The coherence of EUV radiation is extremely important for many experiments based on diffraction or interference and thus knowledge of coherence degree is one of the key parameter of all real partially coherent sources. We present a single shot method based on evaluation of far field diffraction pattern from specially designed diffraction masks. Several measurements of a beam of Ne-like Zn X-ray laser were performed with 1D and 2D masks showing an expected pattern of degree of coherence for this type of source.
We present lasing in Ni-like molybdenum x-ray laser (18.9 nm) demonstrated with grazing incidence pumping and complete diagnostics of the generated EUV beam. This source of EUV radiation was the first experimental realization of transient x-ray laser at the PALS laboratory. The experiment was performed on a 10 Hz Ti:Sapphire laser system with highly efficient grazing incidence pumping by single beam with profiled laser pulse which included a long prepulse followed by a short main pump pulse. The plasma column was created by focusing of the pumping laser beam on a slab target by a spherical mirror in two different off-axis configurations. Lasing close to saturation with EUV pulses of energy around 100 nJ was demonstrated with less than 500 mJ pumping energy on target. Experimental data from far-field images were analyzed by applying the generalized Van Cittert-Zernike theorem which in general relates field correlation function at the source with intensity in the far-field and can give information about the source size.
We will present data on a various X-ray production schemes from laser driven plasmas at the PALS Research Center and discuss the plan for the ELI Beamlines project. One of the approaches, how to generate ultrashort pulses of incoherent X-ray radiation, is based on interaction of femtosecond laser pulses with solid or liquid targets. So-called K-alpha source depending on used targets emits in hard X-ray region from micrometric source size. The source exhibits sufficient spatial coherence to observe phase contrast. Detailed characterization of various sources including the x-ray spectrum and the x-ray average yield along with phase contrast images of test objects will be presented. Other method, known as laser wakefield electron acceleration (LWFA), can produce up to GeV electron beams emitting radiation in collimated beam with a femtosecnond pulse duration. This approach was theoretically and experimentally examined at the PALS Center. The parameters of the PALS Ti:S laser interaction were studied by extensive particle-in-cell simulations with radiation post-processors in order to evaluate the capabilities of our system in this field. The extensions of those methods at the ELI Beamlines facility will enable to generate either higher X-ray energies or higher repetition rate. The architecture of such sources and their considered applications will be proposed.
Laser-driven heavy-ion accelerator represents a possible compact table-top device with a potential to applications, in particular,
ion implantation of PN junctions in semiconductors. We present generation of heavy ion beams, Ti and Fe with an energy of 210
keV and 440 keV, respectively. Such beams were accelerated from a front size of thick foils by p-polarized 600mJ, 50fs laser
pulse. Ion energies were measured by time-of-flight spectrometers. Shot-to-shot stability of obtained energies was better than 30%.
Relativistic electron beams accelerated by laser wakefield have the ability to serve as sources of collimated,
point-like and femtosecond X-ray radiation. Experimental conditions for generation of stable quasi-monoenergetic
electron bunches using a femtosecond few-terawatt laser pulse (600 mJ, 50 fs) were investigated as they are crucial
for generation of stable betatron radiation and X-ray pulses from inverse Compton scattering. A mixture of helium
with argon, and helium with an admixture of synthetic air were tested for this purpose using different backing
pressures and the obtained results are compared. The approach to use synthetic air was previously proven to stabilize
the energy and energy spread of the generated electron beams at the given laser power. The accelerator was operated
in nonlinear regime with forced self-injection and resulted in the generation of stable relativistic electron beams with
an energy of tens of MeV and betatron X-ray radiation was generated in the keV range. A razor blade was tested to
create a steep density gradient in order to improve the stability of electron injection and to increase the total electron
bunch charge. It was proven that the stable electron and X-ray source can be built at small-scale facilities, which
readily opens possibilities for various applications due to availability of such few-terawatt laser systems in many
laboratories around the world.
In a plasma wakefield accelerator, an intense laser pulse propagates in an under-dense plasma that drives a relativistic plasma wave in which electrons can be injected and accelerated to relativistic energies within a short distance. These accelerated electrons undergo betatron oscillation and emit a collimated X-ray beam along the direction of electron velocity. This X-ray source is characterised with a source size of the order of a micrometer, a pulse duration of the order of femtosecond, and with a high spectral brightness. This novel X-ray source provides an excellent imaging tool to achieve unprecedented high-resolution image through phase contrast imaging. The phase contrast technique has the potential to reveal structures which are invisible with the conventional absorption imaging. In the X-ray phase contrast imaging, the image contrast is obtained thanks to phase shifts induced on the X-rays passing through the sample. It involves the real part of refractive index of the object. Here we present high-resolution phase contrast X-ray images of two biological samples using laser-driven Betatron X-ray source.
Increasing the laser-induced damage resistance of optical components is one of the major challenges in the development of Peta-watt (PW) class laser systems. The extreme light infrastructure (ELI) beamlines project will provide ultrafast laser systems with peak powers up to 10 PW available every minute and PW class beams at 10 Hz complemented by a 5-TW, 1-kHz beamline. Sustainable performance of PW class laser systems relies on the durability of the employed optical components. As part of an effort to evaluate the damage resistance of components utilized in ELI beamlines systems, damage thresholds of several optical multilayer dielectric coatings were measured with different laser parameters and in different environments. Three coatings were tested with 10 Hz and 1 kHz pulse repetition rates, and the effect of a cleaning treatment on their damage resistance was examined. To explore the damage threshold behavior at different vacuum levels, one coating was subject to tests at various residual gas pressures. No change of damage threshold in a high vacuum with respect to ambient pressure was recorded. The effect of the cleaning treatment was found to be inconsistent, suggesting that development of the optimal cleaning treatment for a given coating requires consideration of its specific properties.
The ELI Beamlines project will deliver ultrafast laser pulses with peak powers up to 10PW available every minute and PW class beams at 10Hz complemented by a 10TW 1kHz beamline. To properly determine damage thresholds of involved optical components in conditions similar to the operational environment and with expected laser parameters, a high vacuum LIDT test station was constructed at PALS facility. Our study presents results of ISO based S-on-1 and R-on-1 tests in femtosecond regime (50fs, 800nm, 10Hz/1kHz) performed on two different types of coatings: a) highabsorption black coatings with low outgassing rates, intended for use as a beam dump surface; and b) high-reflectivity, low-dispersion 45° AOI ultrafast mirror coatings. Testing of absorptive coatings was accompanied with QMS residual gas analysis to verify, that high intensity laser radiation approaching the damage threshold does not increase concentration of volatile organic compounds in the vacuum chamber. In case of HR mirror coatings, we also investigate the effect of cleaning on LIDT value, comparing characteristic S-on-1 curves of given sample with values obtained after 12h immersion in ethanol-water solution.
The ELI-beamlines project will use high power ultrafast lasers with high average powers up to 1kW and peak powers up
to 10PW. The project presents a major challenge in terms of damage threshold of ultrafast mirror coatings and gratings.
In order to assess the LIDT of ultrafast coatings in the expected environments a test station was constructed in the PALS
facility. The setup can use beams from a 25TW Ti:Sapphire laser system with a 10Hz repetition rate. Testing is
performed mainly with a secondary 1 kHz 40fs while contamination levels are investigated.
A PW Ti:Sapphire laser with 30-J energy and 30-fs pulse duration has been developed at GIST and applied to generate
x-rays and energetic charged particles. We present the status and plan of developing ultrashort x-ray sources and their
applications. We successfully demonstrated x-ray lasers and their applications to high-resolution imaging. In addition,
we plan to generate high flux x-ray/gamma-ray sources using the PW laser.
The results of development and applications of the secondary sources at PALS Center will be presented. Currently the
iodine system and the Ti:Sapphire system are operating at the PALS Center as driving lasers for generation of secondary
sources. The iodine system with net energy of 1kJ is used for QSS X-ray lasing schemes. The most robust and most
energetic QSS scheme with this driver is the Ne-like Zn X-ray laser, which is working here as standard user beamline for
diverse applications. Recent experiment devoted to temporal coherence measurement shows possibility to amplify short
duration X-rays. The second system with high rep rate is Ti: Sapphire laser chain with peak power 20TW. This laser
system is used for generation high order harmonics and transient collisionally excited X-ray lasers.
The ELI-beamlines project is expected to reach state of the art parameters in its laser systems. The Laser Induced Damage Threshold of the corresponding optical systems will have to sustain the expected fluences and repetition rates. The LIDT requirements for the ultrafast pulse compressors, vacuum transport mirrors and high average power optics are presented together with the current and planned capabilities for LIDT testing with a 25TW laser system at 800 and 1060nm.
We report spatial and spectral characterization an optical-field-ionized high-order harmonic-seeded soft-x-ray laser. We
show that it can be controlled between a regular Gaussian shape and a Bessel profile exhibiting several rings via the IR
laser pump intensity. The temporal coherence and spectral linewidth of both the seeded and unseeded soft-x-ray lasers
were experimentally measured using a varying path difference interferometer. It showed that the high-order harmonic is
subject to a strong spectral narrowing during its propagation in the plasma amplifier without rebroadening at saturation.
Also, we present a new method to generate ultra-short x-ray laser pulses by using the laser-driven betatron source to
photo-pump inner-shell transitions.
B. Rus, F. Batysta, J. Čáp, M. Divoký, M. Fibrich, M. Griffiths, R. Haley, T. Havlíček, M. Hlavác, J. Hřebíček, P. Homer, P. Hříbek, J. Jand'ourek, L. Juha, G. Korn, P. Korouš, M. Košelja, M. Kozlová, D. Kramer, M. Krůs, J. C. Lagron, J. Limpouch, L. MacFarlane, M. Malý, D. Margarone, P. Matlas, L. Mindl, J. Moravec, T. Mocek, J. Nejdl, J. Novák, V. Olšovcová, M. Palatka, J. P. Perin, M. Pešlo, J. Polan, J. Prokůpek, J. Řídký, K. Rohlena, V. Růžička, M. Sawicka, L. Scholzová, D. Snopek, P. Strkula, L. Švéda
ELI-Beamlines will be a high-energy, repetition-rate laser pillar of the ELI (Extreme Light Infrastructure) project. It will
be an international facility for both academic and applied research, slated to provide user capability since the beginning
of 2016. The main objective of the ELI-Beamlines Project is delivery of ultra-short high-energy pulses for the
generation and applications of high-brightness X-ray sources and accelerated particles. The laser system will be
delivering pulses with length ranging between 10 and 150 fs and will provide high-energy petawatt and 10-PW peak
powers. For high-field physics experiments it will be able to provide focused intensities attaining 1024 Wcm-2, while this
value can be upgraded in a later phase without the need to upgrade the building infrastructure. In this paper we describe
the overall conception and layout of the designed ELI-Beamlines facility, and review some essential elements of the
design.
Inertial Confinement Fusion with Shock Ignition relies on a very strong shock created by a laser pulse at an intensity of
the order of 1016W/cm2. In this context, an experimental campaign at the Prague Asterix Laser System (PALS) has been
carried out within the frame of the HiPER project. Two beams have been used, the first to create an extended preformed
plasma (scale length of the order of hundreds of micrometers) on a planar target, the second to generate a strong shock
wave. Different diagnostics were used to study both the shock breakout at the rear surface of the target and the laserplasma
coupling and parametric instabilities. This paper is focused on back-scattering analysis to measure the backreflected
energy and to characterize parametric instabilities such as stimulated Brillouin and Raman scattering. Our
experimental data show that parametric instabilities do not play a strong role in the laser plasma coupling. Moreover,
preliminary analysis of the back reflected light from the interaction region shows that less than 5% of the total incident
laser energy was back-reflected, with only a small fraction of that light was originating from parametric instabilities.
We present the results of an experiment concerning laser-plasma interaction in the regime relevant to shock ignition. The
interaction of high-intensity frequency tripled laser pulse with CH plasma preformed by lower intensity pre-pulse on
fundamental wavelength of the kJ-class iodine laser was investigated in the planar geometry in order to estimate the
coupling of the laser energy to the shock wave or parametric instabilities such as stimulated Raman or Brillouin
scattering, or to the fast electrons. First the complete characterization of the hydrodynamic parameters of preformed
plasma was made using crystal spectrometer to estimate the electron temperature and XUV probe to resolve the electron
density profile close to the critical density region. The other part of the experiment consisted of the shock chronometry,
calorimetry of the back-scattered light and hard X-ray spectrometry to evaluate the coupling to different processes. The
preliminary analysis of the measurements showed rather low energy transfer of the high-intensity pulse to back-scattered
light (< 5%) and no traces of any significant hot electron production were found in the X-ray spectra.
Bedrich Rus, Chris Edwards, Mike Tyldesley, Mike Griffiths, Bruno Le Garrec, Manolo Perlado, Jean-Paul Perin, Didier Guillaume, David Neely, Jiří Polan, Michaela Kozlová, Pavel Homer, Jaroslav Nejdl, Steve Sanders, Petr Havlík, Martin Kopecký, Vladimír Kolařík, Tomáš Papírek, Martin Hlaváč, Richard Haley, Lewis MacFarlane, Neil Alexander
We review development in the repetition-rate target area systems and technologies within the Work Package 15 of the
HiPER Preparatory Phase project. The activities carried out in 2009-2010 have been involving analysis of solutions and
baseline design of major elements of the repetition-rated fusion chamber, analysis of prospective injector technologies,
numerical modelling of target survival during acceleration phase and during flight in the environment of fusion
chamber, analysis of options of remote handling, systems of mitigation of fusion debris, and others. The suggested
solutions assume operation at the repetition rate of 10 Hz and fusion yield between 20 and 100 MJ. Shock ignition is
assumed as the baseline ignition scenario, although some technologies are applicable in the fast ignition; a number of
the technologies identified are exploitable as well in the indirect drive. The operation of the HiPER repetition-rate
chamber will contribute to technology development for the Demonstration Reactor HiPER facility.
This paper presents the goals and some of the results of experiments conducted within the Working Package 10 (Fusion
Experimental Programme) of the HiPER Project. These experiments concern the study of the physics connected to
"Advanced Ignition Schemes", i.e. the Fast Ignition and the Shock Ignition Approaches to Inertial Fusion. Such schemes
are aimed at achieving a higher gain, as compared to the classical approach which is used in NIF, as required for future
reactors, and making fusion possible with smaller facilities.
In particular, a series of experiments related to Fast Ignition were performed at the RAL (UK) and LULI (France)
Laboratories and were addressed to study the propagation of fast electrons (created by a short-pulse ultra-high-intensity
beam) in compressed matter, created either by cylindrical implosions or by compression of planar targets by (planar)
laser-driven shock waves. A more recent experiment was performed at PALS and investigated the laser-plasma coupling
in the 1016 W/cm2 intensity regime of interest for Shock Ignition.
We performed measurement of the HHG (High-order Harmonics Generation) beam wavefront by the PDI (Point
Diffraction Interferometer) sensor recently. The XUV HHG beam operates at wavelength 30nm with 1kHz pulse
frequency delivering average energy 1nJ in the beam. This beamline is located in the PALS laser centre facility in
Prague. The PDI sensor is a self-referencing monolithic device and it consists of a thin foil and a very small pinhole. The
foil is semi-transparent for the XUV radiation and it is used as a density filter. The pinhole is located on the axis of the
XUV beam focal spot and works as a diffraction aperture generating a reference spherical wave. If the XUV detector is
placed behind the PDI sensor interferogram can be captured. It represents an interference between the spherical reference
wave and the original measured wave which passed the thin foil. From the information encoded in this pattern it is
possible to sequentially reconstruct the beam wavefront profile. In this paper we will discuss obtained results as well as
design and setup of the XUV PDI sensor.
We present an experimentally simple technique for the measurement of electron density gradients in dense laser plasmas (the plasma region of electron density up to 1024 cm-3 can be investigated with the use of available XRLs). The distortion
of the XRL wave-front caused by the gradients of the electron density is measured using Talbot pattern deformation. The plasma probed by the XRL is imaged on the CCD plane, then a 2D grating is put in front of the chip so that the Talbot plane of this grating fits on the CCD. The compromise between the spatial resolution and the sensitivity for the given wavelength of the probe must be set within the grating design. The main advantages of this method are low
requirements on spatial coherence of the probing beam as well as the simple alignment, which are the main difficulties of interferometry using radiation of XRLs.
We report on the x-ray absorption of Warm Dense Matter experiment at the FLASH Free Electron Laser (FEL) facility at DESY. The FEL beam is used to produce Warm Dense Matter with soft x-ray absorption as the probe of electronic structure. A multilayer-coated parabolic mirror focuses the FEL radiation, to spot sizes as small as 0.3μm in a ~15fs pulse of containing >1012 photons at 13.5 nm wavelength, onto a thin sample. Silicon photodiodes measure the transmitted and reflected beams, while spectroscopy provides detailed measurement of the temperature of the sample. The goal is to measure over a range of intensities approaching 1018 W/cm2. Experimental results will be presented along with theoretical calculations. A brief report on future FEL efforts will be given.
High-harmonic-seeded x-ray laser became an important issue in x-ray laser development due to the possibility to obtain a
highly coherent and polarized soft x-ray source. We performed theoretical investigations into amplification of high
harmonic pulses in an x-ray lasing medium by using a model based on Maxwell-Bloch equations. From the theoretical
works, we analyze characteristics of energy extraction and temporal profile of output pulse. In addition, preliminary
experimental results and ongoing experiments related the harmonic-seeded x-ray lasers are reported.
Irradiation experiments were conducted at Prague Asterix Laser System (PALS) with the Ne-like zinc soft x-ray laser
(SXRL) at 21.2 nm (58.5 eV) delivering up to 4 mJ (~4 x 1014 photons), 100-ps pulses in a narrowly collimated beam.
The SXRL beam was focused using a 1 inch diameter off-axis parabolic mirror (f = 253 mm at 14 degrees) with a Mo:Si
multilayer coating (R = 30% at 21 nm) placed 2825 mm from the SXRL. The diameter of the SXRL beam incident on
the mirror was about 11 mm. Ablation experiments with a gradually attenuated beam were performed to determine the
single-shot damage threshold of various materials. In this case, the sample was positioned at the tightest focus of the
SXRL whose pulse energy was attenuated by aluminum filters of various thickness to adjust the fluence. Both the focal
spot area and single-shot damage threshold were determined from the plot of damaged surface areas as a function of a
pulse energy logarithm to dete. For PMMA, the focal spot area and the ablation threshold inferred from the data are
Sfoc = (1172±230) μm2 and Fth = (1.25±0.4) J/cm2, respectively. Inorganic materials have thresholds significantly higher
than organic polymers, e.g., amorphous and monocrystalline silicon gave values 2.5 J/cm2 and 4.2 J/cm2, respectively.
For prospective SASE FEL optical elements, the SiC coating is of great interest. Its damage threshold is of 20 J/cm2, i.e.,
slightly lower than that of monocrystalline silicon. The thresholds determined with the 100-ps pulses from plasma-based,
quasi-steady state SXRL are significantly higher than the thresholds obtained for 20-fs pulses provided by the SXR freeelectron
laser in Hamburg. There is a difference in PMMA thresholds of two orders of magnitude for these two sources.
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.
We present a review of recent development and applications of soft x-ray lasers, undertaken at the PALS Centre. The applications benefit from up to 10-mJ pulses at the wavelength of 21.2 nm. We describe the pumping regimes used to produce this soft x-ray laser, and outline its emission characteristics. A significant fraction of applications carried out using this device includes probing of dense plasmas produced by IR laser pulses and high-energy-density-in-matter experiments. Results obtained in these experiments are reviewed, including x-ray laser probing of dense plasmas, measurements of transmission of focused soft x-ray radiation at intensities of up to 1012 Wcm-2, measurements of IR laser ablation rates of thin foils, and probing high density plasmas by x-ray laser Thomson scattering
Recent experiments were carried out on the Prague Asterix Laser System (PALS) towards the
demonstration of a soft x-ray laser Thomson scattering diagnostic for a laser-produced exploding foil. The
Thomson probe utilized the Ne-like zinc x-ray laser which was
double-passed to deliver ~1 mJ of focused
energy at 21.2 nm wavelength and lasting ~100 ps. The plasma under study was heated single-sided using a
Gaussian 300-ps pulse of 438-nm light (3ω of the PALS iodine laser) at laser irradiances of 1013-1014 W
cm-2. Electron densities of
1020-1022 cm-3 and electron temperatures from 200 to 500 eV were probed at
0.5 or 1 ns after the peak of the heating pulse during the foil plasma expansion. A flat-field 1200 line mm-1
variable-spaced grating spectrometer with a cooled charge-coupled device readout viewed the plasma in the
forward direction at 30° with respect to the x-ray laser probe. We show results from plasmas generated
from ~1 μm thick targets of Al and polypropylene (C3H6). Numerical simulations of the Thomson
scattering cross-sections will be presented. These simulations show electron peaks in addition to a narrow
ion feature due to collective (incoherent) Thomson scattering. The electron features are shifted from the
frequency of the scattered radiation approximately by the electron plasma frequency ±ωpe and scale as ne1/2.
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.
The ablation of plain aluminium foil and aluminium foil with a thin (50 nm) iron coating was observed using a neon-like zinc x-ray laser. The 21.2 nm x-ray laser was produced by a double pass of a 3 cm long zinc target at the PALS centre in Prague. The x-ray laser was used to probe the sample targets as they were heated by a separate laser beam of 10 J, focussed to a 100 micron diameter spot. The data from the experiment are presented and compared with Ehybrid simulations and simple ablation rate calculations.
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.
Experimental measurements of the opacity of plasmas at densities close to solid state and temperatures ~ 60 - 300 eV using a probing X-ray laser are presented. Utilizing thin targets, opacities of iron have been measured using x-ray lasers of photon energy 89 eV created by pumping with the VULCAN RAL laser. The thin targets are separately heated by spot focus laser pulses. We have demonstrated that X-ray laser brightness is sufficient to overcome the self-emission of hot plasma so that useful opacity measurements can be made. Due to their high brightness, x-ray lasers can fulfill a useful niche in measuring opacity and other phenomena associated with laser-plasma interactions. Quantities such as opacity measured in laser-plasmas are useful elsewhere. For example, plasma opacity is important in understanding radiative transfer in the sun.
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.
The multi-mJ, 21-nm soft-x-ray laser at the PALS facility was focused on the surface of amorphous carbon (a-C) coating, developed for heavily loaded XUV/x-ray optical elements. AFM (Atomic Force Microscopy) images show 3-micrometer expansion of the irradiated material. Raman spectra, measured with an Ar+ laser microbeam in both irradiated and unirradiated areas, confirm a high degree of graphitization in the irradiated layer. In addition to this highfluence (~ 1 J/cm2), single-shot experiment, it was necessary to carry out an experiment to investigate consequences of prolonged XUV irradiation at relatively low fluence. High-order harmonic (HH) beam generated at the LUCA facility in CEA/Saclay Research Center was used as a source of short-wavelength radiation delivering high-energy photons on the surface at a low single-shot fluence but with high-average power. a-C irradiated at a low fluence, i.e., < 0.1 mJ/cm2 by many HH shots exhibits an expansion for several nanometers. Although it is less dramatic change of surface morphology than that due to single-hot x-ray-laser exposure even the observed nanometer-sized changes caused by the HH beam on a-C surface could influence reflectivity of a grazing incidence optical element. These results seem to be important for estimating damages to the surfaces of highly irradiated optical elements developed for guiding and focusing the ultraintense XUV/x-ray beams provided by new generation sources (i.e., VUV FEL and XFEL in Hamburg; LCLS in Stanford) because, up to now, only melting and vaporization, but not graphitization, have been taken into account.
For conventional wavelength (UV-vis-IR) lasers delivering radiation energy to the surface of materials, ablation thresholds, ablation (etch) rates, and the quality of ablated structures often differ dramatically between short (typically nanosecond) and ultrashort (typically femtosecond) pulses. Various short-wavelength (<100 nm) lasers emitting pulses with durations ranging from ~10 fs to ~1 ns have recently been put into routine operation. This makes it possible to investigate how ablation characteristics depend on pulse duration in the XUV spectral region. Four sources of intense short-wavelength radiation available in the authors' laboratories, including XUV and soft x-ray lasers, are used for the ablation experiments. Based on the results of the experiments, the etch rates for three different pulse durations are compared using the XUV-ABLATOR code to compensate for the wavelength difference. Comparing the values of etch rates calculated for nanosecond pulses with those measured for shorter pulses, we can study the influence of pulse duration on XUV ablation efficiency. The results of the experiments also show that the ablation rate increases while the wavelength decreases from the XUV spectral region toward x-rays, mainly due to increase of attenuation lengths at short wavelengths.
Radiation from the Ne-like Zn soft x-ray laser (λ=21.2 nm, τ< 100 ps) driven by PALS (Prague Asterix Laser System) was successfully focused with a spherical Si/Mo multilayer-coated mirror to ablate poly(methyl methacrylate), monocrystalline silicon, and amorphous carbon. To our knowledge, this was the first observation of material ablation with a laser working in the soft x-ray region, i.e. λ<30 nm.
Program of development of deeply saturated Ne-like zinc soft X-ray laser at the PALS (Prague Asterix Laser System) Centre, employing as a pump device a kilojoule high-power iodine laser, is reviewed. The active medium giving rise to laser action at 21.2 nm is generated using a sequence of multiple-100-ns IR pump pulses, consisting of a weak prepulse (<10J), followed after 10 or 50 ns by the main pump pulse (~500 J). The population inversion in the resulting long scale-length density plasma allows to generate an extremely bright and narrowly collimated X-ray laser beam, providing up to ~10 mJ pulses and ~100 MW of peak power, which is the most powerful soft X-ray laser yet implemented. This device was recently used as radiation source in pilot radiobiology study of DNA damage in the soft X-ray region, and in material ablation. A novel interferometric device, based on double Lloyd's mirror, is being developed for surface nanometric probing with teh soft X-ray laser as a source. A test experiment was performed to assess focusing properties of the X-ray laser beam down to a narrow spot, with the ultimate goal of achieving 1013 Wcm-2 for novel applications relevant to e.g. laboratory astrophysics.
For conventional wavelength (UV-Vis-IR) lasers delivering radiation energy to the surface of materials, ablation thresholds, ablation (etch) rates, and the quality of ablated structures often differ dramatically between short (typically nanosecond) and ultrashort (typically femtosecond) pulses. Various short-wavelength (l < 100 nm) lasers emitting pulses with durations ranging from ~ 10 fs to ~ 1 ns have recently been put into a routine operation. This makes it possible to investigate how the ablation characteristics depend on the pulse duration in the XUV spectral region. 1.2-ns pulses of 46.9-nm radiation delivered from a capillary-discharge Ne-like Ar laser (Colorado State University, Fort Collins), focused by a spherical Sc/Si multilayer-coated mirror were used for an ablation of organic polymers and silicon. Various materials were irradiated with ellipsoidal-mirror-focused XUV radiation (λ = 86 nm, τ = 30-100 fs) generated by the free-electron laser (FEL) operated at the TESLA Test Facility (TTF1 FEL) in Hamburg. The beam of the Ne-like Zn XUV laser (λ = 21.2 nm, τ < 100 ps) driven by the Prague Asterix Laser System (PALS) was also successfully focused by a spherical Si/Mo multilayer-coated mirror to ablate various materials. Based on the results of the experiments, the etch rates for three different pulse durations are compared using the XUV-ABLATOR code to compensate for the wavelength difference. Comparing the values of etch rates calculated for short pulses with those measured for ultrashort pulses, we can study the influence of pulse duration on XUV ablation efficiency. Ablation efficiencies measured with short pulses at various wavelengths (i.e. 86/46.9/21.2 nm from the above-mentioned lasers and ~ 1 nm from the double stream gas-puff Xe plasma source driven by PALS) show that the wavelength influences the etch rate mainly through the different attenuation lengths.
Ablation thresholds, etch rates, and quality of ablated structures often differ dramatically if a conventional, UV-Vis-IR laser delivers radiation energy onto a material surface in a short (nanosecond) or ultra-short (picosecond/femtosecond) pulses. Various short-wavelength (λ < 100 nm) lasers emitting pulses with durations ranging from ~ 10 fs to ~ 1 ns have recently been put into a routine operation. This makes possible to investigate how the ablation characteristics depends on the pulse duration in the XUV spectral region. 1.2-ns pulses of 46.9-nm radiation delivered from a capillary-discharge Ne-like Ar laser, focused by a spherical Sc/Si multilayer-coated mirror were used for an ablation of organic polymers and silicon. Various materials were irradiated with an ellipsoidal-mirror-focused XUV radiation (λ = 86 nm, τ = 30-100 fs) generated by the free-electron laser (FEL) operated at the TESLA Test Facility (TTF1 FEL) in Hamburg. The beam of the Ne-like Zn XUV laser (λ = 21.2 nm, τ < 100 ps) driven by the Prague Asterix Laser System (PALS) was also successfully focused by a spherical Si/Mo multilayer-coated mirror to ablate various materials. Based on the results of the experiment the etch rates for three different pulse durations are compared using the XUV-ABLATOR code to compensate for the wavelength difference. Comparing the values of etch rates calculated for short pulses with the measured ones for ultrashort pulses we may study the influence of pulse duration on the XUV ablation efficiency.
Karel Jungwirth, Karel Rohlena, Jiri Ullschmied, A. Cejnarova, Libor Juha, M. Kozlova, Bozena Kralikova, Josef Krasa, E. Krousky, P. Kubat, Leos Laska, Karel Masek, Tomas Mocek, Miroslav Pfeifer, A. Prag, Oldrich Renner, Bedrich Rus, P. Severova, Jiri Skala, Petr Straka, Hana Turcicova
The PALS multi-user laser facility has been offering the beam time to the groups of both domestic and foreign rsearchers since September 2000. During the past two years of operation of its terawatt iodine laser system, a number of technical innovations and new diagnostic options were implemented, the most important of which are described in the paper. A brief survey of the current PALS research program is also given. Laser plasma sources of x-radiation and of highly stripped ions represent the two main lines followed. Recent highlights include the development and application of a highly coherent double-pass XUV laser based on Ne-like zinc. The reported studies of material response to the XUV pulses are mainly motivated by a potential use of the observed ablation phenomena e.g. in nanotechnology, while the x-ray contact microscopy permitted to image living biological objects with a resolution comparable to that of the electron microscopy. The PALS laser system is now in a routine operation, which opens the way to its new upgrades. The progress reached with the key ones -- application of elements of adaptive optics, replacement the original iodine oscillator by a solid-state based one, and, most important, implementation of the optical parametric chirped pulse amplification (OPCPA) technique -- is also reported.
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).
The analysis of available data about properties and possibilities of the CycolorTM reproduction technology for preparing color images using the nonsilver light-sensitive media containing microcapsules with photopolymerizable compositions is carried out. The results of our own investigations in the field of the peculiarities of polymerization processes, the method of preparation for microcapsules based on modified liposomes, and compositions for receiver sheet are discussed.
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