This issue of Laser and Particle Beams contains 25 papers that were presented at the conference on “Matter in Super-Intense Laser Fields: Short Pulse, Superstrong Laser-Plasma Interactions,” held in S. Feliu de Guixols, Spain, from 29 September to 4 October 2001. Almost 100 scientists and young researchers (Ph.D. students and Post Docs) took part in this successful event, despite the terrible terrorist acts of September 11, 2001 in New York, which prevented many speakers from taking part, especially those coming from the United States.

New experimental results of investigations of the interactions of an intense 4.3-GeV electron beam with an aligned single crystal are given in the case when the electrons travel close to the crystalline planes (110). The growth of low energy radiation yields with the intensity of the electron beam and the peculiar properties of the produced γ-quanta at their passage through a small collimator aperture were observed. Large angular asymmetry at the scattering of low energy γ-quanta, which is evidence of high polarization of the radiation, was also detected. These unexpected phenomena may be stimulated by strong correlations of atomic excitations in crystals. The obtained results will be of use for understanding of the properties of both the excited crystalline medium and the mechanism of intense radiation under these conditions and also for practical applications of the radiation.

We have studied the interaction of soft X-ray thermal radiation with foam-layered metal targets. The X-radiation was produced by focusing a high energy laser inside a small size hohlraum. An increment in shock pressure was observed with the foam layer as compared to bare metal targets.

Two experiments have been performed to investigate heating by high-intensity laser-generated electrons, in the context of studies of the fast ignitor approach to inertial confinement fusion (ICF). A new spectrometer and layered targets have been used to detect Kα emission from aluminum heated by a fast electron beam. Results show that a temperature of about 40 eV is reached in solid density aluminum up to a depth of about 100 μm.

A treatment is given of harmonics generation resulting from nonlinear inverse bremsstrahlung in a plasma with an anisotropic bi-Maxwellian electron velocity distribution function. A complete characterization of the process is reported. In particular, analytically and numerically we established how the efficiency of the odd harmonics generation and their polarization depend on such process parameters as: (1) the degree of effective temperature anisotropy, (2) the frequency and the intensity of the fundamental wave, and (3) the angle between the fundamental wave field direction and the symmetry axis of the electron distribution function.

Based on realistic numerical simulations of the three-dimensional time-dependent Schrödinger equation describing a hydrogen atom interacting with a strong electromagnetic field, the influence of the magnetic component is studied. The same computational techniques can be applied to the case of strong field photoionization as well as to the case of ionization by an incident relativistic heavy ion. One of the main consequences, in the strong-laser-field case, is the presence of true even harmonics of the incoming field. In the heavy ion impact, the asymmetry of the wave function reveals the importance of the nondipole nature of the interaction.

Recently, two novel techniques for the extraction of the phase-shift map (Tomassini et al. (2001). Applied Optics40, 35) and the electronic density map estimation (Tomassini P. & Giulietti A. (2001). Optics Communication199, 143–148) have been proposed. In this article, we apply both methods to a sample laser–plasma interferogram obtained with femtoseconds probe pulse, in an experimental setup devoted to laser particle acceleration studies.

Recent experiments undertaken at the Rutherford Appleton Laboratory to produce X-ray lasing over the 5–30 nm wavelength range are reviewed. The efficiency of lasing is optimized when the main pumping pulse interacts with a preformed plasma. Experiments using double 75-ps pulses and picosecond pulses superimposed on 300-ps background pulses are described. The use of travelling wave pumping with the approximately picosecond pulse experiments is necessary as the gain duration becomes comparable to the time for the X-ray laser pulse to propagate along the target length. Results from a model taking account of laser saturation and deviations from the speed of light c of the travelling wave and X-ray laser group velocity are presented. We show that X-ray laser pulses as short as 2–3 ps can be produced with optical pumping pulses of ≈1-ps.

We propose a new approach to the calculation of ion populations in the LTE dense plasmas in the superconfiguration approximation. The screening of plasma ions is obtained using the same free-electron chemical potential for each ion charge. This chemical potential is determined by iteration keeping the total density constant and varying the volume of each ion. In such a way our approach not only gives the charge neutrality of the plasma, but also assures that the free-electron density is equal in the vicinity of each ion. The resulting ion charge distribution is different with respect to the one obtained without imposing the constraint of equal chemical potential. For instance, the effective charge is a little bit higher, which can change the shape of the absorption spectra, especially at higher densities. Examples of absorption spectra of medium-Z plasmas in a temperature and density range close to the present opacity measurements are shown.

Plasmas irradiated by an intense laser beam have recently been demonstrated to be sources for particles (electrons, ions, positrons). During this interaction, it has been found that these charged particles can be efficiently accelerated. An overview of these results as well as some perspectives are presented here.

High dynamic range, space-resolved X-ray spectra, obtained using a TlAP crystal and a cooled CCD camera as a detector, were used to investigate the electron density and temperature profiles of an aluminum laser plasma with micrometer resolution. The electron density profile retrieved from the measurements is compared with numerical predictions from the two hydrodynamics codes MEDUSA (1D) and POLLUX (2D). It is shown that 2D density profiles can be successfully reproduced by 1D simulations using a spherical geometry with an ad hoc initial radius, leading to similar electron temperature profiles.

A nonequilibrium kinetic model is used for predicting the time evolution of the Li atom concentrations (ground and excited states) in the plasma produced by excimer laser ablation of a LiNbO3 target. The model predicts a very high ionization degree (∼0.97) that agrees well with the one obtained experimentally (∼1). These results together with the prediction of high (and close to local thermodynamic equilibrium) population densities of the electronically excited Li upper energy levels provide an indirect support for an electronic rather than thermal ablation mechanism of Li atoms.

The intrinsically nonperturbative “vacuum persistence probability” P for e + e− production in the overlap region of a pair of high-intensity lasers is estimated in the context of three models, each of which adapt and simplify the exact Fradkin representation for the logarithm of the fermion determinant in the fields of the crossed lasers. In each case, one finds for P an expression resembling Schwinger's 1951 result for the probability of pair production in a constant electric field, proportional to an exponential factor which contains an essential singularity and hence does not admit a perturbative expansion about zero coupling. Qualitative estimates of these models suggest that realistic yields for this form of e + e− production must await lasers of intensity 1029 W/m2. The possibility of producing a quark–antiquark pair in this way is noted, in particular, with temporary, but large separations of the qq.

Using the technique of tight-binding electron–ion dynamics, we have calculated the response of crystalline GaAs when a femtosecond laser pulse excites 1–20% of the valence electrons. Above a threshold fluence, which corresponds to promotion of about 12% of the valence electrons to the conduction band, the lattice is destabilized and the band gap collapses to zero. This result supports the conclusion that structural changes on a subpicosecond time scale observed in pump-probe experiments are of a nonthermal nature.

The emission of high-energy protons in laser–solid interactions and the theories that have been used to explain it are briefly reviewed. To these theories we add a further possibility: the acceleration of protons inside the target by the electric field generated by fast electrons. This is considered using a simple one-dimensional model. It is found that for relativistic laser intensities and sufficiently long pulse durations, the proton energy gain is typically several times the fast electron temperature. The results are very similar to those obtained for proton acceleration by electron expansion into vacuum.

Supersonic jet production in conical wire array Z-pinches is modeled using a two-dimensional (2D) resistive magneto-hydrodynamic (MHD) code. In conical wire arrays, the converging plasma ablated from the wires stagnates on axis, forming a standing conical shock which redirects and collimates the flow into a jet. As the jet exits the collimator shock, it is radiatively cooled and accelerated by the steep thermal gradients present. Purely hydrodynamic simulations using conditions relevant to the MAGPIE facility show good agreement with the experiments (Lebedev et al., 2002), indicating that narrow, high Mach number (M ∼ 20), radiatively cooled tungsten jets of astrophysical relevance can be obtained. To investigate the effects of lower radiative cooling on jet collimation, we modeled an aluminum conical wire array. When radiative losses are less significant, lower Mach number (M ∼ 10), less collimated jets are obtained. MHD simulations relevant to the “Z” facility were carried out to investigate the scaling of jet parameters. The resulting hypersonic (M ∼ 40), high density jets should allow the investigation of a wider range of astrophysical jet conditions.

In this article, we present a laboratory astrophysics experiment on radiative shocks and its interpretation using simple modelization. The experiment is performed with a 100-J laser (pulse duration of about 0.5 ns) which irradiates a 1-mm3 xenon gas-filled cell. Descriptions of both the experiment and the associated diagnostics are given. The apparition of a radiation precursor in the unshocked material is evidenced from interferometry diagrams. A model including self-similar solutions and numerical ones is derived and fairly good agreements are obtained between the theoretical and the experimental results.

One of the most exciting results recently obtained in the ultraintense interaction research area is the observation of beams of protons with energies up to several tens of megaelectron volts, generated during the interaction of ultraintense picosecond pulses with solid targets. The particular properties of these beams (high brilliance, small source size, high degree of collimation, short duration) make them of exceptional interest in view of diagnostic applications. In a series of experiments carried out at the Rutherford Appleton Laboratory (RAL) and at the Lawrence Livermore National Laboratory (LLNL), the laser-produced proton beams have been characterized in view of their application as a particle probe for high-density matter, and applied to diagnose ultraintense laser–plasma interactions. In general, the intensity cross section of a proton beam traversing matter will be modified both by collisional stopping/scattering, and deflections caused by electric/magnetic fields. With a suitable choice of irradiation geometry and target parameters, the proton probe can be made mainly sensitive to the electric field distribution in the object probed. Therefore, point projection proton imaging appears as a powerful and unique technique for electric field detection in laser-irradiated targets and plasmas. The first measurements of transient electric fields in high-intensity laser-plasma interactions have been obtained with this technique.

Strong ionization of the gas medium has significant effects in the process of medium-order harmonic generation. The combined effect of neutral atom depletion and defocusing of the pump beam due to the intensity-dependent density of free electrons, significantly modifies the conversion characteristics and efficiency. For moderate harmonic orders, the yield is optimized for well-defined values of the pump laser intensity that do not depend on the order or on the focusing geometry, but only on the ionization potential of the gas. In particular focusing conditions, the ionization-induced defocusing can effectively guide the pump beam along channels of optimum intensity, thus enhancing the overall conversion efficiency. We demonstrate that a very simple model is able to reproduce all our experimental results in a surprisingly good way.

We have performed a kinetic study of the electron dynamic relaxation inside a Au film subjected to a subpicosecond laser pulse. For this purpose, we have developed a time-dependent numerical solution of the Boltzmann equation for the electrons inside the film considering the collision integrals due to electron–electron and electron–phonon collisions and a perturbation term due to the laser pulse. Our results show that, after the pulse excitation, electron distributions are very far from equilibrium. Therefore it is not possible, especially in the first part of the temporal evolution, to describe the relaxation of the electron distribution through a two-temperature model.