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The temporal evolution of the ablation plume of cerium was investigated by absorption spectroscopy. Cerium oxide pellets were ablated in a helium atmosphere by second-harmonic radiation (532 nm) from a Nd:YAG laser at a fluence of 0.5 J/cm2. The lateral velocity (expansion velocity horizontal to the sample surface) of the plume was determined from the magnitude of the Doppler splitting of the absorption spectra measured close to the sample surface. The lateral velocities of neutral and singly ionized atoms were systematically investigated by varying several parameters, such as ambient gas pressure, ablation laser fluence, observation timing, and observation height. In addition, temporal profiles of the absorption signal were measured by detuning the probe laser frequency from the atomic resonant frequency in order to obtain the temporal variation of the velocity. On the basis of the drag force model, the slowing coefficients for atomic and ionic species in a helium atmosphere were evaluated along with lateral velocity in a vacuum. This study may help in understanding the plume dynamics effect on deposited film properties as well as optimizing experimental conditions for ablation-based spectroscopic analysis. [ABSTRACT FROM AUTHOR]

To determine experimental conditions suitable for isotope analysis, we studied the plume dynamics of uranium. A uranium oxide sample was ablated by 2nd harmonic radiation from a Nd:YAG laser at a fluence of 0.5 J/cm. The temporal evolution of the ablation plume was investigated in vacuum and helium environments. In vacuum, the flow velocity perpendicular to the sample surface was determined to be 2.7 km/s for neutral atoms and 4.0 km/s for singly charged atoms. These velocities are about 20 % lower than those of cerium measured under similar conditions. From the evolution of the plume in helium, we found that an observation time of 3-5 μs and an observation height of about 2.5 mm are most suited for obtaining higher sensitivity. Observation times less than 3 μs were unsuitable for precise isotope analysis since the spectral modifications arising from the Doppler splitting effect are different between the two uranium isotopes. Using the established conditions, we evaluated the calibration curve linearity, limit of detection, and precision for three samples having different abundances of U. [ABSTRACT FROM AUTHOR]

The first report on a model experiment of cosmic ray acceleration by using intense laser pulses is presented. Large amplitude light waves are considered to be excited in the upstream regions of relativistic astrophysical shocks and the wakefield acceleration of cosmic rays can take place. By substituting an intense laser pulse for the large amplitude light waves, such shock environments were modeled in a laboratory plasma. A plasma tube, which is created by imploding a hollow polystyrene cylinder, was irradiated by an intense laser pulse. Nonthermal electrons were generated by the wakefield acceleration and the energy distribution functions of the electrons have a power-law component with an index of ∼2. The maximum attainable energy of the electrons in the experiment is discussed by a simple analytic model. In the incoherent wakefield the maximum energy can be much larger than one in the coherent field due to the momentum space diffusion or the energy diffusion of electrons. [ABSTRACT FROM AUTHOR]

High density energetic electrons that are created by intense laser plasma interactions drive MeV proton acceleration. The correlation between accelerated MeV protons and escaped electrons is experimentally investigated at laser intensities in the range of 1018–1019 W/cm2 with S-polarization. Observed proton maximum energies are linearly proportional to escaped electron slope temperatures with a scaling coefficient of about 10. In the context of the simple analytical fluid model for transverse normal sheath acceleration, hot electron sheath density near the target rear surface can be estimated if an empirical acceleration time is assumed. [ABSTRACT FROM AUTHOR]

This article reports about controlling laser-accelerated proton beams with respect to beam divergence and energy. The particles are captured by a pulsed high field solenoid with a magnetic field strength of 8.6 T directly behind a flat target foil that is irradiated by a high intensity laser pulse. Proton beams with energies around 2.3 MeV and particle numbers of 1012 could be collimated and transported over a distance of more than 300 mm. In contrast to the protons the comoving electrons are strongly deflected by the solenoid field. They propagate at a submillimeter gyroradius around the solenoid’s axis which could be experimentally verified. The originated high flux electron beam produces a high space charge resulting in a stronger focusing of the proton beam than expected by tracking results. Leadoff particle-in-cell simulations show qualitatively that this effect is caused by space charge attraction due to the comoving electrons. The collimation and transport of laser-accelerated protons is the first step to provide these unique beams for further applications such as postacceleration by conventional accelerator structures. [ABSTRACT FROM AUTHOR]

By using a cone attached to a capillary, electrons generated through a laser interaction were autoinjected and accelerated in a low-density wake field. The cone attached to the entrance of the capillary serves as an electron supplier. It increases the number of electrons from below the detection limit to 1.1 pC and the energy from 4 to 30 MeV. A two-dimensional particle-in-cell simulation reveals that a significant number of energetic electrons are extracted from the surface of the cone and are subsequently trapped in the wake field and accelerated in the capillary. [ABSTRACT FROM AUTHOR]

Generation of high energy negative ions from laser plasmas by Coulomb implosion mechanism is investigated. When clusters or underdense plasmas are irradiated by an intense laser pulse, positive ions are accelerated inside the clusters or in the self-focusing channel by the Coulomb explosion. This could lead to the acceleration of negative ions toward target center. The maximum energy of negative ions is typically several times lower than that of positive ions. A theoretical description and corresponding particle-in-cell simulations of Coulomb implosion mechanism are presented. We show the evidence of the negative ion acceleration observed in our experiments using high intensity laser pulse and the cluster-gas targets. [ABSTRACT FROM AUTHOR]

It has been shown that a femtosecond plasma of cluster targets is an almost isotropic source of fast ions and, hence, can be used to obtain ionographic images with a wide field of view. The spatial resolution of the resulting ionographic images is no worse than 600 nm, which corresponds to a uniquely high value of about 105 of the ratio of the field of view to the resolution. The use of 100–300-keV ion fluxes ensures the sensitivity of the method to the sample thickness of no worse than 100 nm even for samples consisting of light chemical elements (C, H). The proposed method can be used to obtain images of low-contrast biological objects, thin films, membranes, and other nanostructured objects. [ABSTRACT FROM AUTHOR]

Energy spectra of fast electrons, generated when high-intensity laser pulses irradiated hollow conical targets, have been measured experimentally. It is shown here that the slope temperature of the fast electrons is strongly dependent on the opening angle of the cone, and has a maximum value at 25°. The data confirms optical guiding of the laser pulse, by comparison of the measured electron temperature with ray-tracing calculations that include absorption in plasmas. The enhanced energy flow and intensity induced by optical guiding of the laser pulse inside the cone as a function of the opening angle as well as the f-number of the focusing optics is discussed. [ABSTRACT FROM AUTHOR]

A large number of hot electrons exceeding the Alfvén current can be produced when an ultraintense laser pulse irradiates a solid target. Self-excited extreme electrostatic and magnetic fields at the target rear could influence the electron trajectory. In order to investigate the influence, we measure the hot electrons when a plasma was created on the target rear surface in advance and observe an increase of the electron number by a factor of 2. This increase may be due to changes in the electrostatic potential formation process with the rear plasma. Using a one-dimensional particle-in-cell simulation, it is shown that the retardation in the electrostatic potential formation lengthens the gate time when electrons can escape from the target. The electron number escaping within the lengthened time window appears to be much smaller than the net produced number and is consistent with our estimation using the Alfvén limit. [ABSTRACT FROM AUTHOR]

We have developed an advanced design for a time-of-flight (TOF) neutron spectrometer applied to the study of ultraintense laser-plasma interactions in the area of fast ignition research. The interaction of an ultraintense laser pulse with plasma creates intense gamma rays, which generate a high level of background noise in the thermal neutron detection process. High energy-density ions, which are generated by strong electric fields associated with the laser interactions, produce beamlike fusion neutrons with an angular distribution that is dependent on the momentum distribution of the ions. These beamlike fusion neutrons also act as a source of significant background noise in the thermal neutron detection process. In order to improve the signal-to-noise ratio (S/N), we have proposed and applied a wave-coincidence method to the neutron detection process. We have demonstrated the advantages of this advanced method in ultraintense laser-plasma interaction experiments, where an improvement in S/N by a factor of 7–8 compared to the conventional TOF method was achieved. We have calculated that the value of S/N increases with the number of detectors used as well as with the initial value of S/N for a single detector. [ABSTRACT FROM AUTHOR]

The generation of extremely bright coherent X-ray pulses in the femtosecond and attosecond regime is currently one of the most exciting frontiers of physics–allowing, for the first time, measurements with unprecedented temporal resolution. Harmonics from laser–solid target interactions have been identified as a means of achieving fields as high as the Schwinger limit (E=1.3×1016 V m−1) and as a highly promising route to high-efficiency attosecond (10−18 s) pulses owing to their intrinsically phase-locked nature. The key steps to attain these goals are achieving high conversion efficiencies and a slow decay of harmonic efficiency to high orders by driving harmonic production to the relativistic limit. Here we present the first experimental demonstration of high harmonic generation in the relativistic limit, obtained on the Vulcan Petawatt laser. High conversion efficiencies (η>10−6 per harmonic) and bright emission (>1022 photons s−1 mm−2 mrad−2 (0.1% bandwidth)) are observed at wavelengths <4 nm (the `water-window' region of particular interest for bio-microscopy). [ABSTRACT FROM AUTHOR]

Origins and acceleration directions of accelerated ions inside solid LiF, CH-LiF, and LiF-CH targets irradiated by a 450 fs, 20 J, 1053 nm laser at an intensity of 3×1018 W/cm2 have been investigated by neutron spectroscopy. The irradiated targets generate neutrons through the reaction 7Li (p,n)7Be between accelerated protons and background 7Li ions inside the target. The produced neutron spectra observed from two different observation angles 20° and 120° to the target rear-side normal. From the measured and calculated spectra, by three-dimensional Monte Carlo code, the maximum energy, the total number, and the slope temperature of the accelerated ions are investigated. The results indicate that ions are not only accelerated from the front surface toward the rear surface, but also from the rear surface toward the front surface with comparable maximum energy and higher number. [ABSTRACT FROM AUTHOR]

Proton acceleration inside solid LiF and CH–LiF targets irradiated by a 450-fs, 20-J, 1053-nm laser at an intensity of 3×1018 W/cm2 has been studied via neutron spectroscopy. Neutron spectra produced through the 7Li(p,n)7Be reaction that occurs between accelerated protons, at the front surface, and background 7Li ions inside the target. From measured and calculated spectra, by three–dimensional Monte Carlo code, the maximum energy, total number, and slope temperature of the accelerated protons are investigated. The study indicates that protons originate at the front surface and are accelerated to a maximum energy that is reasonably consistent with the calculated one due to the ponderomotive force. [ABSTRACT FROM AUTHOR]

Detailed neutron energy spectra produced from a CD2 target irradiated by a 450 fs, 20 J, 1053 nm laser at an intensity of 3×1018 W/cm2 have been studied. Wide-ranging neutron spectra were observed from two different observation angles 20° and 70° relative to the rear-side target normal. The experiment and numerically calculated spectra, by a three-dimensional Monte Carlo code, indicate that the range of the measured spectra is larger than that produced by the D(d,n)3He reaction. An interpretation for the measured spectra is introduced by considering the 12C(d,n)13N and D(12c,n)13N reactions. In addition, the study revealed that the neutron spectra produced by the D–C and C–D reactions can overlap that produced by the D–D reaction, and due to their high cross sections, comparing to the D–D reaction, both of them effectively participate in the neutron yield. [ABSTRACT FROM AUTHOR]

Discusses a study which examined fast heating of ultrahigh density plasma as a step toward laser fusion ignition. How the propagation of relavistic electrons in solid is measured; Details on the experiments conducted on plasma at the University of Osaka in Japan; Process used in investigating the heating efficiency.

An intense isotropic source of multicharged carbon and oxygen ions with energy above 300 keV and particle number >108 per shot was obtained by femtosecond Ti:Sa laser irradiation of submicron clusters. The source was employed for high-contrast contact ionography images with 600 nm spatial resolution. A variation in object thickness of 100 nm was well resolved for both Zr and polymer foils. [ABSTRACT FROM AUTHOR]

We report the demonstrated irradiation effect of laser-accelerated protons on human cancer cells. In vitro (living) A549 cells are irradiated with quasimonoenergetic proton bunches of 0.8–2.4 MeV with a single bunch duration of 15 ns. Irradiation with the proton dose of 20 Gy results in a distinct formation of γ-H2AX foci as an indicator of DNA double-strand breaks generated in the cancer cells. This is a pioneering result that points to future investigations of the radiobiological effects of laser-driven ion beams. Unique high-current and short-bunch features make laser-driven proton bunches an excitation source for time-resolved determination of radical yields. [ABSTRACT FROM AUTHOR]

Energetic electrons were generated by the interaction of a high-intensity laser pulse with a plasma preformed from a hollow plastic cylinder via laser-driven implosion. The spectra of a comparatively high-density plasma ∼1019 cm-3 had a bump around 10 MeV. Simple numerical calculations explained the spectra obtained in this experiment. This indicates that the plasma tube has sufficient potential to convert a Maxwellian spectrum to a comparatively narrow spectrum. [ABSTRACT FROM AUTHOR]

The application of high intensity laser-produced gamma rays is discussed with regard to picosecond resolution deep-penetration radiography. The spectrum and angular distribution of these gamma rays is measured using an array of thermoluminescent detectors for both an underdense (gas) target and an overdense (solid) target. It is found that the use of an underdense target in a laser plasma accelerator configuration produces a much more intense and directional source. The peak dose is also increased significantly. Radiography is demonstrated in these experiments and the source size is also estimated. [ABSTRACT FROM AUTHOR]