Design, simulation and application of phase plates

In this paper we analyze the use of phase plates to obtain homogeneous laser intensity profiles. We studied the dependence of intensity distribution on phase plates characteristics, we obtained analytical solution for the intensity profile in the focal plane for plane waves and developed a numerical simulator to calculate the intensity distribution with a generic initial beam and at any propagation plane. We defined criteria to evaluate the quality of profiles produced by different phase plates. Finally we compared experimental results obtained at the Max-Planck Institut für Quantenoptik of Garching with our numerical simulations. Received 23 July 2001 and Received in final form 8 January 2002     

On a magnetic confinement of femtosecond laser pulse plasmas

Perspectives of magnetic confinement for the increase of life times of laser plasmas generated by femtosecond laser pulses are considered. Possibilities that are provided by miniature magnetic cusp configurations with magnetic fields of moderate intensities (of order of Teslas) are investigated. The construction of micro-traps with permanent magnets, making it possible to increase neutron yield, seems to be very simple and possible for most modern “table top" laser experiments.     

Collisionless absorption in an overdense plasma with anisotropic electron distribution function

Collisionless absorption of linearly polarized electromagnetic wave in a plasma with anisotropic bi-Maxwellian electron velocity distribution is investigated. Due to the wave magnetic field influence on the electron kinetics in the skin layer, the wave absorption is found to significantly depend on the degree of the electron temperature anisotropy. Depending on the value of the skin layer anomaly parameter, and on the electron temperature anisotropy degree, the conditions are found when a significant decrease or increase of the collisionless absorption is expected. Received 25 January 2002     

Resonant coupling of small size-controlled lead clusters with an intense laser field

We exposed small size-controlled lead clusters with a few hundreds of atoms to laser pulses with peak intensities up to 10¹⁵ W cm^-2 and durations between 60 fs to 2.5 ps. We measured kinetic energies and ionic charge of fragments as a function of the laser intensity and pulse duration. Highly charged Pbⁿ⁺ ions up to n = 26 have been detected presenting kinetic energies up to 15 keV. For comparison with our experimental results, we have performed simulations of the laser coupling with a cluster-sized lead nanoplasma using a qualitative model that was initially proposed by Ditmire and co-workers at LLNL for the case of rare gas clusters. From these simulations we conclude that two mechanisms are responsible for the explosion dynamics of small lead clusters. As already observed for large rare gas clusters (n = 10⁶), fragments with charge states below +10 are driven by Coulomb forces, whereas the higher charged fragments are accelerated by hydrodynamic forces. The latter mechanism is a direct consequence of the strong laser heating of the electron cloud in the nanoplasma arising from a plasmon-like resonance occurring at n _e = 3n _c. In order to obtain an optimized laser-nanoplasma coupling, our results suggest that the plasma resonance should occur at the peak intensity of the laser pulse. Due to inertial effects, even for such small-sized clusters, the observed optimum pulse duration is in the order of 1 ps which is in good agreement with our theoretical results. Received 18 March 2002 Published online 19 July 2002     

Measurements of sound velocity of laser-irradiated iron foils relevant to Earth core condition

We present a novel scheme to measure sound velocity of shock-compressed iron of geophysical interest. The sound velocity of laser-irradiated iron foils was obtained with side-on X-ray radiograph technique from measured rarefaction wave velocity of shocked iron. Iron foils were irradiated with a two-stepped square laser pulse to reach Earth's core condition by double compression. The experimental parameters of temperature and pressure were very close to the Earth's core condition.     

Radiation reflection by a plasma with electron temperature anisotropy

The reflection of a test electromagnetic wave normally impinging on a plasma surface is investigated within the formalism of the surface impedance. The plasma is assumed to possess an anisotropic two-temperature bi-Maxwellian electron velocity distribution function. The linearly polarized impinging wave during reflection transforms into an elliptically polarized one, the degree of ellipticity depending on the electron temperature anisotropy. Polarization modifications of the reflected wave are particularly important in the conditions of the anomalous skin-effect, when the influence of the wave magnetic field on the electron kinetics in the skin layer is strong. Relations are reported connecting the reflected wave basic parameters to those of the reflecting plasma surface, making possible, through the experimental determination of the reflected wave characteristics, to find the plasma electron concentration and the two effective temperatures. Received 21 May 2002 / Received in final form 21 August 2002 Published online 6 November 2002 RID="a" ID="a"e-mail: zarcone@unipa.it     

The use of high energy laser-plasma sources in soft X-ray contact microscopy of living biological samples

In this paper the results of an experiment on soft X-ray contact microscopy using a laser-plasma source are presented. A resolution of 50 nm has been achieved imaging pig sperm cells, while other specimens, such as algae and yeast cells, showed internal details, proving the technique to be a powerful tool for biological investigations. Original biological information has been obtained and the conditions for optimal image formation have been studied. Received 5 June 2002 Published online 24 September 2002 RID="a" ID="a"e-mail: batani@mib.infn.it     

Study of the conditions for the effective energy transfer in a process of acceleration and collision of the thin metal disks with the massive target

Efficiency studies of laser driven thin metal disks acceleration using the first harmonic (λ₁=1.315 μm) of the Prague Asterix Laser System (PALS) and subsequent craters creation produced by collisions of these disks with massive targets are presented. Several different disks made of aluminium and copper foils with diameters of 300 μm and 600 μm and thicknesses of 11 μm (Al) and 3.6 μ m (Cu) were employed. Disks were placed at the distance of either 100 μ m or 300 μm in front of aluminium massive targets. The following irradiation conditions were used: the laser beam energy of 120 J, the focal spot diameter of 200 μm, and the pulse duration of 0.4 ns (FWHM). A three-frame interferometric system was employed to determine electron density distributions in plasma corona. Shape and volume of craters were obtained by crater replica technology and microscopy measurements. The aim of these investigations was to analyse conditions leading to the most effective energy transfer in the process of collision of the accelerated disks with solid targets. The overall efficiency of these processes was characterized by the volume of craters produced in such targets.     

A plasma channel scheme for the interaction of an intense femtosecond laser pulse with a deuterium cluster jet

We propose a plasma channel scheme to obtain an improved table-top laser driven fusion neutron yield as a result of explosions of large deuterium clusters irradiated by an intense laser pulse. A cylindrical plasma channel is created by two moderate intensity laser prepulses at the edge of a deuterium cluster jet along which an intense main laser pulse propagates several nanoseconds later. With the aid of this plasma channel, the main laser pulse will be allowed to deposit its energy into the central region of the deuterium gas jet where the cluster sizes are larger and the atomic density is higher. The plasma channel formation and its impact on the deuterium ion energy spectrum and the consequent fusion neutron yield have been investigated. The calculated results show that a remarkable increase of the table-top laser driven fusion neutron yield would be expected.     

Influence of laser pulse width on absolute EUV-yield from Xe-clusters

Using 50 fs ( ∼ 2×10¹⁸ W/cm²) and 2 ps ( ∼ 5×10¹⁶ W/cm²) pulses from a Ti:Sa multi-TW laser at 800 nm wavelength large Xe-clusters ( 10⁵...10⁶ atoms per cluster) have been excited. Absolute yield measurements of EUV-emission in a wavelength range between 10 nm and 15 nm in combination with cluster target variation were carried out. The ps-laser pulse has resulted in about 30% enhanced and spatially more uniform EUV-emission compared to fs-laser excitation. Circularly polarized laser light instead of linear polarization results in enhanced emission which is probably caused by electrons gaining higher energies by the polarization dependent optical field ionization process. An absolute emission efficiency at 13.4 nm of up to 0.8% in 2π sr and 2.2% bandwidth has been obtained. Received 11 January 2001 and Received in final form 27 March 2001     

The effect of volumetric weighting in the interaction of intense laser fields with clusters

Silver clusters embedded in helium nanodroplets are exposed to intense femtosecond laser pulses (10¹³ - 10¹⁶ W/cm²). The signal of highly charged (q≤11) atomic fragments is maximized by delayed plasmon enhanced ionization using stretched laser pulses. Further details with respect to the dynamics of the charging process can be obtained, when the intensity distribution within the laser focus is taken into account. For the first time, the z-scan method is applied to clusters which offers a route to investigate the explicit dependence of the ion signals with respect to the laser intensity. By taking advantage of the volumetric weighting effect ionization thresholds are determined, yielding values well below 10¹⁴ W/cm² for Ag^q+ ions with q≤11.     

Dynamics of free and embedded lead clusters in intense laser fields

Lead clusters are exposed to strong femtosecond light pulses. The dependence of the recoil energy on the charge state of the atomic ion is now investigated using a new detection setup, i.e., a Thomson analyser. First results show that in contrast to laser-induced overdense plasmas at surfaces the recoil energy distribution appears much narrower. Comparing free lead clusters with lead clusters embedded in large helium droplets, the charging dynamics show distinct differences on the femtosecond time scale. In the embedded case the maximum ionization enhancement is reached earlier.     

Modelling dynamics of samples exposed to free-electron-laser radiation with Boltzmann equations

We apply Boltzmann equations for modelling the radiation damage in samples irradiated by photons from free electron lasers (FELs). We test this method in a study case of a spherically symmetric xenon cluster irradiated with VUV FEL photons. Qualitative agreement between the model predictions and experimental data is found. The results obtained demonstrate the potential of the Boltzmann method for describing the complex and non-equilibrium dynamics of samples exposed to FEL radiation.     

Photodissociation and photoionization of sodium coated C $_\mathsf{60}$ clusters

At the Prague Asterix Laser System Center (PALS) the Asterix iodine laser delivering up to 700 J/0.5 ns is used as a pump source for X-ray laser experiments and applications. The prepulse technique was applied which is known to improve the neon-like X-ray laser output at the J = 0-1 transition dramatically. Since Zn slab targets were used the operating wavelength was 21.2 nm. A prepulse having up to 20 J precedes the main pulse by 10 ns. The main beam and the prepulse beam are focussed by two different optical systems separately and their foci are superimposed at the target surface. By implementing a half-cavity set-up for double-pass amplification using a Mo/Si multilayer mirror - which can be used for more than 100 shots - the X-ray laser output was more than 10 times stronger than at the single pass in a 30 mm long plasma. Double-pass amplification was observed to be most efficient when the pump pulse duration was at least 150 ps longer than the round trip time ( ≈ 260 ps) in the half-cavity. Under this fundamental condition the X-ray laser reached saturation in the double-pass regime containing approx. 4 mJ energy which has been proved to be enough for future applications. In this contribution, the X-ray laser features like divergence in two dimensions, the beam quality (symmetry), the pointing angle and the integrated intensity giving an estimation of the output energy are investigated over 110 shots. To characterize the stability of the X-ray laser the shot distribution, the mean value and the standard deviation for these parameters are evaluated. For 18 shots in a series - what was achievable during one day - the corresponding values are given, and a statistical analysis carrying out a chi-squared test characterize the Zn X-ray laser as a robust tool suitable for applications. In the future it is planned to allocate X-ray laser beam time to external research groups. Received 17 May 2002 / Received in final form 10 September 2002 Published online 6 November 2002 RID="a" ID="a"e-mail: praeg@fzu.cz     

Analysis of the impedance mismatch effect in foam-solid targets compressed by laser-driven shock waves

The impedance mismatch effect in a two-layer (low density plastic foam, and solid aluminum, respectively) plane target compressed by a laser driven shock wave is considered. In such targets the ablative pressure generated by absorption of laser light in the foam layer is amplified when crossing the foam-aluminum interface. In this paper an analytical model is developed to evaluate the shock pressure in the aluminum layer as a function of the density and thickness of the foam layer and of the laser parameters. The model is in good agreement with previously published experimental results [A. Benuzzi et al., Phys. Plasmas 5, 2827 (1998)]. Received 20 January 2000 and Received in final form 16 May 2000     

Plasma-based X-ray laser at 21 nm for multidisciplinary applications

An overview of recent advances in applications of currently the most energetic X-ray laser at 21 nm is given. The unique parameters of this half-cavity based X-ray laser such as record output energy of 10 mJ, highly symmetric beam, robustness and reproducibility, have made it possible to carry out a number of multidisciplinary scientific projects featuring novel applications of intense coherent X-ray radiation. Selected results obtained in these experiments are reviewed, including X-ray laser probing of dense plasmas, measurements of transmission of focused soft X-ray radiation at intensities of up to 10¹² W cm^-2, measurements of infrared laser ablation rates of thin foils, and ablative microstructuring of solids.     

Nonlinear interactions between drift waves and zonal flows

Phase coherent interactions between drift waves and zonal flows are considered. For this purpose, mode coupling equations are derived by using a two-fluid model and the guiding center drifts. The equations are then Fourier analyzed to deduce the nonlinear dispersion relations. The latter depict the excitation of zonal flows due to the ponderomotive forces of drift waves. The flute-like zonal flows with insignificant density fluctuations have faster growth rates than those which have a finite wavelength along the magnetic field direction. The relevance of our investigation to drift wave driven zonal flows in computer simulations and laboratory plasmas is discussed. Received 5 April 2002 Published online 28 June 2002     

Modeling of clusters by a molecular dynamics model using a fast tree method

The dynamics of clusters irradiated by a high-intensity ultrashort pulse laser has been studied using a fully relativistic three-dimensional Molecular Dynamics Model. A fast three-dimensional tree algorithm for computing the electrostatic force has been developed and compared with the conventional particle-particle method. The particle-particle method requires computation time, which scales as O(N_p ²), and it is faster for small number of particles N_p <10³. In the opposite case of relatively large ensemble of particles N_p >10³, the preferred method is the tree algorithm whose computation time scales as O(N_p log N_p). The tree algorithm has been benchmarked against the particle-particle method for clusters composed of xenon and deuterium atoms and its accuracy and computation time have been analyzed. The optimum free parameter of the tree method has been determined to be θ≈0.5. We addressed the effects of boundary conditions by studying the contribution of adjacent clusters to the total electromagnetic force exerted on individual particles. We found that the adjacent clusters play a minor role in the overall cluster dynamics.     

Solitary waves in the Madelung's fluid: Connection between the nonlinear Schrödinger equation and the Korteweg-de Vries equation

An investigation to deepen the connection between the family of nonlinear Schr?dinger equations and the one of Korteweg-de Vries equations is carried out within the context of the Madelung's fluid picture. In particular, under suitable hypothesis for the current velocity, it is proven that the cubic nonlinear Schr?dinger equation, whose solution is a complex wave function, can be put in correspondence with the standard Korteweg-de Vries equation, is such a way that the soliton solutions of the latter are the squared modulus of the envelope soliton solution of the former. Under suitable physical hypothesis for the current velocity, this correspondence allows us to find envelope soliton solutions of the cubic nonlinear Schr?dinger equation, starting from the soliton solutions of the associated Korteweg-de Vries equation. In particular, in the case of constant current velocities, the solitary waves have the amplitude independent of the envelope velocity (which coincides with the constant current velocity). They are bright or dark envelope solitons and have a phase linearly depending both on space and on time coordinates. In the case of an arbitrarily large stationary-profile perturbation of the current velocity, envelope solitons are grey or dark and they relate the velocity u₀ with the amplitude; in fact, they exist for a limited range of velocities and have a phase nonlinearly depending on the combined variable x-u_{0 s} (s being a time-like variable). This novel method in solving the nonlinear Schr?dinger equation starting from the Korteweg-de Vries equation give new insights and represents an alternative key of reading of the dark/grey envelope solitons based on the fluid language. Moreover, a comparison between the solutions found in the present paper and the ones already known in literature is also presented. Received 20 February 2002 and Received in final form 22 April 2002 Published online 6 June 2002     

Progress in direct-drive inertial confinement fusion research at the laboratory for laser energetics

Direct-drive inertial confinement fusion (ICF) is expected to demonstrate high gain on the National Ignition Facility (NIF) in the next decade and is a leading candidate for inertial fusion energy production. The demonstration of high areal densities in hydrodynamically scaled cryogenic DT or D₂ implosions with neutron yields that are a significant fraction of the “clean” 1-D predictions will validate the ignition-equivalent direct-drive target performance on the OMEGA laser at the Laboratory for Laser Energetics (LLE). This paper highlights the recent experimental and theoretical progress leading toward achieving this validation in the next few years. The NIF will initially be configured for X-ray drive and with no beams placed at the target equator to provide a symmetric irradiation of a direct-drive capsule. LLE is developing the “polar-direct-drive” (PDD) approach that repoints beams toward the target equator. Initial 2-D simulations have shown ignition. A unique “Saturn-like” plastic ring around the equator refracts the laser light incident near the equator toward the target, improving the drive uniformity. LLE is currently constructing the multibeam, 2.6-kJ/beam, petawatt laser system OMEGA EP. Integrated fast-ignition experiments, combining the OMEGA EP and OMEGA Laser Systems, will begin in FY08.