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The present study was aimed at investigating whether imatinib pharmacogenetics is related to its pharmacodynamics in patients affected by chronic myeloid leukemia. Through a procedure based on a sequence of classical statistics methods, we investigated the possible relationships between treatment efficacy/tolerability and combinations of time-independent variables as gender and genetic covariates in the form of single nucleotide polymorphisms (SNPs) or combinations thereof. Moreover, since the drug tolerability has a strong incidence on the discontinuation of the therapy, we investigated whether the time of manifestation of the most frequent toxic effects can be related to time-independent patients’ characteristics or not. We found that a combination of two polymorphisms, namely hOCT1 c.480C>G (rs683369) and ABCB1 c.3435C>T (rs1045642), seems to play the role of predictor for imatinib in both efficacy and toxicity. Furthermore, the time of manifestation of edema toxicity is found to be associated to a combination of gender and ABCB1 c.3435C>T, whereas the time of manifestation of cramp toxicity appears related to gender. The novelty of this study is dual: the achievement of results that potentially have a significant clinical interest and the demonstration that the adoption of composed covariates may represent a unique tool to study different aspects of the treatment with imatinib.

Multiphase flow models represent valuable tools for the study of the complex, non-equilibrium dynamics of pyroclastic density currents. Particle sedimentation, flow stratification and rheological changes, depending on the flow regime, interaction with topographic obstacles, turbulent air entrainment, buoyancy reversal, and other complex features of pyroclastic currents can be simulated in two and three dimensions, by exploiting efficient numerical solvers and the improved computational capability of modern supercomputers. However, numerical simulations of polydisperse gas-particle mixtures are quite computationally expensive, so that their use in hazard assessment studies (where there is the need of evaluating the probability of hazardous actions over hundreds of possible scenarios) is still challenging. To this aim, a simplified integral (box) model can be used, under the appropriate hypotheses, to describe the kinematics of pyroclastic density currents over a flat topography, their scaling properties and their depositional features. In this work, multiphase flow simulations are used to evaluate integral model approximations, to calibrate its free parameters and to assess the influence of the input data on the results. Two-dimensional numerical simulations describe the generation and decoupling of a dense, basal layer (formed by progressive particle sedimentation) from the dilute transport system. In the Boussinesq regime (i.e., for solid mass fractions below about 0.1), the current Froude number (i.e., the ratio between the current inertia and buoyancy) does not strongly depend on initial conditions and it is consistent to that measured in laboratory experiments (i.e., between 1.05 and 1.2). For higher density ratios (solid mass fraction in the range 0.1–0.9) but still in a relatively dilute regime (particle volume fraction lower than 0.01), numerical simulations demonstrate that the box model is still applicable, but the Froude number depends on the reduced gravity. When the box model is opportunely calibrated with the numerical simulation results, the prediction of the flow runout is fairly accurate and the model predicts a rapid, non-linear decay of the flow kinetic energy (or dynamic pressure) with the distance from the source. The capability of PDC to overcome topographic obstacles can thus be analysed in the framework of the energy-conoid approach, in which the predicted kinetic energy of the flow front is compared with the potential energy jump associated with the elevated topography to derive a condition for blocking. Model results show that, although preferable to the energy-cone, the energy-conoid approach still has some serious limitations, mostly associated with the behaviour of the flow head. Implications of these outcomes are discussed in the context of probabilistic hazard assessment studies, in which a calibrated box model can be used as a fast pyroclastic density current emulator for Monte Carlo simulations.

We discuss the expansion of finite plasma bunches in two different geometries: planar and spherical. We show that, soon after the plasma formation, a steady state configuration between the hot electrons and the cold ions is reached in a few electron plasma times. Such an equilibrium configuration plays a crucial role in the subsequent plasma expansion and ion acceleration. We provide a description of this state and we investigate the ion dynamics by both analytical models and numerical simulations.

We discuss harmonic generation in the case of laser field-dressed Hamiltonians that are invariant under so-called dynamical symmetry operations. Examples for such systems are molecules which exhibit a discrete rotational symmetry of order N (e.g. benzene with N=6) interacting with a circularly polarized laser field and single atoms in a bichromatic field, with the two lasers having circular polarizations. Within a general group theory approach we study the harmonics one obtains from the interaction of a laser pulse and a circular molecule. When the system is in a pure field-dressed state the known selection rule kN \pm 1, k=1,2,3, ... results. However, other lines are observed when recombinations with states of a symmetry different from the initial one become important. This is the case for realistic laser pulses (i.e., with a finite duration). In particular when the fundamental laser frequency (or one of its multiples) is resonant with a transition between field-dressed states. Numerical ab initio simulations, confirming our analytical calculations and illustrating the power of the group theory approach, are presented., Comment: Submitted to Journal of Physics B

The problem of the Thomas-Fermi atom in strong static homogeneous electric fields is revisited. The static field situation is a reasonable approximation to the otherwise fully time-dependent problem of an atom in a laser pulse, provided that the laser frequency is not too high. Exact results for the ionization degree versus the applied electric field, obtained numerically, are compared with those from an approximate model which is much easier to solve. Very good agreement is found. Although the approximate model was published ten years ago, an unexplained discrepancy with both experimental results and the exact numerical solution of about one order of magnitude in the ionization degree had remained. In this paper it is shown that this discrepancy does not exist. A comparison with the simpler `over the barrier ionization' model and available experimental data for rare gases is made. A very appealing feature of the Thomas-Fermi model is its universality thanks to the scaling behaviour with respect to the nuclear charge Z .

A class of nonlinear optical effects related to fast field ionization in an interference pattern is investigated by numerical simulations. Interference between counter-propagating ultra-short pulses slightly below the ionization threshold produces a layered distribution of free-electron density. In a dense dielectric target, this effect allows us to trap light between plasma layers creating a sort of optical microcavity. Other peculiar features include frequency upshift, pulse lengthening and self-generated relativistic ionization fronts.

In this paper, we want to discuss different hydrodynamic schemes for fast ignition and some of their basic effects. Relativistic hydrodynamics and a covariant generalized Ohm's law are presented. Matter perforation (hole boring) by an intense laser pulse is investigated by 2D numerical simulations and a simple formula for the perforation speed is derived. Furthermore, we study macroparticle acceleration by an intense laser beam and the possibility to provide an energetic and massive projectile for ballistic ignition. The dynamics of ignition of a precompressed D–T mixture is illustrated by numerical simulations in planar 2D geometry using direct laser irradiation and macroparticle impact as external drivers. Electron heat conduction is found to be responsible for an efficient burn wave propagation which prevents the hot gas bubble of burning fuel from deflagrating into vacuum.

Absorption of intense femtosecond laser pulses in targets having either self-generated or imposed deformations is investigated with Vlasov simulations. Absorption attains high values and scales strongly with the deformation depth but weakly with density and intensity. Fast electron transport is mainly lateral for thin foils with small ratios of target thickness to deformation depth. For thick foils with larger ratios a single magnetic channel forms.

Using particle-in-cell simulations we show that a steady, megagauss magnetic field can be generated due to ionization dephasing in the interaction of an ultrashort laser pulse with a dielectric target. The magnetic field amplitude is limited by the screening of the laser-produced plasma and depends upon the ionization threshold of the target material rather than the laser intensity.

A fluid model that takes the field ionization energy correctly into account is presented for the first time by introducing an energy conserving ionization current as a source term in the wave equation. Nonstandard type fluid equations result from the finite ejection energy of the electrons in the field ionization process. The energy and momentum distributions of the ejected electrons are obtained from the time-dependent Schrodinger equation and classical Monte Carlo calculations. Characteristic results of how field ionization influences the pulse propagation, and some extremely nonlinear features caused by the ionization current are given.

The stationary self-action of an intense electromagnetic beam propagating in a moving medium is investigated. Taking into account the ponderomotive force, the fluid motion is solved and the beam's waist equation is obtained and discussed in a quite general form. A detailed analysis of the beam threshold intensity and geometry for self-focusing is carried out. Finally, these general results are applied to the specific case in which an approximate representation of the fluid equations is adopted.

Femtosecond laser pulses are capable of producing very high light intensities at moderate pulse energies. We study the laser light absorption at flux densities of 1017 W/cm2 and higher during the plasma formation and the plasma heating process. In the first stage multiphoton and collisional ionization dominate. Modification of the inverse bremsstrahlung absorption occur owing to ionization dephasing. In the second stage beam energy conversion is mainly by collisional absorption at normal incidence, and by resonance absorption at oblique illumination. Simultaneously strong electron heat conduction provides for plasma formation and heating in deeper layers of the solid not accessible to the laser light. The penetration depth of the laser beam is modified by the anomalous skin effect. The electron-ion collision frequency at the high laser intensities under consideration is determined by the oscillation energy of the electrons rather than by their thermal motion. Although it reaches high values (1014-1016 S-1), all kinds of solid targets become strongly reflecting (>60%) owing to the formation of electron densities largely exceeding those of solid-state plasmas. Effects modifying the degree of absorption are briefly discussed.

Ponderomotive (or radiation pressure) acceleration is a route to the generation of high-energy ions in laser–solid interactions, which does not rely on the generation of fast electrons. It may be optimized by the use of circularly polarized (CP) laser pulses rather than linearly polarized (LP) ones, since for CP, fast electron generation is strongly inhibited, quenching competing mechanisms such as target normal sheath acceleration. We present a comparison of 1-D and 2-D particle-in-cell simulations for CP and LP, which shows the potential of CP pulses for ion acceleration. The comparison also enables us to discriminate “pure” ponderomotive effects from those due to fast electrons, aiding the understanding of basic mechanisms of ion acceleration and surface rippling.

A semi-analytical treatment of elliptic gaussian beam propagation in a nonlinear medium, in the WKB, paraxial approximation is presented. It is shown that the self-trapping threshold increases with ellipticity. This suggests the possibility of controlling diffraction and self-focusing collapse by varying the beam geometry. It is also shown that, at fixed wavelength and intensity, four intervals of ellipticity, in which the propagation properties show marked differences in the longitudinal intensity profile of the beam, can be identified.

In recent experiments of laser pulse interaction at relativistic intensities with a low density plasma, the proton radiography technique showed evidence of long--lived field structures generated after the self-channeling of the laser pulse. We present 2D particle-in-cell simulations of this interaction regime, where the dynamics of similar structures has been resolved with high temporal and spatial resolution. An axially symmetrical field pattern, resembling both soliton-like and vortex structures, has been observed. A study of the physics of such structures and a comparison with experimental data is in progress., To appear in the Proceedings of the International Workshop "Collective phenomena in macroscopic systems", Villa Olmo, Como, Italy, December 4-6, 2006; 9 pages, 8 figures submitted in GIF format

The acceleration of ions in the interaction of ultrashort, high intensity, circularly polarized laser pulses with overdense plasmas has been theoretically investigated. By using particle-in-cell (PIC) simulations it is found that high-density, short duration ion bunches moving into the plasma are promptly generated at the laser-plasma interaction surface. This regime is qualitatively different from ion acceleration regimes driven by fast electrons such as sheath acceleration at the rear side of the target. A simple analytical model accounts for the numerical observations and provides scaling laws for the ion bunch velocity and generation time as a function of pulse intensity and plasma density. The ion bunches have moderate energies (100 keV-1 MeV) but very high density and low beam divergence, and might be of interest for problems of ultrafast compression, acceleration or heating of high–density matter. In particular, we have studied their application to the development of compact sources of fusion neutrons. We analyzed two target schemes showing that intense neutron bursts with femtosecond duration are produced.

The propagation of a superintense laser pulse in an underdense, inhomogeneous plasma has been studied numerically by two-dimensional particle-in-cell simulations on a time scale extending up to several picoseconds. The effects of the ion dynamics following the charge-displacement self-channeling of the laser pulse have been addressed. Radial ion acceleration leads to the ``breaking'' of the plasma channel walls, causing an inversion of the radial space-charge field and the filamentation of the laser pulse. At later times a number of long-lived, quasi-periodic field structures are observed and their dynamics is characterized with high resolution. Inside the plasma channel, a pattern of electric and magnetic fields resembling both soliton- and vortex-like structures is observed., Comment: 10 pages, 5 figures (visit http://www.df.unipi.it/~macchi to download a high-resolution version), to appear in Plasma Physics and Controlled Fusion (Dec. 2007), special issue containing invited papers from the 34th EPS Conference on Plasma Physics (Warsaw, July 2007)

Background. In spite of the dramatically positive changes in the outcome of Chronic Myeloid Leukemia (CML) patients due to the introduction of imatinib, several aspects of both the pharmacokinetics and pharmacodynamics of this TKI remain unclear. In particular, the possible relations between patients' specific characteristics and the efficacy and toxicity of treatment are still matter of debate. Aim of the study. The purpose of this study was twofold: 1) set up an innovative and unsupervised mathematical model that helps in explaining the variability of imatinib plasma levels through the analysis of the main pharmacokinetic parameters in terms of patient-specific features as BMI, renal and hepatic function, alpha1-acid glycoprotein, age, sex and polymorphisms of transporters; 2) set up a specifically tailored analysis of variance technique which allows to detect in limited databases also possible correlations between efficacy, toxicity and genetic covariates either as single SNP or in the form of more complex combinations of different polymorphisms. Patients and Methods. The study has been carried out on 53 CML adult patients (30 males and 23 females) receiving imatinib as first-line therapy at the Section of Hematology of the Department of Clinical and Experimental Medicine of the University of Pisa and from the Department of Internal Medicine of Orbassano, Turin. Patients gave their informed consent to the participation to the study before enrollment. The pharmacokinetic model has been constructed following the indication of the Factor Analysis of Mixed Data (FAMD), a procedure that permitted us to select the possible important covariates and eventually which of them aggregate in a single factor using an unsupervised approach. For what concerns imatinib pharmacodynamics, we studied first the possible relation between treatment efficacy and genetic covariates (either as single SNPs or diplotypes) and then whether the pharmacogenetic variables obtained are also relevant for the tolerability of the drug. The genetic variables considered were represented by the following polymorphisms: rs72552763 [MI420I], rs12208357 [c.181C>T] and rs683369 [c.480C>G] (hOCT1), rs1128503 [c.1236C>T], rs2032582 [c.2677G>T/A] and rs1045642 [c.3435C>T] (ABCB1), rs4149117 [c.334G>T] (SLCO1B3). Results. The FAMD procedure allowed us to identify three main factors (single or aggregated covariates) which properly included in the pharmacokinetic model reduced significantly the uncertainties on all pharmacokinetic parameters. The selected factors are: hOCT1 polymorphism c.480C>G, a combination of BMI and gender and the glomerular filtration rate. Such a procedure led to ka = 0.786 ± 5.9 %, c1 = 0.0282 ± 24.5 %, where ka and c1 are the absorption and elimination rate, respectively. Through our investigation of the pharmacodynamics, we found that both the complete cytogenetic response (CCyR) and the maximum toxicity are related to a particular combination of hOCT1 rs683369 [c.480C>G] and ABCB1 rs1045642 [c.3435C>T] polymorphisms. Furthermore, since the toxic effects of the drug have a strong incidence on the discontinuation of the therapy, we have studied whether the manifestation and the onset time of the most frequent specific toxic effects of Imatinib can be associated to the genetic characteristics of the patients and/or to a combination of them. Edema toxicity has resulted to be correlated with combinations between c.3435C>T and gender or ABCB1 rs1128503 [c.1236C>T]. Cramp toxicity has resulted to be correlated to gender alone or in combination with SCLO1B3 rs4149117 [c.334G>T]. Discussion. Results of this study obtained provide relevant insights on the role of covariates in imatinib treatment and suggest that FAMD analyses in combination with ANOVA tests may allow the unsupervised identification of covariates that significantly affect drug disposition and pharmacodynamics. Figure 1. Color map of the mean values (left) and coefficient of variation (right) of contributions of ten covariates on the first five dimensions. In the color map of the mean values, greater is the value (darker color), more important is the role of the covariate in the dimension. Figure 1. Color map of the mean values (left) and coefficient of variation (right) of contributions of ten covariates on the first five dimensions. In the color map of the mean values, greater is the value (darker color), more important is the role of the covariate in the dimension. Disclosures No relevant conflicts of interest to declare.

We have theoretically investigated the acceleration of ions in the interaction of high intensity, circularly polarized laser pulses with overdense plasmas. By using 1D and 2D particle‐in‐cell (PIC) simulations we find that high‐density, short duration ion bunches moving into the plasma are promptly generated at the laser‐plasma interaction surface. This regime is qualitatively different from ion acceleration regimes driven by fast electrons, such as sheath acceleration at the back of the target or shock acceleration at the front, which occur for linear polarization. A simple analytical model accounts for the numerical observations and provides scaling laws for the ion bunch velocity and generation time as a function of pulse intensity and plasma density. The present mechanism based on circular polarization of the laser pulse leads to moderate ion energies (in the 100 keV–1 MeV range) but very high ion densities and low beam divergence. These ion bunches might be of interest for problems of compression and...

The main features of codes for inertial confinement fusion studies are outlined, and a few recent simulation results are presented. The two-dimensional Lagrangian fluid code DUED is used to study target evolution, including beam-driven compression, hydrodynamic stability, hot spot formation, ignition and burn. An electro-magnetic particle-in-cell (PIC) code is applied to the study of ultraintense laser–plasma interaction and generation of fast electron jets. A relativistic 3D collisionless fluid model addresses relativistic electron beam propagation in a dense plasma.

The stationary state of a spherically symmetric plasma configuration is investigated in the limit of immobile ions and weak collisions. Configurations with small radii are positively charged as a significant fraction of the electron population evaporates during the equilibration process, leaving behind an electron distribution function with an energy cutoff. Such charged plasma configurations are of interest for the study of Coulomb explosions and ion acceleration from small clusters irradiated by ultraintense laser pulses and for the investigation of ion bunches propagation in a plasma.

The expansion dynamics of a finite size plasma is examined from an analytical perspective. Results regarding the charge distribution as well as the electrostatic potential are presented. The acceleration of the ions and the associated cooling of the electrons that takes place during the plasma expansion is described. An extensive analysis of the transition between the semi infinite and the finite size plasma behaviour is carried out. Finally, a test of the analytical results, performed through numerical simulations, is presented., Comment: 4 pages with 5 figures

The generation of harmonics by atoms interacting with two circularly polarized and frequency related laser fields is addressed through ab initio numerical simulations. A detailed charaterization of a few specific harmonics is given. In particular, the two different cases where the total energy absorbed through photons is far-off or close to the energy gap between different atomic states are investigated. It is found that the conversion efficiency in the harmonic generation is strongly dependent on the inner atomic structure and in certain specific cases it can be significantly enhanced within a small frequency range., Submitted to Appl. Phys. B, 4 pages

Two-surface wave decay (TSWD), i.e. the parametric excitation of electron surface waves, was recently proposed as an absorption mechanism in the interaction of ultrashort, intense laser pulses with solid targets. We present an extension of the fluid theory of TSWD to a warm plasma which treats boundary effects consistently. We also present test-particle simulations showing localized enhancement of electron acceleration by TSWD fields; this effect leads to a modulation of the current density entering into the target and may seed current filamentation instabilities., 4 figures, submitted to Appl.Phys.B (special issue from HFSW X conference, Biarritz, France, Oct 12-15 2003); slightly revised text

In recent years, several schemes for laser-driven fast ignition (FI) of inertial confinement fusion targets have been proposed. In all schemes, a key element is the conversion of the energy of a Petawatt laser pulse into a beam of strongly relativistic electrons and the transport of the latter into a dense plasma or a solid target. The electron beam may either drive ignition directly or be used to accelerate a proton beam which is in turn used to ignite. Both ignition scenarios involve a number of physical processes which are widely unexplored and challenging for theory and simulation. In this contribution, we present theoretical and numerical investigations of several fundamental issues of relevance to FI from the stage of electron generation and transport to that of proton energy deposition, including electron beam instabilities, electron transport in solid-density plasma, proton transport in the coronal plasma, and requirements for proton beam driven ignition.

The generation of harmonics by atoms or ions in a two-color, coplanar field configuration with commensurate frequencies is investigated through both, an analytical calculation based on the Lewenstein model and the numerical ab initio solution of the time-dependent Schroedinger equation of a two-dimensional model ion. Through the analytical model, selection rules for the harmonic orders in this field configuration, a generalized cut-off for the harmonic spectra, and an integral expression for the harmonic dipole strength is provided. The numerical results are employed to test the predictions of the analytical model. The scaling of the cut-off as a function of both, one of the laser intensities and frequency ratio $\eta$, as well as entire spectra for different $\eta$ and laser intensities are presented and analyzed. The theoretical cut-off is found to be an upper limit for the numerical results. Other discrepancies between analytical model and numerical results are clarified by taking into account the probabilities of the absorption processes involved., Comment: 8 figures

In all recently proposed schemes for laser-driven Fast Ignition (Fl) of Inertial Confinement Fusion (ICF) targets,two key elements are the conversion of the energy of a Petawatt laser pulse into a beam of strongly relativisticelectrons and its transport through a dense plasma or a solid target. The electron beam may either drive ignitiondirectly or be exploited to accelerate a proton beam which in turn is used to ignite the target. Both approaches to FT involve a number of physical processes that are challenging for theory and simulation. In this paper, theoretical and numerical investigations are presented concerning several fundamental issues of relevance to Fl,including electron beam instabilities, electron transport in solid-density materials, and requirements for protonbeam driven ignition.Keywords: Fast Ignition, energy conversion, electron and ion beam transport, proton beam driven ignition. 1. INTRODUCTION An essential element of Fast Ignition (Fl) of ICF targets is the conversion of the energy of a Petawatt laser pulseinto a beam of strongly relativistic electrons. In the original proposal,' the electron beam is generated by directinteraction of the laser pulse with the coronal plasma and reaches the precompressed fuel core after propagatingin the dense plasma region. The electron beam is expected to carry a current of several MegaAmperes, and itstransport properties are thus affected by the self-generated electric and magnetic fields, by beam instabilities andby inefficient charge neutralization due to low conductivity of the background electrons. Efficient energy transportis also a key factor in alternative FT schemes.2 Electric field generation by relativistic electrons penetrating in

Computational studies of many-electron systems in strong laser fields are presented. The ionization and nonlinear dipole response of the ball-shaped C60 fullerene molecule are investigated with a time-dependent density functional approach and a jellium approximation for the ionic background. We find that C60 ionization at 800 nm wavelength occurs multiphoton-like rather than via excitation of a “giant” resonance. Harmonic generation from the interaction of a circularly polarized laser field and a molecule with a discrete rotational symmetry is also studied, and spectra and selection rules are interpreted via a general group theory approach.

The dynamics and generation of surface structures in intense laser interaction with overdense plasmas is studied numerically and analytically. The dynamics of two-dimensional (2D) electron surface oscillations (ESOs) has been resolved in particle-in-cell simulations, showing a “period doubling” effect with respect to the driving magnetic force of the laser. A new parametric process, in which a 1D electrostatic oscillation decays into two electromagnetic surface waves, has been investigated analytically and provides a mechanism for the generation of ESOs. For oblique laser incidence and p-polarization generation of subharmonic surface waves is also predicted. The possible impact of surface deformations on fast electron generation and disruption of plasma “moving mirrors” for high harmonic generation is discussed.

Using an analytical model we discuss the parametric excitation of pairs of electron surface waves (ESW) in the interaction of an ultrashort, intense laser pulse with an overdense plasma which has a step-like density profile. The ESWs can be excited either by the electric or by the magnetic part of the Lorentz force exerted by the laser and, correspondingly, have frequencies around $\omega/2$ or $\omega$, where $\omega$ is the laser frequency., Comment: 4 EPS figures, Revtex

By analytical modeling and numerical simulation, we show that surface modes in moderately overdense plasmas may be excited parametrically by an intense, ultrashort laser pulse. This process has a feedback effect on fast electron generation and may seed a fast distortion of plasma “moving mirrors.”

SUMMARY We study the interaction of strong femtosecond laser pulses with the C$_{60}$ molecule employing time-dependent density functional theory with the ionic background treated in a jellium approximation. The laser intensities considered are below the threshold of strong fragmentation but too high for perturbative treatments such as linear response. The nonlinear response of the model to excitations by short pulses of frequencies up to 45eV is presented and analyzed with the help of Kohn-Sham orbital resolved dipole spectra. In femtosecond laser pulses of 800nm wavelength ionization is found to occur multiphoton-like rather than via excitation of a ``giant'' resonance.

The generation of electron surface oscillations in overdense plasmas irradiated at normal incidence by an intense laser pulse is investigated. Two-dimensional (2D) particle-in-cell simulations show a transition from a planar, electrostatic oscillation at $2\omega$, with $\omega$ the laser frequency, to a 2D electromagnetic oscillation at frequency $\omega$ and wavevector $k>\omega/c$. A new electron parametric instability, involving the decay of a 1D electrostatic oscillation into two surface waves, is introduced to explain the basic features of the 2D oscillations. This effect leads to the rippling of the plasma surface within a few laser cycles, and is likely to have a strong impact on laser interaction with solid targets., Comment: 9 pages (LaTeX, Revtex4), 4 GIF color figures, accepted for publication in Phys. Rev. Lett

We investigate numerically effects related to ``single-cycle'' ionization of dense matter by an ultra-short laser pulse. The strongly non-adiabatic response of electrons leads to generation of a megagauss steady magnetic field in laser-solid interaction. By using two-beam interference, it is possible to create periodic density structures able to trap light and to generate relativistic ionization fronts, 12 pages, 6 figures, to be published in Laser and Particle Beams

The effect of surface deformations on laser absorption and transport in overdense plasma targets is investigated with 2D Vlasov simulations. Fractional absorption increases strongly with the deformation depth and scales weakly with density or intensity. Fast electrons generated in the skin layer have high tangential velocity directed toward the deformation center. In the case of a single, smooth deformation, they may be collimated by magnetic fields into a single jet which penetrates deeply into the plasma. Small-scale surface corrugations are able to destroy the collimation of a single jet leading to multiple jets or diffuse electron sprays.

By discussing the results of Yu et al. [Phys. Rev. E 59, 3583 (1999)] on the near-total transmission of laser pulses through highly overdense plasma foils, we show that they actually cannot explain the experimental results of Giulietti et al. [Phys. Rev. Lett. 79, 3194 (1997)]. Simple analytical calculations as well as particle-in-cell simulations support our assertion.

Ponderomotive (or radiation pressure) acceleration is a route to the generation of high-energy ions in laser-solid interactions, which does not rely on the generation of fast electrons. It may be optimized by the use of circularly polarized (CP) laser pulses rather than linearly polarized (LP) ones, since for CP, fast electron generation is strongly inhibited, quenching competing mechanisms such as target normal sheath acceleration. We present a comparison of 1-D and 2-D particle-in-cell simulations for CP and LP, which shows the potential of CP pulses for ion acceleration. The comparison also enables us to discriminate "pure" ponderomotive effects from those due to fast electrons, aiding the understanding of basic mechanisms of ion acceleration and surface rippling.

Relativistic Vlasov simulation is employed for the study of ultrashort, superintense laser-solid interaction. A novel absorption mechanism is evidenced in 1D2V simulations. Analytical calculations elucidate the simulation results. Implementation of the Vlasov code on a parallel platform and progress towards a full Boltzmann approach are discussed.

We discuss a kinetic model for a system of free and bound electrons undergoing field ionization. Fluid equations are derived and used to study transient ionization effects on short laser pulse propagation. We finally discuss a suitable semiclassical fluid approach to heavy atoms ionization.

In the variety of processes occurring in matter under the action of superstrong laser fields rapid field ionization and ponderomotive effects play a dominant role. We investigate field ionization by solving the time-dependent Schrodinger equation numerically and compare the results with classical calculations. We obtain the ionization rates of hydrogen-like ions, the ejection energy spectrum of the electrons, and we show the transition to classical behavior at high energies. In addition, we are able to give a physical explanation of non-sequential ionization. Our ionization rates differ by more than an order of magnitude from the standard expressions. A model is presented which allows to calculate the back action of field ionization on the laser beam. Finally, we present the most general relativistic derivation of ponderomotive forces on single particles and present their transformation properties. Ponderomotive forces are of non-Newtonian character.

The dynamics of electric field generation and radial acceleration of ions by a laser pulse of relativistic intensity propagating in an underdense plasma has been investigated using an one-dimensional electrostatic, ponderomotive model developed to interpret experimental measurements of electric fields [S. Kar et al, New J. Phys. *9*, 402 (2007)]. Ions are spatially focused at the edge of the charge-displacement channel, leading to hydrodynamical breaking, which in turns causes the heating of electrons and an "echo" effect in the electric field. The onset of complete electron depletion in the central region of the channel leads to a smooth transition to a "Coulomb explosion" regime and a saturation of ion acceleration., Comment: 9 pages, 7 figures, final revised version, to appear on Plasma Phys. Contr. Fus., special issue on "Laser and Plasma Accelerators", scheduled for February, 2009

We tested the effects produced by a converging shock wave propagating through a plasma. Experimental data gave the first indications on the interaction of a weak converging front with a fairly inhomogeneous, intermediate-density (∼1018 cm−3) plasma at a temperature of few eV. It is found that most of the energy of the converging shock wave is devoted to the ionization of the cold external layers of the plasma. In hydrogen the residual shock energy produced a stop in natural adiabatic cooling, while in helium the lower temperature and the higher ionization energies avoided any appreciable temperature effect.

Large-scale self-focusing effects are evidenced by focusing a 300 MW Nd laser beam with aF/8 spherical lens in a fairly low-density gas (such thatne≈nc/100). The include important beam recollimation and formation of an «empty» channel which was visualized by holographic interferometry. In these conditions plasma expansion and laser energy deposition were both larger ahead than behind the focal point.

Our aim is to study the two-photon induced luminescence in CdS crystals by using a low power CW laser (5 mW). We discuss briefly the possibility given by the electron-pulse-counting technique to detect weak optical signals. This technique enables us to investigate the luminescence spectra (which show two bands at 2.4 and 2.0 eV) and the dependence of the luminescence intensity on the laser power. The discussion of the experimental results makes it clear that the level responsible for the red emission is partially saturated at a laser intensity of 1023 photon/cm2 s.

Nonlinear impurity screening in metals has been widely studied in the literature, and the differences with respect to the linear results have been found to be important. The screening theories available for semiconductors were until very recently obtained in $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}$ space, and this prevented the study of nonlinear effects. We present here for the first time an investigation of nonlinear impurity screening in a model semiconductor. The Thomas-Fermi theory of dielectric screening, recently developed by one of us, has been used. Nonlinearity effects are found to be of the same order of magnitude as in metals. A remarkable donoracceptor asymmetry is also found.

The interaction of a GW laser beam ( λ= 1. 06 µm) with a plasma far from critical density (ne /nc ≤ 10-2) has been studied at an intensity of 1013 W/cm2. Experimental results showed a net redistribution of the laser energy both in space and time; the formation of a low density channel was also detected. These results are discussed in terms of self-focusing in an inhomogeneous plasma, where inhomogeneities are produced by hydrodynamic motion.

Electron density distribution and shock front profiles are obtained, by holographic interferometry, from a plasma produced by a laser in a 500 Torr helium atmosphere, with an f N = 1.4 lens. For a long time, compared with the duration of the laser pulse, interesting features are observed, such as a three-folded shock front and hot plasma jets. A model based on Mach reflection of two spherical shocks allows us to explain those structures.