Evolution of elastic x-ray scattering in laser-shocked warm dense lithium

We have studied the dynamics of warm dense Li with near-elastic x-ray scattering. Li foils were heated and compressed using shock waves driven by 4-ns-long laser pulses. Separate 1-ns-long laser pulses were used to generate a bright source of 2.96 keV Cl Ly-alpha photons for x-ray scattering, and the spectrum of scattered photons was recorded at a scattering angle of 120 degrees using a highly oriented pyrolytic graphite crystal operated in the von Hamos geometry. A variable delay between the heater and backlighter laser beams measured the scattering time evolution. Comparison with radiation-hydrodynamics simulations shows that the plasma is highly coupled during the first several nanoseconds, then relaxes to a moderate coupling state at later times. Near-elastic scattering amplitudes have been successfully simulated using the screened one-component plasma model. Our main finding is that the near-elastic scattering amplitudes are quite sensitive to the mean ionization state Z and by extension to the choice of ionization model in the radiation-hydrodynamics simulations used to predict plasma properties within the shocked Li.

Observation of K-Shell Soft X Ray Emission of Nitrogen Irradiated by XUV-Free Electron Laser FLASH at Intensities Greater than 10(16) W/cm(2)

In the past few years, the development of light sources of the 4(th) generation, namely XUV/X-ray Free Electron Lasers provides to the scientific community outstanding tools to investigate matter under extreme conditions never obtained in laboratories so far. As theory is at its infancy, the analysis of matter via the self-emission of the target is of central importance. The characterization of such dense matter is possible if photons can escape the medium. As the absorption of K-shell X-ray transitions is minimal, it plays a key role in this study. We report here the first successful observation of K-shell emission of Nitrogen at 430 eV using an XUV-Free Electron Laser to irradiate solid Boron Nitride targets under exceptional conditions: photon energy of 92 eV, pulse duration of similar to 20 fs, micro focusing leading to intensities larger than 10(16) W/cm(2). Using a Bragg crystal of THM coupled to a CCD, we resolved K-shell line emission from different charge states. We demonstrate that the spectroscopic data allow characterization of electron heating processes when X-ray radiation is interacting with solid matter. As energy transport is non-trivial because the light source is monochromatic, these results have an important impact on the theory. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Transient pumping of a Ni-like Ag x-ray laser with a subpicosecond pump pulse in a traveling-wave irradiation geometry

We report what is to our knowledge the first demonstration of a transient x-ray laser pumped by a 350-fs pulse in a traveling-wave irradiation geometry. For a 500-fs pump pulse the traveling-wave irradiation was found to have a strong effect on enhancing the Ni-like silver 4d-4p lasing emission at 13.9 nm. The signal enhancement was significantly less when the pulse duration was lengthened to 1.7 ps. The experimental observations are well reproduced by a simple model when the duration of gain is taken of the order of 15-20 ps. For the 500-fs pulse a gain coefficient of 14.5 cm(-1) was measured for plasma lengths up to 7 mm. Refraction of the amplified photons is believed to be the main cause of the limitation of the effective amplification length. (C) 2000 Optical Society of America.

MEASUREMENT OF THE PHOTO-PUMP STRENGTH OF THE 3D-5F TRANSITIONS IN THE AUTOMATICALLY LINE MATCHED NI-LIKE SM PHOTOPUMPED X-RAY LASER

The photo-pump strengths of both the ((3 (d) over bar(4))(0)(3d(6))(0))(0)-(((3 (d) over bar(3))(3/2)(3d(6))(0))(3/2)(5 (f) over bar)(5/2))(1) and the ((3 (d) over bar(4))(0)(3d(6))0)0-(((3 (d) over bar(4))(0)(3d(5))(5/2))(5/2)(5f)(7/2))(1) transitions in Ni-like Sm34+ have been measured to be 2.0 x 10(-4) and 2.4 x 10(-4) photons/mode respectively. The implications of the measurement are briefly discussed in a comparison of the merits of automatically line matched photo-pump scheme to those of the collisional excitation Ni-like Sm+34 scheme.

Fragmentation of Neutral Amino Acids and Small Peptides by Intense, Femtosecond Laser Pulses

High power femtosecond laser pulses have unique properties that could lead to their application as ionization or activation sources in mass spectrometry. By concentrating many photons into pulse lengths approaching the timescales associated with atomic motion, very strong electric field strengths are generated, which can efficiently ionize and fragment molecules without the need for resonant absorption. However, the complex interaction between these pulses and biomolecular species is not well understood. To address this issue, we have studied the interaction of intense, femtosecond pulses with a number of amino acids and small peptides. Unlike previous studies, we have used neutral forms of these molecular targets, which allowed us to investigate dissociation of radical cations without the spectra being complicated by the action of mobile protons. We found fragmentation was dominated by fast, radical-initiated dissociation close to the charge site generated by the initial ionization or from subsequent ultrafast migration of this charge. Fragments with lower yields, which are useful for structural determinations, were also observed and attributed to radical migration caused by hydrogen atom transfer within the molecule.

Laser-driven generation of high-quality ultra-relativistic positron beams

An ultra-relativistic electron beam propagating through a high-Z solid triggersan electromagnetic cascade, whereby a large number of high-energy photons andelectron–positron pairs are produced mainly via the bremsstrahlung and Bethe–Heitler processes, respectively. These mechanisms are routinely used to generatepositron beams in conventional accelerators such as the electron–positron collider(LEP). Here we show that the application of similar physical mechanisms to a laserdrivenelectron source allows for the generation of high-quality positron beams in amuch more compact and cheaper configuration. We anticipate that the applicationof these results to the next generation of lasers might open the pathway for therealization of an all-optical high-energy electron–positron collider.

A table-top laser-based source of short, collimated, ultra-relativistic positron beams

An ultra-relativistic electron beam passing through a thick, high-Z solid target triggers an electromagnetic cascade, whereby a large number of high energy photons and electron-positron pairs are produced. By exploiting this physical process, we present here the first experimental evidence of the generation of ultra-short, highly collimated and ultra-relativistic positron beams following the interaction of a laser-wake field accelerated electron beam with high-Z solid targets. Clear evidence has also been obtained of the generation of GeV electron-positron jets with variable composition depending on the solid target material and thickness. The percentage of positrons in the overall leptonic beam has been observed to vary from a few per cent up to almost fifty per cent, implying a quasi-neutral electron-positron beam. We anticipate that these beams will be of direct relevance to the laboratory study of astrophysical leptonic jets and their interaction with the interstellar medium.

Photo-ionized neon plasmas induced by radiation pulses of a laser-plasma EUV source and a free electron laser FLASH

In this work, a laser-produced plasma extreme ultraviolet source and a free electron laser were used to create Ne photo-ionized plasmas. In both cases, a radiation beam was focused onto a gas stream injected into a vacuum chamber synchronously with the radiation pulse. Extreme ultraviolet radiation from the plasma spanned a wide spectral range with pronounced maximum centered at lambda = 11 +/- 1 nm while the free electron laser pulses were emitted at a wavelength of 32 nm. The power density of the focused plasma radiation was approximately 2 x 10(7) W/cm(2) and was seven orders of magnitude lower compared with the focused free electron laser beam. Radiation fluences in both experimental conditions were comparable. Despite quite different spectral characteristics and extremely different power densities, emission spectra of both photo-ionized plasmas consist of the same spectral lines within a wavelength range of 20 to 50 nm, however, with different relative intensities of the corresponding lines. The dominating spectral lines originated from singly charged ions (Ne II); however, Ne III lines were also detected. Additionally, computer simulations of the emission spectra, obtained for photo-ionized plasmas, driven by the plasma extreme ultraviolet source, were performed. The corresponding measured and calculated spectra are presented. An electron temperature and ionic composition were estimated. Differences between the experimental spectra, obtained for both irradiation conditions, were analyzed. The differences were attributed mainly to different energies of driving photons.

Fragmentation of CD+ induced by intense ultrashort laser pulses

The fragmentation of CD+ in intense ultrashort laser pulses was investigated using a coincidence three-dimensional momentum imaging technique improved by employing both transverse and longitudinal electric fields. This allowed clear separation of all fragmentation channels and the determination of the kinetic energy release down to nearly zero, for a molecule with significant mass asymmetry. The most probable dissociation pathways for the two lowest dissociation limits, C+ + D and C+ D+, were identified for both 22-fs, 798-nm and 50-fs, 392-nm pulses. Curiously, the charge asymmetric dissociation of CD2+ was not observed for 392-nm photons, even though it was clearly visible for the fundamental 798 nm at the same peak intensity.

Gamma-ray emission in near critical density plasmas at laser intensities of 1021 W/cm2

We study synchrotron radiation emission from laser interaction with near critical density (NCD) plasmas at intensities of 1021 W∕cm2 using three-dimensional particle-in-cell simulations. It is found that the electron dynamics depend on the laser shaping process in NCD plasmas, and thus the angular distribution of the emitted photons changes as the laser pulse evolves in space and time. The final properties of the resulting synchrotron radiation, such as its overall energy, the critical photon energy, and the radiation angular distribution, are strongly affected by the laser polarization and plasma density. By using a 420 TW∕50 fs laser pulse at the optimal plasma density (∼1nc ), about 108 photons/0.1% bandwidth are produced at multi-MeV photon energies, providing a route to ultraintense, femtosecond gamma ray pulses.

Evaluating laser-driven Bremsstrahlung radiation sources for imaging and analysis of nuclear waste packages

A small scale sample nuclear waste package, consisting of a 28 mm diameter uranium penny encased in grout, was imaged by absorption contrast radiography using a single pulse exposure from an X-ray source driven by a high-power laser. The Vulcan laser was used to deliver a focused pulse of photons to a tantalum foil, in order to generate a bright burst of highly penetrating X-rays (with energy >500 keV), with a source size of <0.5 mm. BAS-TR and BAS-SR image plates were used for image capture, alongside a newly developed Thalium doped Caesium Iodide scintillator-based detector coupled to CCD chips. The uranium penny was clearly resolved to sub-mm accuracy over a 30 cm2 scan area from a single shot acquisition. In addition, neutron generation was demonstrated in situ with the X-ray beam, with a single shot, thus demonstrating the potential for multi-modal criticality testing of waste materials. This feasibility study successfully demonstrated non-destructive radiography of encapsulated, high density, nuclear material. With recent developments of high-power laser systems, to 10 Hz operation, a laser-driven multi-modal beamline for waste monitoring applications is envisioned.