As early as 1997, we decided to promote a short series of conferences devoted to a field that was showing dramatic progress and in which an increasing number of young researchers were participating. The Euroconference format of the “TMR Programme of the Commission of the European Union” was the most appropriate to fulfill the two objectives of a quasi real-time presentation and discussion of novel results and a tutorial support of young scientists from eminent lecturers active in the field. The success of the two meetings, held in two authentic capitals of the European cultural heritage, went well beyond any optimistic expectation. More than 200 enthusiastic participants, including approximately 100 young scientists, made those events a unique experience of scientific progress and debate. I wish to express my gratitude to Dimitris Charalambidis, who divided with me this experience, to the Scientific Advisory Committee, who guaranteed the high scientific level of the meetings, and to the Local Organizers, who made all this possible. The ULIA participants strongly appreciated the initiative of Laser and Particle Beams to edit two special issues with a collection of selected works presented at the ULIA meetings. Antonio Giulietti was the Scientific Promoter of the ULIA Conferences

By this time, we trust that you have received issues from Volume 18 of Laser and Particle Beams which used the new, large-page size, double-column format. As I indicated in an earlier preface, this format provides additional type-space, which will hopefully allow us to speed up publication of articles in LPB.

Detailed investigations of the propagation of an ultraintense picosecond laser pulse through preformed plasmas have been carried out. An underdense plasma with peak density around 0.1nc was generated by exploding a thin foil target with an intense nanosecond laser pulse. The formation of plasma channels with an ultraintense laser pulse due to ponderomotive expulsion of elections and the subsequent Coulomb explosion were investigated. The laser transmission through underdense plasmas was measured for a picosecond pulse at intensities above 1019 W/cm2 with and without a plasma channel preformed with an ultraintense prepulse. The energy transmitted through the plasma increased from the few percent transmittance measured in absence of the preformed channel to almost 100% transmission with the channelling to main pulse delay at around 100 ps. The propagation of a relativistic laser pulse through overdense plasmas was also investigated. A well-characterized plasma with an electron density up to 8nc was generated by soft X-ray irradiation of a low-density foam target. The propagation of the laser pulse was observed via X-ray imaging and monitoring the energy transmission through the plasma. Evidence of collimated laser transport was obtained.

Relatively small-scale laser-driven sources of short wavelength radiation covering a range from the extreme ultraviolet to the hard X-ray regime are now available. Because the duration of the X-ray pulses is comparable to, or shorter than the laser pulse width, it is possible to carry out X-ray measurements with picosecond or femtosecond time resolution.

Collective absorption, so far determined by numerical simulations, is explained in physical terms for cold and warm plasma. After deducing a few general relations for flat targets, interface phase mixing and nonadiabatic electron acceleration in the skin layer are identified as the main physical processes leading to irreversibility, that is, to absorption.

We report the results of measurements aimed to check the phase lock of time-delayed, collinear, harmonic pulses and to verify the possibility of performing Ramsey-like spectroscopy in the XUV. We demonstrate that for harmonics of medium order and for the peak intensities available with our laser system, the generation of collinear phase-locked harmonic pulses is indeed possible and that such pulses can be used to achieve high-resolution spectroscopy in the short-wavelength region.

In this paper, we present and compare the two different XUV interferometric techniques using high-order harmonics that have been developed so far. The first scheme is based on the interference between two spatially separated phase-locked harmonic sources while the second uses two temporally separated harmonic sources. These techniques have been applied to plasma diagnostics in feasibility experiments where electron densities up to a few 1020 e−/cm3 have been measured with a temporal resolution of 200 fs. We present the main characteristics of each technique and discuss their respective potentials and limitations.

The high-order harmonic generation process was studied using laser pump pulses of different duration, down to 7 fs. The time profile of this kind of pulse corresponds to a few optical cycles at the fundamental frequency, and therefore a cycle-to-cycle variation of the maximum amplitude occurs. These facts violate the adiabatic approximation of the generation process, giving rise to new aspects of the physical process. The comparison of results with pulses in the two regimes are reported and discussed. The modeling of the complete generation and propagation process in three dimensions is also presented.

A laser–plasma interaction experiment was performed in order to match effective conditions for wake-field electron acceleration. A 30-fs pulse was made to interact with a preformed plasma generated via the exploding foil technique from a 1-μm-thick plastic film. The irradiance of the femtosecond pulse in the plasma was 1020 Wcm−2. The interaction conditions were investigated via imaging and spectroscopy at the fundamental and the second harmonic of the laser frequency, both forward and backward. Our data clearly show that conditions suitable for electron acceleration are achieved close to the propagation axis and can be easily reproduced from shot to shot. In contrast, significant growth of instabilities occurs at the boundaries of the interaction region. These observations are consistent with a preliminary evidence of forward acceleration of high-energy electrons. Optical, X-ray, and γ-ray data obtained for different positions of the foil target with respect to the laser focal plane further support this promising scenario.

New fields of X-ray source applications (X-ray laser and high order harmonic generation) could appear if an intensity higher than 1012 Wcm−2 is reached. Following this goal, we have started a complete investigation of the X-ray beam wavefront both numerically and experimentally. The first XUV wavefront sensor has been developed and tested on different XUV sources. For a better comprehension of the experimental results, a numerical work (ray-trace code) has been performed. We present and discuss the first results obtained on the X-ray laser at 21.2 nm.

We present some experimental results which demonstrate the presence of electric inhibition in the propagation of relativistic electrons generated by intense laser pulses, depending on target conductivity. The use of transparent targets and shadowgraphic techniques has made it possible to evidence electron jets moving at the speed of light, an indication of the presence of self-generated strong magnetic fields.

We use both a Fourier-transform-based analysis and time-retrieval calculations to get time-resolved measurements of the reflectivity and the phase shift of a chirped probe pulse interacting with a fs-laser-produced plasma by spectral interferometry in a single shot. We have devised a novel dual-quadrature Mach–Zehnder configuration in which wavefront division is used to obtain spatial fringes from the perturbed and unperturbed probe simultaneously. We demonstrate the capability of this technique with fully analyzed experimental data on the interaction of a femtosecond laser with C3H6 and C samples, showing 35-fs time resolution.

The influence of absorption and ionization on high-order harmonic generation in a gas-filled hollow fiber is analyzed within the framework of slowly varying envelope approximation. Harmonic spectra and pressure dependencies are calculated for high-order harmonic generation in hollow fibers filled with different rare gases. Ionization of the gas filling the fiber gives rise to self-phase modulation of the pump pulse, changing the phase mismatch within the pump pulse and shifting the maxima in pressure dependencies of high-order harmonics.

Harmonics spectra in an n-type InP bulk semiconductor showing up to the 13th harmonic are reported. The external field is linearly polarized with the frequency ν = 100 GHz. Calculations are based on a Monte Carlo simulation for the electron motion in the conducting band and on an electrodynamics equation for the harmonics generation. The effect of a significant reduction of efficiency in the generation of particular harmonics is found, and is traced back to a sort of alternating field Gunn effect. A short analysis of the different physical mechanisms giving rise to harmonics generation is presented.

When a laser pulse impinges on a molecule which is invariant under certain symmetry operations selection rules for harmonic generation (HG) arise. In other words, symmetry controls which channels are open for the deposition and emission of laser energy—with the possible application of filtering or amplification. We review the derivation of HG selection rules and study numerically the interaction of laser pulses with an effectively one-dimensional ring-shaped model molecule. The harmonic yields obtained from that model and their dependence on laser frequency and intensity are discussed. In a real experiment obvious candidates for such molecules are benzene, other aromatic compounds, or even nanotubes.

Characteristic X-rays generated by high-intensity laser interaction with solids were investigated and used for determining the resulting hot electron populations. Spectrally as well as spatially resolved data are evaluated for this purpose. The experimental data were compared with Monte Carlo simulations to determine the electron energy distribution and geometric features of the electron beam. These results are in good agreement with PIC simulations. The self-generated low-resistivity channel of the electron beam results in a distinct difference in the generated rear-side X-ray spot when the electron beam propagates in an insulator rather than a metal. Self-generated electric fields prevent electron propagation into the vacuum. This fact is used for demonstrating photopumping of cobalt with copper Kα radiation, an experiment relevant to innershell X-ray laser schemes. The emission of cobalt is compared with that of nickel, which is not photopumped by copper Kα, and is found to be enhanced by a factor of 2.5.

Charge-exchange collisions are one of the effective pumping methods for soft X-ray lasers. Experiments are performed to investigate charge-exchange collisions between highly charged Mg ions in colliding laser-produced magnesium plasmas. Pinhole photography and XUV spectroscopy are used as diagnostic tools. Spectroscopic studies show selective population of n = 3 levels of Mg IX ions, which results in enhancement of respective line intensities. Theoretical calculations also give a large cross section as high as 10−15 cm2 for these charge-exchange collisions when the relative velocities of the colliding ions are of the order of 107 cm s−1. XUV pinhole pictures are taken in early stages, which give more insight into the expansion dynamics of the colliding magnesium plasmas.

Usage of a cavity with a high quality factor in a streak camera (SC) allows us to increase the energy of electrons up to 100 keV and therefore to reduce the influence of some of the effects that limit the SC time resolution. Time resolution up to 1.4 ps on a prototype of a radio-frequency-based SC (RFC) with circular scanning was demonstrated earlier. Following this, the new RFC was produced. The possibility of achieving a time resolution of 0.6 ps for a 350-ps single light signal is demonstrated. Preparation for experiments with spiral scanning (sub-ps time resolution for 10-ns process duration) is in progress.

We present some preliminary results on the equation of state (EOS) of water in a pressure regime of astrophysical interest. In the experiments, structured targets made of an aluminum step followed by a water layer are irradiated by the laser at an intensity up to 4·1014 W·cm−2 to generate a shock wave. Velocities are measured in the two materials using a VISAR interferometric diagnostic for water, and a streak camera to measure target self-emission for Al. EOS points for water are obtained with the impedance mismatch method using Al EOS as a reference. Water reflectivity was also measured.

High dynamic range, space-resolved X-ray spectra of an aluminum laser–plasma in the 5.5–8 Å range were obtained using a TlAP crystal and a cooled CCD camera as a detector. This technique was used to investigate the emission region in the longitudinal direction over a distance of approximately 350 μm from the solid target surface. These data show that the electron density profile varies by two orders of magnitude with the temperature ranging from about 180 eV in the overdense region to about 650 eV in the underdense region. Accordingly, different equilibria take place across the explored region which can be identified with this experimental technique. Detailed studies on highly ionized atomic species in different plasma conditions can therefore be performed simultaneously under controlled conditions.