The recent development of intense sources in the XUV range (10-100 nm), such as X-ray laser, Free Electron Laser and
High order Harmonics (HoH), allows the study of high flux processes and ultra-fast dynamics in various domains.
At the SLIC facility of CEA-Saclay, we have built a gas-harmonic beamline to investigate the interaction of intense
XUV pulse with solids. High Harmonics of an IR laser (Ti:Sa at 800 nm, 35 fs, 13 mJ/pulse, 1 kHz) are generated in a
rare gas cell (Xe). The useful XUV range (40-60 nm) is selected with metallic filters. The harmonic beam is focused with
a parabolic mirror to a 10 μm focal spot on sample, leading to a fluence per shot of up to 1 mJ/cm2 (within a typical 10 fs
pulse duration).
Studies aimed at understanding the damaging mechanisms caused by XUV irradiation on surface of various samples by
systematically varying of fluence and exposure time.
For PMMA irradiated in the desorption regime (fluence/shot ≤ 0.2 mJ/cm2), the surface presents craters whose profile
depends on the dose (Grey [Gy] = 1 J/kg). The crater evolution proceeds from the competition between two main
degradation processes, that is chain scission and cross linking. Namely, at low dose (≤ 1 GGy) polymer chain scission is
followed by the blow up of the volatile, molecular fragments, forming the crater. At high dose (> 10 GGy) the broken
chain-ends, in the near-surface layer of the remaining material, recombine by cross-linking, opposing desorption by
surface hardening.
In a recent experiment at LCLS FEL facility, PMMA was irradiated at high fluence; the cross-linking signature was
identified from Raman spectroscopy. A kinetic model could be adapted for interpreting these original and very promising
results.
The beam of Free-Electron Laser in Hamburg (FLASH) tuned at either 32.5 nm or 13.7 nm was focused by a grazing
incidence elliptical mirror and an off-axis parabolic mirror coated by Si/Mo multilayer on 20-micron and 1-micron spot,
respectively. The grazing incidence and normal incidence focusing of ~10-fs pulses carrying an energy of 10 μJ lead at
the surface of various solids (Si, Al, Ti, Ta, Si3N4, BN, a-C/Si, Ni/Si, Cr/Si, Rh/Si, Ce:YAG, poly(methyl methacrylate)
- PMMA, stainless steel, etc.) to an irradiance of 1013 W/cm2 and 1016 W/cm2, respectively. The optical emission of the
plasmas produced under these conditions was registered by grating (1200 lines/mm and/or 150 lines/mm) spectrometer
MS257 (Oriel) equipped with iCCD head (iStar 720, Andor). Surprisingly, only lines belonging to the neutral atoms
were observed at intensities around 1013 W/cm2. No lines of atomic ions have been identified in UV-vis spectra emitted
from the plasmas formed by the FLASH beam focused in a 20-micron spot. At intensities around 1016 W/cm2, the OE
spectra are again dominated by the atomic lines. However, a weak emission of Al+ and Al2+ was registered as well. The
abundance ratio of Al/Al+ should be at least 100. The plasma is really cold, an excitation temperature equivalent to 0.8 eV was found by a computer simulation of the aluminum plasma OE spectrum. A broadband emission was also
registered, both from the plasmas (typical is for carbon; there were no spectral lines) and the scintillators (on Ce:YAG
crystal, both the luminescence bands and the line plasma emission were recorded by the spectrometer).
The new XUV sources, which deliver spatially coherent pulses of high peak power, allow to study elementary
processes in the light/solid interaction in the high intensity regime (⩾1011W/cm2). Here, we report two
studies which have used high-order laser harmonics (HH) generated in gas as the excitation source. Firstly, we
have investigated the dynamics of electron relaxation in the wide gap CdWO4 dielectric crystal, an efficient
scintillator material, using time-resolved luminescence spectroscopy. The kinetics decay of luminescence shows
evidence of non radiative relaxation of the self-trapped excitons at the &mgr;s damage to surfaces of poly(methyl
methacrylate) - PMMA, induced by a multi-shot XUV-irradiation (1 kHz reprate) for given fluence, below
damage threshold range of ≈mJ/cm2. The main processes participating in the surface modification, polymer
chain scission followed by the blow up of the volatile, molecular fragments and cross-linking in the near-surface
layer of remaining material, are tentatively identified and associated to, crater formation for short-time exposure
(< 1min) and surface hardening for long-time exposure (⩾1min).
An advanced time integrated method has been developed for soft X-ray pulsed laser beam characterization. A technique
based on poly (methyl methacrylate) - PMMA laser induced ablation has been used for beam investigations of soft X-ray
laser sources like FLASH (Free-electron LASer in Hamburg; formerly known as VUV FEL and/or TTF2 FEL) and
plasma-based Ne-like Zn laser performed at PALS (Prague Asterix Laser System). For the interaction experiments reported here, the FLASH system provided ultra-short pulses (~10-fs) of 21.7-nm radiation. The PMMA ablation was
also induced by plasma-based Ne-like Zn soft X-ray laser pumped by NIR beams at the PALS facility. This quasi-steady-state
(QSS) soft X-ray laser provides 100-ps pulses of 21.2-nm radiation, i.e. at a wavelength very close to that of
FLASH but with about 5,000 times longer pulses. In both cases, the PMMA samples were irradiated by a single shot
with a focused beam under normal incidence conditions. Characteristics of ablated craters obtained with AFM (Atomic
Force Microscope) and Nomarski microscopes were utilized for profile reconstruction and diameter determination of the
focused laser beams ablating the PMMA surface.
An ultra fast, sub-picosecond resolution streak camera has been recently developed at the LLNL. The camera is a versatile instrument with a wide operating wavelength range. The temporal resolution of up to 300 fs can be achieved, with routine operation at 500 fs. The streak camera has been operated in a wide wavelength range from IR to x-rays up to 2 keV. In this paper we briefly review the main design features that result in the unique properties of the streak camera and present its several scientific applications: (1) Streak camera characterization using a Michelson interferometer in visible range, (2) temporally resolved study of a transient x-ray laser at 14.7 nm, which enabled us to vary the x-ray laser pulse duration from ~2-6 ps by changing the pump laser parameters, and (3) an example of a time-resolved spectroscopy experiment with the streak camera.
During recent months we have continued investigations of many different aspects of x-ray lasers to characterize and improve the source and applications. This work has included temporal characterization of existing laser-heated x-ray lasers under a wide range of pumping conditions. We have also looked into more details at different applications of x-ray lasers among which was the interferometry of laser-produced and capillary discharge plasmas in several irradiation conditions for different target Z materials. The reduction of pump energy remains the most important for the generation of new compact x-ray lasers. Numerical studies show that there are some ways to improve several of the key parameters of x-ray lasers specifically repetition rates and efficiency.
The Picosecond Laser-Electron Inter-Action for the Dynamic Evaluation of Structures (PLEIADES) facility, is a unique, novel, tunable (10-200 keV), ultrafast (ps-fs), hard x-ray source that greatly extends the parameter range reached by existing 3rd generation sources, both in terms of x-ray energy range, pulse duration, and peak brightness at high energies. First light was observed at 70 keV early in 2003, and the experimental data agrees with 3D codes developed at LLNL. The x-rays are generated by the interaction of a 50 fs Fourier-transform-limited laser pulse produced by the TW-class FALCON CPA laser and a highly focused, relativistic (20-100 MeV), high brightness (1 nC, 0.3-5 ps, 5 mm.mrad 0.2% energy spread) photo-electron bunch. The resulting x-ray brightness is expected to exceed 1020 ph/mm2/s/mrad2/0.1% BW. The beam is well-collimated (10 mrad divergence over the full spectrum, 1 mrad for a single color), and the source is a unique tool for time-resolved dynamic measurements in matter, including high-Z materials.
The use of ultrafast laser pulses to generate very high brightness, ultrashort (10-14 to 10-12 s) pulses of x-rays is a topic of great interest to the x-ray user community. In principle, femtosecond-scale pump-probe experiments can be used to temporally resolve structural dynamics of materials on the time scale of atomic motion. However, further development of this field is severely hindered by the absence of a suitably intense x-ray source that would drive the development of improved experimental techniques and establish a broader range of applicability. We report on a project at the Lawrence Livermore National Laboratory to produce a novel x-ray source and essential experimental techniques that will enable unprecedented dynamic measurements in matter. Based on scattering of a sub-50-fs, multi-terawatt, multi-beam laser from a co-synchronous and highly focused relativistic electron bunch, PLEIADES (Picosecond Laser Electron Interaction for Dynamic Evaluation of Structures) will produce tunable, ultrafast, hard x-ray (10- 200 keV) probes that greatly exceed existing 3rd generation synchrotron sources in speed (100 fs - 1 ps), peak brightness (1020 ph/mm2s mrad2 0.1% BW, and >109 ph/pulse), and simplicity (100-fold smaller). Such bright, ultrafast high energy x-rays will enable pump-probe experiments using radiography, dynamic diffraction, and spectroscopy to address the equation of state and dynamics of phase transitions and structure in laser heated and compressed heavy dense metals of interest for materials science.
This paper summarizes our recent progress achieved in the characterization and understanding of the Ni-like Ag transient x-ray laser pumped under traveling wave irradiation. At the Rutherford Laboratory CPA laser facility, we measured the temporal history of the 13.9 nm laser pulse with a high-resolution streak camera. A very short, approximately 2 ps x-ray laser pulse was directly demonstrated for the first time. More recently we carried out an experiment at the LULI CPA laser facility. Several diagnostics that recorded the plasma emission at the XRL wavelength or in the keV range indicate the presence of small-scale spatial structures in the emitting XRL source. Single-shot Fresnel interferograms at 13.9 nm were successfully obtained with a good fringe visibility. Strong lasing was also observed on the Ni-like 4f-4d line at 16 nm.
We present a review of new progress performed in several laboratories (Laboratoire pour l'Utilisation des lasers Intenses, Rutherford Appleton Laboratory, Prague Asterix Laser System, Institute of Laser Engineering, Laboratoire d'Optique Appliquee). Concerning the realization of x-ray lasers sources, using different laser pumping techniques (600 ps, 100 ps, ns/ps, OFI) and the optimization of their optical properties, using curved and plane half-cavity mirrors. In parallel of these developments, we present the main results obtained with x-ray laser in interferometry applications. These studies concern on the one hand the Michelson interferometry with an x-ray laser emitting at 13.9 nm (recently realized at LULI), and on the other hand the Fresnel bi-mirror with an x-ray laser emitting at 21.2 nm (recently realized at PALS).
Recent high temporal resolution Ni-like x-ray laser experiments have yielded important insights into the output characteristics of picosecond pumped x-ray lasers. However, current experimental observations do not fully explain the plasma dynamics which are critical to the gain generation within the x-ray laser medium. A theoretical study of the Ni-like Silver x-ray laser has therefore been undertaken to compliment our experimental results, in an attempt to further our understanding of the processes at play in yielding the observed x-ray laser output. Preliminary findings are presented within this paper.
Annie Klisnick, Antoine Carillon, Gerard Jamelot, Pierre Jaegle, David Ros, Philippe Zeitoun, F. Albert, P. Fourcade, Jaroslav Kuba, Jean-Luc Miquel, Nathalie Blanchot, J. Wyart, Pierre Agostini, P. Breger, David Garzella, Heimo Mueller-Seelich, Denis Joyeux, Daniel Phalippou, E. Bechir, S. Hubert, G. De Lacheze-Murel, Hiroyuki Daido
We present new progress in the optimization and understanding of the transient collisional pumping scheme using an ultra-short sub-ps heating pulse. The effect of traveling-wave irradiation in enhancing the lasing output of the 4d-4p Ni-like Ag line is studied in detail. A new irradiation scheme using a frequency-doubled 600 ps pulse to preform a plasma is tested. Strong lasing is also obtained on a new line at 16.05 nm that we identify to a 4f-4d transition in Ni-like Ag. Finally we review our recent work in the development of applications of the 21.2 nm zinc laser for imaging or exciting matter. New experiments include the probing of a plasma by imaging Fresnel interferometry and a first attempts to demonstrate two-photon ionization in a xenon gas.
Recent experiments, performed at the C.E.A./Limeil-Valenton P102 laser facility on the Ni-like transient collisional scheme, are reported in this paper. They mainly aimed at enhancing the efficiency and improving the optical properties of the already demonstrated 4d J equals 0/4p J equals 1 Ag19+ x-ray laser at 13.9 nm. The now classical 2- stage traveling-wave irradiation of slab targets was used, the illumination sequence being constituted of a long (600 ps) low-flux (0.5 - 11 J) laser pulse followed (200 ps later) by a short (< 1 ps) high intensity (1 - 20 J) one. The work novelty was the use of frequency-doubled pulses, either for the pre-forming or the pumping one. Various combinations ((omega) -(omega) , 2(omega) -(omega) , (omega) - 2(omega) ) have been investigated in terms of lasing performances. High gains, around 34/cm, have been measured and saturation achieved for target lengths above 4 mm. A strong enhancement, up to a few (mu) J, of the x-ray laser output has been observed, due to traveling-wave irradiation method, while the emission duration was decreased to less than 10 ps, resulting in a 300 kW source. Moreover, under specific laser conditions, a second lasing line at 16 nm was detected. Finally, the possibility of cavity operating transient collisional x-ray lasers has been demonstrated.
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