Overview of progress in construction and testing of the laser systems of ELI-Beamlines, accomplished since 2015, is presented. Good progress has been achieved in construction of all four lasers based largely on the technology of diode-pumped solid state lasers (DPSSL). The first part of the L1 laser, designed to provide 200 mJ <15 fs pulses at 1 kHz repetition rate, is up and running. The L2 is a development line employing a 10 J / 10 Hz cryogenic gas-cooled pump laser which has recently been equipped with an advanced cryogenic engine. Operation of the L3-HAPLS system, using a gas-cooled DPSSL pump laser and a Ti:sapphire broadband amplifier, was recently demonstrated at 16 J / 28 fs, at 3.33 Hz rep rate. Finally, the 5 Hz OPCPA front end of the L4 kJ laser is up running and amplification in the Nd:glass large-aperture power amplifiers was demonstrated.
We present an overview of the projected and/or implemented laser systems for ELI-Beamlines. The ELI-Beamlines
facility will be a high-energy, high repetition-rate laser pillar of the ELI (Extreme Light Infrastructure) project. The
facility will make available high-brightness multi-TW ultrashort laser pulses at kHz repetition rate, PW 10 Hz repetition
rate laser pulses, and kilojoule nanosecond laser pulses that will be used for generation of 10 PW, and potentially higher,
peak power. These systems will allow meeting user requirements for cutting-edge laser resources for programmatic
research in generation and applications of high-intensity X-ray sources, in electron and proton/ion acceleration, and in
dense plasma and high-field frontier physics.
Extensive measurements of wavefront profile of the coherent XUV (eXtreme Ultra-Violet) HHG (High-order Harmonics Generation) beam at the wavelength of 30 nm have been performed. Unique results have been achieved using the PDI (Point Diffraction Interferometer) technique. The basic principle of the PDI is straightforward – ultrathin aluminium foil with a miniature pinhole – and it benefits from the self-referencing feature which is very important due to the measured wavelength. On the other hand, fabrication and experimental measurements are in general difficult in this spectral domain. In this paper we present basic principles, experimental setup, alignment techniques, obtained data and their analysis.
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 10<sup>24</sup> Wcm<sup>-2</sup>, 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
This paper presents a short analysis of possible techniques for fusion targets tracking in rep-rate regime. Target tracking
solution is limited with necessity of high speed, high precise and long-distance measurement combined with a harsh
environment of the vacuum fusion chamber. The only optical measurement seems to be usable to meet required
conditions to measurement system. Few standards and less traditional methods are presented in this paper. Its possibility
to meet the target goal resolution is discussed. Preparation of experimental techniques for verification of measurement
conditions of suggested methods is shown too.
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.
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.
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 10<sup>14</sup> 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
S<sub>foc</sub> = (1172±230) μm<sup>2</sup> and F<sub>th</sub> = (1.25±0.4) J/cm<sup>2</sup>, respectively. Inorganic materials have thresholds significantly higher
than organic polymers, e.g., amorphous and monocrystalline silicon gave values 2.5 J/cm<sup>2</sup> and 4.2 J/cm<sup>2</sup>, respectively.
For prospective SASE FEL optical elements, the SiC coating is of great interest. Its damage threshold is of 20 J/cm<sup>2</sup>, 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/cm<sup>2</sup>. 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 demonstrate a novel experimental method for efficient structural surface modification of various solids (PMMA,
amorphous carbon) achieved by simultaneous action of XUV (21.6 nm), obtained from High-order Harmonic Generation
(HHG), and Vis-NIR (410/820 nm) laser pulses. Although the fluence of each individual pulse was far below the surface
ablation threshold, very efficient and specific material modification was observed after irradiation even by a single shot
of mixed XUV/Vis-NIR radiation.
For the purpose of the wavefront profile measurement of XUV beams emitting at 21.2 nm and 30 nm, we designed the
PDI (Point Diffraction Interferometer) wavefront sensor. PDI is a self-referencing monolithic device consisting of a thin
neutral filter and a very small pinhole located near the axis of the XUV beam focal spot. The small pinhole works as a
diffraction aperture generating a reference spherical wave, and working as well as a spatial filter. The material of the thin
foil is partially transparent for the XUV radiation, and it determines the visibility of the interference fringes. The
interference pattern is recorded by an XUV detector placed behind the foil. From the information encoded in the pattern
it is possible sequentially to reconstruct the beam wavefront profile. We will discuss the design and optimization of the
PDI wavefront sensor setup.
The point diffraction interferometer (PDI) is a simple
self-referencing interferometer, designed here to measure the
wavefront profile of a soft X-ray laser emitting at the wavelength of 21.2 nm. It is a monolithic device consisting of a
thin filter and a very small pinhole (~1 μm), located near the axis of the X-ray laser focal spot. The foil material around
the hole is semitransparent for the X-ray radiation of interest, acting like a neutral density filter. The small pinhole is a
diffraction aperture and plays a spatial filtering role, generating spherical wave that acts a reference. The interferometric
pattern is produced on a detector placed a few centimeters behind the foil. The beam wavefront profile is reconstructed
from the information encoded in the pattern. In this paper we discuss the overall design of the PDI wavefront sensor
setup, namely its geometry, fringe contrast, level of the detected signal, size of the pinhole, and candidate materials for
the PDI foil.
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 10<sup>13</sup>-10<sup>14</sup> W
cm<sup>-2</sup>. Electron densities of
10<sup>20</sup>-10<sup>22</sup> cm<sup>-3</sup> 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<sup>-1</sup>
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 (C<sub>3</sub>H<sub>6</sub>). 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 ±ω<sub><i>pe</i></sub> and scale as <i>n<sub>e</sub></i><sup>1/2</sup>.
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 10<sup>12</sup> Wcm<sup>-2</sup>, measurements of IR laser ablation rates of thin foils, and probing high density plasmas by x-ray laser Thomson scattering
We report on the development of ultrafast coherent soft X-ray beamline at the Prague Asterix Laser System
(PALS) Research Center intended for interdisciplinary applications such as ablation and controlled surface modification
of solid materials for a micro/nano-pattering, soft X-ray interferometry and holography for surface probing with
nanometric resolution, and improvement of focusing optics for soft
X-ray beams. The beamline is based on 1 kHz, tabletop,
high-order harmonic generation (HHG) source capable to deliver fully coherent, tunable beam in the 13 - 40 nm
spectral range. Ti:sapphire (810 nm, 1 kHz) laser pulses with a duration of 35 fs and energy 1.5 mJ have been focused
into a gas jet or gas cell containing conversion medium (Ar). To achieve highly efficient HHG we will apply the
technique of guided laser pulses and the two-color laser field. Results on the optimization of HHG near 21 nm are
presented. The beamline consists of a tandem of two vacuum chambers: one for the HHG source and its diagnostics, and
second for the application experiments. After completion, access to this new installation will be opened to external users.
This table-top system will be complementary to the existing, high energy (~10 mJ) Ne-like Zn soft X-ray laser at 21.2
nm developed at PALS. We will also present the first experimental results on the structural surface modifications of
various solid materials (i.e., PMMA - poly(methyl methacrylate); amorphous carbon) caused by a few shot exposure to
the focused HHG beam at 21.6 nm.
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.
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 ~10<sup>13</sup> Wcm<sup>-2</sup>.
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<sup>-2</sup>. 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.
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 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 (SiO<sub>2</sub>), 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.