Results of a novel X-ray laser application, aimed at understanding the microscopic effects involved in formation of laserinduced
damage in optical materials exposed to sub-ns laser pulses, will be presented. Specifically, we studied thin plane
beamsplitters that are presently the weakest element of the next generation of high-energy lasers (LMJ, NIF), with
permanent damage threshold below 20 J/cm2. Standard fused silica substrates and a model system, containing welldefined
micron grooves as seeding sites to trigger damage when irradiated by 438 nm laser pulses, were in situ probed by
a neon-like zinc X-ray laser delivering up to 10 mJ at 21.2 nm. The probing beamline employed a double Lloyd's mirror
interferometer, used in conjunction with an imaging mirror to provide magnification of ~8. In conjunction with an array
of in-situ optical diagnostics, one of the questions addressed was whether the damage (transient or permanent) on the
rear surface of the beamsplitter occurs during or after the laser pulse, i.e. whether it is due to local electrical fields or to
other processes. Another issue, examined by both the X-ray interferometric microscopy and the optical diagnostics, is
whether a local rear-surface modification is associated with non-linear effects (self-focusing, filamentation) of the laser
beam in the bulk.
Since the first seeding of an OFI soft x-ray laser in 2004, we progressed towards the full characterization of the output
beam. The final is to be able to deliver to users well-known beam. Temporal as well as spatial parameters have been
measured for different conditions of amplification. We observed a strong enhancement of the spatial coherence due to
the amplification process with a far-field pattern exhibiting an airy-like shape. The gain zone having strong discontinuity
behaves like a hard pinhole. Spatial filtering has been also observed on the wave front (δ/5 root-mean-square, rms,
before seeding and δ/20 rms after amplification). Temporal coherence has been studied thanks to the use of a Fourier-
Transform spectrometer. Spectral widths, δδ/δ, around 10-5 have been measured for different plasma lengths or gas pressures. Departure from Gaussian shape has been clearly observed on the spectral line for some cases.
LASERIX is a high power laser facility intended to realise and use for applications transient collisional excitation (TCE)
X-ray lasers (XRLs) at various wavelengths, using grazing incidence pumping (GRIP) configuration with 10 Hz
repetition rate. In addition new types of XRL schemes giving rise to emission at short wavelengths will be developed
using the high energy LASERIX driver with 0.1 HZ rep-rate. Thus, this laser facility will both offer Soft X-ray lasers in
the 40-10 nm range and synchronised auxiliary IR beam that could be also used to produce XUV sources. This
experimental configuration highly enhances the scientific opportunities of the facility. Indeed it will be possible to realise
both X-ray laser experiments and more generally pump/probe experiments, mixing IR and XUV sources. Then, this
facility will be useful for the community, opening a large scale of investigations, including imagery and irradiation as
illustrated in the case of laser Interaction with matter investigations using XUV interferometry.
We have developed a double Lloyd's mirror wavefront-splitting interferometer, constituting a compact device for surface probing in the XUV and soft X-ray spectral domain. The device consists of two independently adjustable superpolished flat surfaces, operated under grazing incidence angle to reflect a diverging or parallel beam. When the mirrors are appropriately inclined to each other, the structure produces interference fringes at the required distance and with tuneable fringe period. The double Lloyd's mirror may be used alone for surface topography with nanometric altitude resolution, or in conjunction with an imaging element for interferometric XUV surface microscopy. In the latter case, resolution in the plane of the probed
surface is about micron, which is given by the quality of the imaging element and/or by the detector pixel size. Here, we present results obtained using the double Lloyd's mirror in two separate X-ray laser and high harmonics generation (HHG) application projects. The first
experiment was aimed at understanding microscopic nature of the effects involved in laserinduced optical damage of thin pellicles, exposed to sub-ns laser pulses (438 nm) producing fluence of up to 10 Jcm-2. The probing source in this case was a QSS neon-like zinc soft X-ray laser, proving a few mJ at 21.2 nm in ~100-ps pulses. The second experiment was carried out using a narrowly collimated HHG beam near 30 nm, employed to topographically probe the surface of a semiconductor chip.
We present early results of an application of X-ray laser, aimed at understanding the effects involved in formation of laser-induced damage in optical materials exposed to sub-ns laser pulses. For the purpose of the experiment, a novel interferometric microscopy technique was designed and tested. The interferometric beamline employed a double Lloyd's mirror interferometer, used in conjunction with an imaging mirror to provide magnification of ~8 along a plane
inclined with respect to the propagation direction of the X-ray beam. The objects investigated were thin plane beamsplitters made of fused silica (SiO2), irradiated by damaging laser light at 438 nm and in situ probed by the developed technique of interferometric microscopy. The soft X-ray beam was emitted by neon-like zinc laser, delivering up to 10 mJ at 21.2 nm. In conjunction with an array of in-situ optical diagnostics, one of the questions addressed was whether the damage of the rear surface of the beamsplitter occurs approximately during of much after the laser pulse. Another issue examined by the X-ray interferometric microscopy technique was whether the surface perturbation seen shortly after the impact of the damaging pulse is associated or not with the pattern of permanent surface modifications.
We give an overview of recent advances in development and applications of deeply saturated Ne like zinc soft X-ray laser at PALS, providing strongly saturated emission at 21.2 nm. Population inversion is produced in the regime of long scale-length density plasma, which is achieved by a very large time separation between the prepulse (<10 J) and the main pump pulse (~500 J), of up to 50 ns. This pumping regime is unique in the context of current x-ray laser research. An extremely bright and narrowly collimated double-pass x-ray laser beam is obtained, providing ~10 mJ pulses and ~100 MW of peak power, which is the most powerful soft X-ray laser yet demonstrated. The programme of applications recently undertaken includes precision measurements of the soft X-ray opacity of laser irradiated metals relevant to stellar astrophysics, soft X-ray interferometric probing of optical materials for laser damage studies, soft X-ray material ablation relevant to microfabrication technologies, and pilot radiobiology studies of DNA damage in the soft X-ray region. A concomitant topic is focusing the x-ray laser beam down to a narrow spot, with the final goal of achieving ~1013 Wcm-2.
We present a detailed analysis of an experiment carried out recently in which the temporal coherence of the Ni-like silver transient X-laser at 13.9 nm was measured. Two main consequences of this measurement will be discussed and interpreted with numerical calculations. First we show that the high temporal coherence length measured corresponds to an extremely narrow spectral width of the X-ray laser line. Second we show that the high temporal coherence helps to explain the presence of small-scale structures observed in the cross-section of all transient X-ray laser beams.
Future progress of x-ray lasers call for specific facilities intended to x-ray laser studies and development of a large variety of x-ray laser applications. This paper presents LASERIX, a future x-ray laser facility under construction for optimization of transient collisional excitation x-ray lasers, study of new x-ray laser schemes, and development of applications.
We review our recent progress in the development of transient x-ray lasers and of their application to plasma diagnostic. The first observation of C-ray laser emission at the new PHELIX-GSI facility is reported. This TCE X-ray laser will be a promising tool for heavy-ion spectroscopy. We then present the main results obtained at the LULU-CPA facility with a compact high-resolution X-UV imaging device. This device was used to investigate the spatial source structure of the Ni-like silver transient X-ray laser under different pumping conditions. The key-role of the width of the background laser pulse on the shape of the emitting aperture is demonstrated. Finally the imaging device was used as an interference microscope for interferometry probing of a laser-produced plasma. We describe this experiment performed at APRC-JAERI.
We give an overview of recent results on optical-field-ionization collisional soft X-ray lasers developed at LOA. By focusing a 30-fs, circularly polarized Ti-sapphire laser pulse at an intensity of up to 8 x 1017 Wcm-2 into a low-density gas cell containing xenon or krypton, we produced a few mm long plasma column for soft X-ray amplifier. Saturated amplification has been achieved on the 4d95d(1S0) - 4d95p(1P1) transition at 41.8 nm in Pd-like Xe, and strong lasing at 32.8 nm on the 3d94d(1S0) - 3d94p(1P1) transition in Ni-like Kr has been demonstrated. Under optimum pumping conditions the Xe IX laser provides about 5 x 109 photons per pulse while the Kr IX laser delivers up to (2-3) x 109 photons per shot. The repetition rate of these X-ray lasers is 10 Hz. The experimental results are discussed and compared to computational simulations in which issues related to plasma formation, electron energy distribution, and atomic processes are addressed.
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 report recent investigations on collisional Optical-Field Ionization soft x-ray lasers. The amplifying medium is generated by focusing a high energy circularly polarized, 35 fs 10 Hz Ti: sapphire laser system in a few mm cell filled with gas (xenon or krypton). Using xenon, a gain of 67 cm-1 on the 4d95p-4d95d transition at 41.8 nm in Pd-like xenon and a gain-length product of 15 have been inferred at saturation. This source delivers about 5 109 photons per pulse. Using krypton, a large amplification of the 3d94d-3d94p line at 32.8 nm has been observed for the first time. The influence of the pumping energy and the laser polarization on the lasing output are also presented.
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).
Collisional soft x-ray lasers are brighter by several orders of magnitude than any other available source in the wavelength range comprised between a few tens and a few nanometers. Then, a new line of research has recently appeared besides continuing works aimed to improve optical properties of x-ray lasers and to develop new types of x-ray lasers. It consists in using the actual x-ray lasers as an efficient tool to produce and to study other phenomena in various fields such as atomic physics, plasma physics, or surface science. This paper gives a survey of recent x-ray laser progress and some examples of x-ray laser applications experiments.
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.
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.
We use x-ray laser interferometry to probe defects induced by a strong electric field on niobium surface. Niobium has been chosen on account of its frequent use in superconductive cavities of particle accelerators. The x-ray laser emits bright, 50 ps-duration pulses at (lambda) equals 21.2 nm. The beam is reflected on the niobium surface under grazing incidence. The interferometer is of the wave-front division type. Interferograms are single shot recorded, which enables to probe `instantaneous' defect morphology. We observed appearance and evolution of defects between 14 MV/m and 35 MV/m. The vertical set amplitude is of 10 - 20 nm. The defect structure has been observed to shift by 500 micrometers along the metal surface under a constant 35 MV/m electric field, during the 20 minutes time interval between two laser shots.
This work has consisted in demonstrating that high gain can be achieved by pumping x-ray lasers (XRL) with a combination of a high intensity and short duration driving pulses (approximately 100 ps). Short pulses are very well suited for pumping collisional XRL since a high lasant ion density, electron density and temperature can be achieved simultaneously. We have successfully tested this pumping scheme on the 4d-4p (J equals 0 - 1) transition of Ni-like tin (lambda approximately 11.93 nm) and silver (lambda approximately 13.89 nm) as well as on the 3p-3s (J equals 0 - 1) Ne-like iron (lambda approximately 25.5 nm) at an intensity of approximately 2 X 1013 Wcm-2 (130 ps in duration). The driving laser (lambda equals 1.06 micrometer) was composed of three pulses (a prepulse and two main pulses). Large amplifications were demonstrated in tin and silver (respectively GL approximately 12 and GL approximately 16). Finally, the saturation of the 3p-3s (J equals 0 - 1) transition of Ne-like iron at 25.5 nm was achieved on both pumping pulses, using a prepulse of 109 Wcm-2. A gain coefficient of 15 plus or minus 3 cm-1 (GL approximately 26 plus or minus 5) on the first main pulse and 12 plus or minus cm-1 (GL approximately 23 plus or minus 2) on the second one was measured.
Collisional X-UV lasers are now currently produced world-wide. The LSAI team has developed at LULI an efficient, high brightness laser at 21.2 nm, using neonlike zinc. Beside the effort aimed at improvement of efficiency of the neonlike X-UV lasers, the LSAI recent activity has covered scaling the collisional scheme down to shorter wavelengths, as well as development of applications. In this paper we present results of applications of the 21.2 nm laser in atomic physics, solid state physics, and X-UV interferometry of surfaces. The emphasized message is that the characteristics of the existing X-UV lasers render possible applications in many research areas nowadays.
In this contribution we outline our program aimed at obtaining an efficient soft x-ray laser at the wavelength 25.5 nm, using collisional excitation in a neonlike iron plasma. The goal of the research that we are pursuing is to develop a relatively small-scale soft x-ray laser that would be accessible for a variety of applications. A driver available for us is the laser iodine photodissociation laser system PERUN, providing on the order of 40 joules in on the order of 400 ps (FWHM) pulses at the fundamental wavelength 1.315 micrometer. Theoretical analysis based on Z-scaling as well as detailed computer modeling of neonlike Fe plasmas suggest driving energies of approximately equals 15 Jcm-1 should be sufficient to generate high gain coefficients (up to 7 cm-1) on the J equals 0 minus 1 line at 25.5 nm. The modeling reveals that a 1.315 micrometer iodine driver is more efficient in producing J equals 0 minus 1 amplification in this moderate-Z system than a 1.06 micrometer Nd:glass driver. As optimizing of pumping conditions is imperative for the feasibility of the project, we have started our experimental effort by investigating plasmas analogous to those created by a small-level prepulse, i.e. by intensities ranging from approximately 5 multiplied by 109 to approximately 1.3 multiplied by 1011 Wcm-2. Preliminary results of this study are discussed elsewhere in this Proceedings.
Results of an experiment aimed at a detailed characterization of line plasmas created by intensities equivalent to those generated by low-level prepulses in the collisional excitation soft x-ray lasers are presented. Electron density profile in the plane perpendicular to the line plasma axis, at times 4 and 10 ns subsequent to the laser pulse, time-integrated electron temperature and time-integrated lateral extent of the plasma were obtained. These parameters were studied for zinc, copper and iron plasmas created on slab target by approximately 400 ps (FWHM) pulses of an iodine laser ((lambda) equals 1.315 micrometer) giving rise to irradiances ranging from approximately 4.8 multiplied by 109 to approximately 1.2 multiplied by 1011 Wcm-2.
Through the use of time-integrated space-resolved keV spectroscopy, we investigate line plasmas showing gain for irradiation using the prepulse technique. The experiments were conducted with the LULI laser of the Ecole Polytechnique, Palaiseau (France), at 1.06 micrometer with prepulse-to-main pulse intensity ratio ranging from 10-6 to 10-2. The particular x-ray lasers which were studied were the collisionally excited Ne-like zinc, copper and nickel systems. The effect of the prepulses on plasma conditions are inferred through spectroscopic line ratio diagnostics. It is observed that the value of the electron temperature for each system does not vary significantly with prepulse levels, nor does their spatially resolved profile along the line. The lateral width and density of the Ne-like regions in the plasmas of all three x-ray lasers are seen to increase with the prepulse level.
Current successful approaches for achieving soft x-ray lasing typically require pumping laser pulses of duration approximately ns and energy approximately kJ (collisionally pumped schemes) or approximately ps pulses and powers of approximately several TW (recombination-pumped schemes). For applications, it is important to improve the efficiency of soft x-ray lasers and so reduce the required power of pumping lasers. The effect of pre- pulse on neon-like collisionally pumped lasers has been investigated using the LULI laser (Ecole Polytechnique, France). A small pre-pulse level approximately 10-3 of the main pulse energy was found to increase the J equals 0 minus 1 neon-like zinc laser output at 21 nm by an order-of-magnitude with a comparable increase in efficiency. A double pumping laser pulse on neon-like yttrium lasing output at 15 nm obtained with the VULCAN laser (Rutherford Appleton Laboratory, England) was also found to increase the x-ray lasing efficiency. With adiabatically cooled recombination lasing, it is shown that approximately 2 ps pulses are optimum for achieving the desired ionization balance for lasing output. The possibility of achieving recombination lasing at short wavelengths on lithium-like ions with longer pulse lasers has been investigated using the ASTERIX laser (Max-Planck Quantenoptik, Germany). These results are presented and interpreted to provide possible directions for improving the efficiency of x-ray lasers.
The main feature of x-ray laser research at LULI is the development of a saturated laser at 212 angstrom with a relatively small pump laser of 0.4 kJ in 600 ps. The laser works with the 3p- 3s J equals O yields 1 transition of neon-like zinc, by using the double-pass of amplified radiation in the active medium. Plasma parameters (temperature, density, homogeneity), and x-ray laser emission properties (intensity, pointing angle, divergence, and coherence) have been studied. Lasing action needs the main laser pulse to be preceded by a ten-prepulse train (contrast ratio less than 103) due to the remnant oscillator. The effect of a single prepulse was investigated as a function of contrast ratio and delay between the prepulse and the main pulse.
Line profiles taking into account ion (Stark) broadening, ion dynamic effect, and electron collisions are calculated for the Al10+ lines at 154.7 and 105.7 angstrom, and for the S13+ line at 206.5 angstrom in recombination lasers. The first two lines are formed of three fine structure components, while the third is constituted of nine components, and the resulting gain may be defined accordingly. The electron collisions yield an homogeneous broadening, while the Stark interaction with neighboring ions is responsible for an asymmetry of the whole profile. Consistently with the experimental determination of the gain, we calculate the total intensity involving the population inversions of the set of components which contribute to the lasing radiation, and deduce an effective small-signal gain coefficient. We discuss our results and compare them to experimental gains.
We present our recent efforts to produce X-ray lasers in the 200 angstroms range by using the moderate power drive of the LULI facility in Palaiseau. The 4 - 5 transitions of Li-like sulfur exhibit large gain-length products in recombining plasmas, and appear to be less sensitive to plasma non-uniformity than the 3 - 4 and 3 - 5 transitions previously studied. From numerical simulations this is likely due to smaller radiative and collisional excitation from 4f than from 3d levels. In collisional scheme, neon-like zinc gives analogous results to similar works on other elements for the 3p - 3s, J equals 2 yields 1 transitions, but the J equals 0 yields 1 transition shows a surprisingly large gain coefficient of 4.9 cm-1. From a detailed comparison of time-dependent intensities of the J equals 0 yields 1 and the J equals 2 yields 1 lines, we conclude that transitions from J equals 0 and from J equals 2 are not emitted in the same region of the plasma.
Difficulties involved in maintaining large gain coefficients in plasma columns longer than 1-2 cm are examined with particular reference to the potential use of lithium-like recombination schemes for pumping X-ray lasers with a moderate pumping power. The discussion focuses on the role of plasma nonuniformity along the amplification path due to large-scale and small-scale inhomogeneities of the pumping optical laser. Two methods of smoothing nonuniformities are considered: the use of random phase plates and wall-confined plasmas.
Systematic study of the collisionally pumped Ne-like germanium 3p-3s laser system has led to the observation of saturated output on the J = 2-1 lines at 23.2 and 23.6 nm when a double slab target is double passed using a normal incidence concave mirror. The about-1-MW output power on the same lines from a simple double plasma has been used to record preliminary images with a Schwarzchild condenser and zone plate microscope arrangement. Initial analysis of measurements on the 4d-4p J = 1-0 line at 7.3 nm from nickel-like samarium indicate a gain coefficient of between 0.4/cm and 0.8/cm for an incident irradiance of about 2 x 10 exp 13 W/sq cm using a l.06-micron heating laser on slab targets of SmF3.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.