungsten films deposited by pulsed laser deposition is presented. The films are compact and pore-free, with crystal grain sizes ranging from 14 nm to less than 2 nm. It is shown how, by properly tailoring deposition rate and kinetic energy of ablated species, it is possible to achieve a detailed and separate control of both film morphology and structure. The role of the main process parameters, He background pressure, laser fluence, and energy, is elucidated. In contrast with W films produced with other PVD techniques, β-phase growth is avoided and the presence of impurities and contaminants, like oxygen, is not correlated with film structure. These features make these films interesting for the development of coatings with improved properties, like increased corrosion resistance and enhanced diffusion barriers.

Electrostatic field distribution at the sharp interface between high density matter and vacuum

Ultrahigh intensity lasers are proven to be particularly suitable for ion acceleration to energies above hundreds of keV and even in the multi MeV range, due to their interaction with either planar thin solid foils, or spherically symmetric targets. With reference to these problems, a quasistationary model is developed, where the Poisson equation for the electrostatic potential distribution at the sharp solid target-vacuum interface is solved for a nonrelativistic Maxwellian distribution of trapped electrons. Analytical solutions are given and ion acceleration in the relevant electrostatic field configurations is discussed.

Bulk Cr tips for scanning tunneling microscopy and spin-polarized scanning tunneling microscopy

A simple, reliable method for the preparation of bulk Cr tips for scanning tunneling microscopy (STM) is proposed and its potentialities in performing high-quality and high-resolution STM and spin-polarized STM (SP-STM) are investigated. Cr tips show atomic resolution on ordered surfaces. Contrary to what happens with conventional W tips, rest atoms of the Si(111)-7×7 reconstruction can be routinely observed, probably due to a different electronic structure of the tip apex. SP-STM measurements of the Cr(001) surface showing magnetic contrast are reported. Our results reveal that the peculiar properties of these tips can be suited in a number of STM experimental situations.

Advanced strategies for ion acceleration using high-power lasers

A short overview of laser–plasma acceleration of ions is presented. The focus is on some recent experimental results and the related theoretical work on advanced regimes. These latter include in particular target normal sheath acceleration using ultrashort low-energy pulses and structured targets, radiation pressure acceleration in both thick and ultrathin targets and collisionless shock acceleration in moderate density plasmas. For each approach, open issues and the need and potential for further developments are briefly discussed.