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[en] Complete text of publication follows. We report on recent results in the generation, characterization and applications of energetic attosecond pulse trains and ultra-broad coherent XUV continua: 1) Generation: 1a) We report experimental results confirming contribution of both long and short trajectories in on-axis harmonic generation before, at and after an atomic gas jet, i.e. under three different phase matching conditions. The contribution of both trajectories is manifested through their interference leading to a modulated harmonic (and side band) yield as a function of the driving intensity. 1b) We report the generation of sub-fs pulse trains at the 40 μJ pulse energy level from laser surface plasma, measured through 2nd order intensity volume autocorrelation (2nd order IVAC). 2) Characterization: We present comparative studies between RABITT and 2nd order IVAC in on axis harmonic generation before, at and after an atomic gas jet. We find that the two techniques give fairly different results that are compatible with the differently weighted but unavoidable presence of the long and short trajectory in the generation process in all three phase matching conditions. We show that the relative contributions of the two trajectories can be estimated through RABITT measurements, while spatiotemporal mean pulse durations can be extracted from 2nd order IVAC traces. 3) Applications: 3a) We present time resolved VUV spectroscopy of ultrafast dynamics in molecular ethylene. 3b) We present time resolved XUV spectroscopy at the 1 fs temporal scale and ultra-broad band XUV Fourier Transform Spectroscopy in a manifold of doubly excited autoionizing and inner-shell Auger decaying states excited simultaneously through a coherent broadband XUV continuum. Acknowledgments. This work is supported in part by the European Community's Human Potential Program under contract MTKD-CT-2004-517145 (X-HOMES), the Ultraviolet Laser Facility (ULF) operating at FORTH-IESL (contract PHRI-CT-2001-00139), the ELI research infrastructure preparatory phase program, the FASTQUAST ITN, and the FLUX program of the 7th FP.
[en] Complete text of publication follows. Laser ion acceleration has caught significant attentions due to its various applications such as medical applications, ion beam fast ignition, and proton radiography. For medical applications, laser ion acceleration is expected to realize a compact and reliable ions source for the cancer therapy. One of the critical issues on laser-driven ion source is the enhancement of laser-accelerated ion energy, where protons with maximum energy of 200 MeV are requested from the medical point of view. The mechanism known as a target normal sheath acceleration (TNSa) has been widely investigated theoretically and experimentally, and it is shown that a relatively high power laser such as PW-class laser is needed for generating 200 MeV protons. Recently, high energy ions are observed from gas-cluster targets with relatively small laser energy, where 20 MeV/u ions are generated by using 4 TW laser pulse. We consider that these high energy ions, the energy of which is roughly one order higher than the TNSA energy scaling, are generated via formation and evolutions of magnetic dipole vortex. In this paper, we investigate a detail of the ion acceleration via magnetic dipole vortex and derive an ion energy scaling. By the propagation of an intense laser pulse thorough underdense plasma, a dipole vortex is induced when the laser energy is almost depleted. In Fig. 1(a), an ion distribution is plotted when a magnetic dipole vortex is formed. Electrons and resultant ions are pushed outward from the vortex, forming an ion shell around the vortex and wall along the vortex axis. Electron, ion and electric field distributions along the axis are plotted in Fig. 1.(b). The ion distribution has a sharp spike and electrons are located in front of it, which results in generation of strong electro-static field. This structure moves to the right together with expanding dipole vortex. This leads to an ion acceleration by moving electric field. We modeled the magnetic dipole acceleration and obtained energy scaling, which predicts that a 100 TW laser can generate 200 MeV protons by magnetic dipole acceleration.
[en] Complete text of publication follows. With the intense laser entering the relativistic regime for some time, the vision of laser wakefield acceleration has become a reality. With the energy of relativistic laser further increasing, we now envision a regime of greater accelerated energies as well as new regimes of acceleration, including ions. With the 100 J class laser now soon in hand, we foresee multi-staged acceleration reaching beyond 100 GeV, a serious energy threshold. With this class ion acceleration also enters into a new regime where ions too become relativistic and thus monoenergy. We will discuss TeV and PeV laser acceleration in this talk. In addition to laser-based collider possibility (for which we advocate laser development with high efficiency and high repetition rate), we also envision possibility of conducting fundamental physics research based on non-collider approaches. PeV energies will allow us to feel the texture of vacuum. Meanwhile, high intensity fields of laser could excite and resonate with 'vacuum modes', for which we will elaborate. Finally, I will mention some of societal applications that may be derived from these laser technology development spurred by the interest into fundamental physics.
[en] Complete text of publication follows. Experimental results of the LOASIS (Lasers, Optical Accelerator Systems Integrated Studies) Laboratory of LBNL achieved with the routine operation of a 10 TW (Godzilla) and a 100 TV class laser system (TREX) are reviewed. Recent laser plasma acceleration (LPA) results include GeV electron beam production in 3-cm capillary discharge, controlled injection via longitudinal density tailoring in a plasma channel, LPA electron beam control using quadrupole magnets, and e-beam characterization using optical transition radiation (OTR), phosphor screen monitors, and cavity based beam pointing monitors (BPMs). Progress on the development of an undulator-based e-beam diagnostic will also be discussed. These recent LPA experimental results open up new opportunities to develop compact, extremely bright light sources with wavelengths spanning the extreme edges of the electromagnetic spectrum from the THz waves, through the deep UV and hard X-rays, to multi-MeV gamma radiation. The physical processes in these light generation methods are: coherent transition radiation generated by the LPA e-beam at a plasma-vacuum interface for THz radiation generation; undulator radiation in a LPA-driven free electron laser (FEL) for XUV generation; betatron motion and emission of synchrotron radiation from the accelerated electrons in a plasma channel for keV-range X-ray generation; Thomson scattering of visible laser light on LPA electrons for gamma ray production. Design of experiments and early results of measurements performed in the LOASIS Program at LBNL, as well as the prospects for light source applications based on LPAs will be reviewed. Acknowledgement. This work has been supported by the Director, Office of Science, Office of High Energy Physics, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, and DTRA.
[en] Complete text of publication follows. The shortest - attosecond - light pulses available today are produced by high harmonic generation (HHG) of near-infrared (NIR) laser pulses in noble gas jets, providing a broad spectral plateau of XUV radiation ending in a cutoff. The minimum pulse duration is determined by the achievable bandwidth (i.e. the position of the cutoff), and the chirp of the produced pulses. The extension of the cutoff by increasing the laser intensity is limited by the depletion and phase matching problems of the medium. An alternative method demonstrated to produce higher harmonic orders is by using longer pump pulse wavelength, with the disadvantage of decreased efficiency. Recently it was shown that application of a quasi-DC high strength electric field results in an increase of more than a factor of two in the order of efficiently generated high harmonics. However, the possibility to implement the method proposed in  of using a CO2 laser to create a quasi-DC field for assisting HHG of the NIR laser is questionable, because it's technically very challenging to synchronize pulses from different laser sources. Alternatively, synchronous production of THz pulses with the NIR laser pulse offers a more promising route. The first numerical test of this idea has been reported in . In this contribution we further investigate the method for realistic THz field strengths and short driving pulses, exploring the effect of longer pump laser wavelength on the process. We assume the presence of high intensity THz pulses for supplying the high-strength quasi-DC electric field. The spectrum as well as the chirp of the produced radiation is calculated. We use the non-adiabatic saddle point method to determine the generated radiation described in . We simulate harmonic generation in noble gas atoms, with few cycle NIR pulses of peak intensity at and above 2 x 1014 W/cm2 (388 MV/cm) and wavelengths 800 nm and 1560 nm. The THz field strength is varied from 0 to 100 MV/cm (the highest field strength currently available). The generated spectra is calculated for each half-cycle, and coherently summed. As a result of the presence of the THz field, the half-cycle periodicity of the HHG process is broken, leading to the appearance of both odd and even harmonics and a radiation with a spectrum split to two plateaus. The two cutoffs are set by the radiation produced in the consecutive half-cycles. The higher cutoff increases, whereas the lower cutoff decreases with increasing THz field strength. In cases when THz field is added to a few cycle laser pulse a broad super-continuum part in the spectra can be obtained. The broad spectrum of the produced radiation would support the synthesis of single attosecond pulses in the absence of a strong chirp. The method presented here allows for the production of a broader spectral range of harmonics, leading to the synthesis of even shorter attosecond pulses.
[en] Complete text of publication follows. Attosecond light pulses (1 as 10-18 s) can nowadays be produced by high-harmonic generation (HHG) in the Extreme Ultraviolet (EUV). In this spectral range, optical systems are much more difficult to design than in the visible range, partly because of the absorption of materials. Several techniques were proposed to overcome the problem of attosecond pulse control after the high harmonic source, for example by using filters, plasmas or gas mediums. Multilayer aperiodic chirped mirrors, which efficiently reflect EUV radiation, were recently proposed in order to manipulate such pulses. But the possibility of phase control over large spectral bandwidth has not yet been demonstrated. We designed and manufactured three plane multilayer mirrors with optimized reflectivity and controlled spectral phase in the 35-55 eV range near 45 deg incidence. Their reflectivity was characterized on the Elettra Synchrotron and their spectral phase, on an attosecond pulse source at CEA SPAM, providing a full characterization of their spectral response. We report on the characterization of these mirrors and show how they affect the temporal profile of the attosecond pulses. Such pulses provided by the HHG process are intrinsically chirped. We demonstrate that they can be recompressed using our designed multilayer aperiodic mirrors. Until now attosecond pulse manipulation was restricted to some shaping functions mainly determined by the material of the used optical elements. In this presentation we also propose a more flexible way to shape the pulses giving relatively high intensities. A combination of two may provide us with an active attosecond pulse-shaper. It allows the realization of some simple functions as a tunable pulse compression, or the shaping of a single, double or multiple sub-100 as pulses, the properties of which can be set by simply turning the pulse-shaper. Acknowledgements. This work was supported by the ANR project 07-BLAN-0150. The multilayer fabrication have been carried out on CEMOX (Centrale d'Elaboration et de Metrologie des Optiques X).
[en] Complete text of publication follows. In an Inertial Confinement Fusion (ICF) situation, laser driven ablation front of an imploding capsule is subjected to the fluid instabilities like Rayleigh-Taylor (RT), Richtmyer-Meshkov (RM) and Kelvin-Helmholtz (KH) instability. In this case dense core is compressed and accelerated by low density ablating plasma. During this process laser driven shocks interact the interface and hence it becomes unstable due to the formation of nonlinear structure like bubble and spike. The nonlinear structure is called bubble if the lighter fluid pushes inside the heavier fluid and spike, if opposite takes place. R-M instability causes non-uniform compression of ICF fuel pellets and needs to be mitigated. Scientists and researchers are much more interested on RM instability both from theoretical and experimental points of view. In this article, we have presented the analytical expression for the growth rate and velocity for the nonlinear structures due to the effect of magnetic field of fluid using potential flow model. The magnetic field is assumed to be parallel to the plane of two fluid interfaces. If the magnetic field is restricted only to either side of interface the R-M instability can be stabilized or destabilized depending on whether the magnetic pressure on the interface opposes the instability driving shock pressure or acts in the same direction. An interesting result is that if both the fluids are magnetized, interface as well as velocity of bubble and spike will show oscillating stabilization and R-M instability is mitigated. All analytical results are also supported by numerical results. Numerically it is seen that magnetic field above certain minimum value reduces the instability for compression the target in ICF.
[en] Complete text of publication follows. The filamentation of the high power laser beam by taking off-axial contribution is investigated when ponderomotive nonlinearity is taken into account. The splitted profile of the laser beam is obtained due to uneven focusing of the off-axial rays. It is observed that the weak electron plasma wave (EPW) propagating in the z direction is nonlinearly coupled in the modified filamentary regions of the laser beam. The semi-analytical solution of the nonlinear coupled EPW equation in the presence of laser beam filaments has been found and it is observed that the nonlinear coupling between these two waves leads to localization of the EPW. Stimulated Raman scattering (SRS) of this EPW is studied and back reflectivity has been calculated. Further, the localization of EPW affects the eigen frequency and damping of plasma wave. As a result of this, mismatch and modified enhanced Landau damping lead to the disruption of SRS process and a substantial reduction in the back reflectivity. For the typical laser beam and plasma parameters with wavelength (λ = 1064 nm), power flux (∼ 1016 W cm-2), and plasma density (n/ncr) 0.2; the back reflectivity was found to be suppressed by a factor of around 20%.
[en] Complete text of publication follows. By using Maxwell equations and the nonlinear dielectric permittivity, the electric and magnetic field profiles along with the achieves density profiles in plasma with and without the ramped density are investigated. It is found that the electromagnetic fields are deviated from its normal sinusoidal profile and the wavelength of electric and magnetic field oscillations increases in comparison to the uniform underdense plasma. The effect of the weakly relativistic ponderomotive force generated by intense laser pulses in the underdense plasma with initial density ramp and temperature variations is studied. It is noticed that ponderomotive force modifies the electron density distribution and the steepening of the plasma density increases in the presence of the electron density with ramp profile.
[en] Complete text of publication follows. Temporal contrast of ultrashort pulses is one of the most critical issues for high-intensity laser-plasma interactions. The higher the intensity, the larger contrast is needed for keeping prepulse levels low enough to avoid preplasmas which may modify in an undefined way the initial conditions for the interactions with ultrashort laser pulses. Recently plasma mirrors became the ultimate tool for cleaning ultrashort laser pulses from pedestals in the visible/IR spectral range using solid-state laser systems. For UV lasers as KrF systems as well. The short-pulse KrF system in our laboratory (70 mJ / 610 fs) uses direct and not chirped-pulse amplification, therefore the pedestals originate solely from the ASE of the KrF amplifiers. Although - as a consequence of limited focusability and longer duration of the ASE - intensity contrast as high as 1010 can be obtained, it is still not sufficient in some cases, A prepulse with duration of 15 ns with 107 W/cm2 intensity may cause photoablation and ionization of the solid target, i.e. the main pulse does not hit a pure solid any more in case of 1018 W/cm2 focused intensity. Plasma reflectivity was investigated using 45 deg and 12.4 deg angle of incidence. The antireflection-coated target was moved by stepping motors shot-by-shot. The energy was monitored for each shot and the reflected radiation was measured by calibrated photodiodes. The diode signal was integrated by a peak-hold detector and digitized, controlled by a microprocessor. Fiber-based communication provided the isolation from electronic noises. Plasma mirror effect was demonstrated for ultrashort KrF laser pulses. The reflectivity starts to increase above 1012 W/cm2 and it reaches a maximum at ∼ 1014 W/cm2, above which it saturates and even decreases with large shot-to-shot variations due to the nonlinear phenomena (e.g. harmonics generation) above this intensity. The measured maximum reflectivity was found to be ∼ 33% ror 45 deg angle of incidence, and it was nearly 50% for 12.4 deg. The obtainable 50% reflectivity allows its direct application for high-intensity experiments but new schemes are shown their possible application before the final amplifier using the saturation properties of the KrF amplifiers. Acknowledgements. This work, supported by the European Communities under the contract of Association between EURATOM and the Hungarian Academy of Sciences, was carried out within the framework of the European Fusion Development Agreement. It was also supported by the Hungarian OTKA foundation no. 60531 and by the IAEA CRP on Path-ways to Inertial Fusion - An Integrated Approach, Research Contract no. HUN13759.