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Multi-MeV beams of light ions have been produced using the 300 picosecond, kJ-class iodine laser, operating at the Prague Asterix Laser System facility in Prague. Real-time ion diagnostics have been performed by the use of various time-of-flight (TOF) detectors: ion collectors (ICs) with and without absorber thin films, new prototypes of single-crystal diamond and silicon carbide detectors, and an electrostatic ion mass spectrometer (IEA). In order to suppress the long photopeak induced by soft X-rays and to avoid the overlap with the signal from ultrafast particles, the ICs have been shielded with Al foil filters. The application of large-bandgap semiconductor detectors (>3 eV) ensured cutting of the plasma-emitted visible and soft-UV radiation and enhancing the sensitivity to the very fast proton/ion beams. Employing the IEA spectrometer, various ion species and charge states in the expanding laser-plasma have been determined. Processing of the experimental data based on the TOF technique, including estimation of the plasma fast proton maximum and peak energy, ion beam currents and total charge, total number of fast protons, as well as deconvolution processes, ion stopping power, and ion/photon transmission calculations for the different metallic filters used, are reported. [ABSTRACT FROM AUTHOR]

We present measurements of in situ nanometric-resolution topographical modifications of thin niobium layers subjected to strong electric fields. The Nb layers, deposited on a fused silica substrate, are interferometrically flash probed using soft x-ray laser (XRL) at the wavelength of 21.2 nm. Its pulses are reflected by the probed sample under grazing incidence angle, and the information about surface deformation is obtained by a Fresnel wave-front-division interferometer. It was experimentally established that the probing pulses at the soft x-ray wavelength do neither produce any measurable photoelectric-field emission, nor alter the topographical features of the probed surface. The examined Nb electrodes were periodically probed while the electric field was increased up to 80 MV/m, and alterations of their topographical characteristics with a resolution of ∼2 nm in the relief elevation were obtained. It was found that behavior of the Nb layer strongly depends on the polarity of the applied voltage. Only small modifications are observed with the Nb surface at the positive potential, whereas the negative potential induces significant transient surface perturbations, with peak-to-valley elevation differences ∼50 nm. The qualitative nature of these modifications was observed to be unrelated to intermittent parasitic breakdowns. [ABSTRACT FROM AUTHOR]

Abstract: Fast plasma particle streams have been produced from the interaction of thin solid targets with the recently commissioned 25TW, Ti:Sapphire, femtosecond laser operating at PALS Centre in Prague. A real-time diagnostic system was employed to characterize the produced particles. Low-current proton beams with energies exceeding 2MeV were detected by a silicon-carbide detector and their typical energy spectrum is reported. High-current slow plasma ions were recorded by ion collectors placed at different distances and angles. Particle streams were characterized with the future aim to use them as probe beams in combination with the kJ-class, iodine, sub-nanosecond laser beamline available at PALS. [Copyright &y& Elsevier]

Abstract: The sub-nanosecond laser system available at PALS facility in Prague has been used in order to produce MeV proton beams with typical current density approaching 1A/cm2 at few tens of centimeters from the target surface. In spite of the relatively long pulse duration (0.3ns) and low intensity (∼1016 W/cm2), far away from the forefront laser facilities used for advanced proton beam acceleration in the recent years (from tens of femtoseconds to few picoseconds), the obtained results are promising both in terms of maximum proton energy and fast proton current. Real-time diagnostics systems, mainly in time-of-flight (TOF) configuration, have been used in order to estimate maximum and peak energy of the plasma fast proton component, peak current density, total number of fast protons and conversion efficiency of laser energy into accelerated fast proton total energy. Optimization of the maximum attainable proton energy and current has been carried out by irradiating targets of different composition as well as varying the laser energy and the focal spot diameter. Experimental results, as well as possible applications in material science and nuclear physics, are discussed and compared with literature data. [Copyright &y& Elsevier]

A new and simple experimental technique for the measurement of electron density gradients in dense laser-produced plasmas using an electromagnetic wave probe is presented. The main advantage of this method is the low requirements on coherence of the probing beam. The method is based on measuring the deformation of the Talbot pattern of a two-dimensional grating that stems from the distortion of the probe beam wave-front caused by the gradients of the index of refraction. The compromise between spatial resolution and sensitivity for the given wavelength of the probe beam is set by the experimental design. The proposed technique was experimentally verified on plasmas that were created by either a point focus or a line focus of a laser interacting with various solid targets. In the experiments reported here, all plasmas were probed by a Ne-like Zn x-ray laser beam at 21.2 nm, but the technique is applicable for any wavelength of the probe. [ABSTRACT FROM AUTHOR]

We report on a single-shot micropatterning of an organic polymer achieved by ablation with demagnifying projection using a plasma-based extreme ultraviolet (XUV) laser at 21 nm. A nickel mesh with a period of 100 μ m was 10×demagnified and imprinted on poly(methyl methacrylate) (PMMA) via direct ablation. This first demonstration of single-shot projection, single-step lithography illustrates the great potential of XUV lasers for the direct patterning of materials with a resolution scalable down to the submicrometer domain. In the second part, we present a novel experimental method for improving the efficiency of surface processing of various solids achieved by simultaneous action of XUV, obtained from high-order harmonic generation, and near-infrared (NIR)-VIS laser pulses. The NIR-VIS pulse interacts with free charge carriers produced by the energetic XUV photons, so that its absorption dramatically increases. Laser-induced periodic surface structures were effectively produced using this technique. [ABSTRACT FROM AUTHOR]

The design, construction and development of proper diagnostics systems for multi-MeV proton/ion and relativistic electron beams, typically generated by fs-laser plasmas, are under study in our laboratories. Ultra-fast plasma ion detection through classical ion collectors (ICs) in time-of-flight (TOF) configuration, shielded by a thin metallic absorber, has been theoretically studied and preliminarily tested using the 300 ps, kJ-class iodine laser operating at PALS Research Centre, in order to cut the long photo-peak contribution and analyse the ultra-fast particle signal. Processing of obtained experimental IC-TOF data, including maximum proton/ion energy estimations, is reported. Design, modelling and simulations for relativistic electron bunch detection, by means of a proper magnetic electron spectrometer, are also included. The effort is mainly devoted to match the diagnostics systems with the peculiarity of fast particle streams which could be generated by the 20 TW Ti:sapphire laser (40 fs, 1 J, 10 Hz) recently installed at PALS. [ABSTRACT FROM AUTHOR]

Ultrafast plasma light ion streams have been produced using the 300 ps, kJ-class iodine laser, operating at PALS Centre in Prague. Ion detection was performed through standard ion collectors (IC) in time-of-flight configuration (TOF), shielded by thin metallic absorbers. This new diagnostics technique has been theoretically studied and experimentally tested in order to cut the long photopeak contribution and to analyze the ultrafast particle signal. Processing the obtained experimental IC-TOF data, including deconvolution processes of the TOF signals, UV/soft-x-ray photopeak absorption, and ion transmission calculations for different metallic filters, is shown. Mainly amorphous carbon (graphite) targets have been irradiated in order to limit the maximum number of ion charge states and to focus our study on demonstrating the validity of the proposed investigation technique. Maximum ion energy and acceleration gradient estimations as a function of the laser energy and focal spot diameter are reported. [ABSTRACT FROM AUTHOR]

Abstract: For conventional wavelength (UV–vis–IR) lasers delivering radiation energy to the surface of materials, ablation thresholds, etch (ablation) rates, and the quality of ablated structures often differ dramatically between short (typically nanosecond) and ultrashort (typically femtosecond) pulses. Various very short-wavelength (λ <100nm) lasers, emitting pulses with durations ranging from ∼10fs to ∼1ns, have recently been placed into routine operation. This has facilitated the investigation of how ablation characteristics depend on the pulse duration in the XUV spectral region. Ablation of poly(methyl methacrylate) (PMMA) induced by three particular short-wavelength lasers emitting pulses of various durations, is reported in this contribution. [Copyright &y& Elsevier]

We give an overview of recent experimental 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 8×1017 Wcm-2 into a low-density gas cell containing Xe or Kr, 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 Xe8+, and on the 3d94d(1S0)­3d94p(1P1) transition in Kr8+ at 32.8 nm. Under optimum pumping conditions the Xe8+ laser provides about (5±2)×109 photons per pulse whilst the Kr8+ laser delivers up to (2.5±1)×109 photons per shot. The repetition rate of these soft X-ray lasers is 10 Hz. The beam wavefront of the Xe8+ laser has been measured by a Shack­Hartmann soft X-ray wavefront sensor, and the pulse duration by a cross-correlation technique. [ABSTRACT FROM AUTHOR]

At the Prague asterix laser system (PALS) of the Academy of Sciences of the Czech Republic the 1-TW asterix iodine laser is used as a pump source for soft X-ray laser experiments. The prepulse technique was applied which is known to enhance the X-ray laser output at the J=0-1 transition dramatically. Since Zn slab targets were used the laser wavelength was 21.2 nm. A prepulse beam having 1.6 J was preceding the main pulse by 10 ns. The main and the prepulse beam are focused by two different optical systems separately. Implementing a half-cavity set-up for double-pass amplification using a Mo/Si multilayer mirror the X-ray laser output was 10 times stronger than at single pass amplification in a 3-cm long plasma. Double-pass amplification was observed to be most efficient when the pump pulse duration was at least 120 ps longer than the round trip time in the half-cavity. Under this fundamental condition the X-ray laser reached saturation in the double-pass regime containing 4 mJ energy what is proved to be enough for applications. In this contribution the X-ray laser features like divergence in two dimensions, the beam quality (symmetry), the pointing angle and the time-integrated output energy are investigated over more than 100 shots. To characterize the stability of the X-ray laser the shot distribution, the mean value and the standard deviation for these parameters are evaluated. For 18 shots in a series -- achieved during one single day -- the corresponding values are given and detailed chi-squared tests characterize the Zn X-ray laser as a robust tool suitable for applications. At PALS soft X-ray laser beam time can be reserved for external research groups. [ABSTRACT FROM AUTHOR]

At the Prague Asterix Laser System Center (PALS) the Asterix iodine laser delivering up to 700 J/0.5 ns is used as a pump source for X-ray laser experiments and applications. The prepulse technique was applied which is known to improve the neon-like X-ray laser output at the J = 0-1 transition dramatically. Since Zn slab targets were used the operating wavelength was 21.2 nm. A prepulse having up to 20 J precedes the main pulse by 10 ns. The main beam and the prepulse beam are focussed by two different optical systems separately and their foci are superimposed at the target surface. By implementing a half-cavity set-up for double-pass amplification using a Mo/Si multilayer mirror - which can be used for more than 100 shots - the X-ray laser output was more than 10 times stronger than at the single pass in a 30 mm long plasma. Double-pass amplification was observed to be most efficient when the pump pulse duration was at least 150 ps longer than the round trip time (≈ 260 ps) in the half-cavity. Under this fundamental condition the X-ray laser reached saturation in the double-pass regime containing approx. 4 mJ energy which has been proved to be enough for future applications. In this contribution, the X-ray laser features like divergence in two dimensions, the beam quality (symmetry), the pointing angle and the integrated intensity giving an estimation of the output energy are investigated over 110 shots. To characterize the stability of the X-ray laser the shot distribution, the mean value and the standard deviation for these parameters are evaluated. For 18 shots in a series - what was achievable during one day - the corresponding values are given, and a statistical analysis carrying out a chi-squared test characterize the Zn X-ray laser as a robust tool suitable for applications. In the future it is planned to allocate X-ray laser beam time to external research groups. [ABSTRACT FROM AUTHOR]

A dual-output thin-disk picosecond laser operating at 100 W with 1 kHz repetition rate is reported in this Letter. By electronically adjusting the amplitude of the optical seed pulses that are injected into the laser cavity, the energy extracted from the gain medium can be shared between two pulses. Amplified double pulses are subsequently spatially separated into two independent beams by a fast Pockels cell, compressed in one common compressor, and frequency-doubled with ∼70% efficiency. This approach significantly decreases strain on the optics, as well as nonlinear effects, and is advantageous for power scaling.

The latest advances in petawatt laser technology within the ELI Beamlines project have stimulated the development of large surface area dielectrically coated mirrors meeting all demanding requirements for guiding the compressed 30 J, 25 fs HAPLS laser beam at 10 Hz repetition rate and a center wavelength of 810 nm entirely in vacuum. We describe the production and evaluation of Ta 2 O 5 /HfO 2 /SiO 2 ion beam sputtered coated (440 × 290 × 75) mm 3 beam transport mirrors. No crazing was observed after thirty vacuum-air cycles. A laser induced damage threshold of 0.76 J/cm 2 (fluence on mirror surface) was achieved and maintained at high shot rates.

We report on a 1 kHz, 515 nm laser system, based on a commercially available 230 W average power Yb:YAG thin-disk regenerative amplifier, developed for pumping one of the last optical parametric chirped pulse amplification (OPCPA) stages of the Allegra laser system at ELI Beamlines. To avoid problems with self-focusing of picosecond pulses, the 1030 nm output pulses are compressed and frequency doubled with an LBO crystal in vacuum. Additionally, development of a thermal management system was needed to ensure stable phase matching conditions at high average power. The resulting 515 nm pulses have an energy of more than 120 mJ with SHG efficiency of 60% and an average RMS stability of 1.1% for more than 8 h.

We present a study of the temporal prepulse contrast degradation of high focused intensity pulses produced in CPA laser systems due to imperfections in amplifier design, alignment of amplifier components, and crystal inhomogeneity. Using an extended cross-polarized imaging technique, we demonstrate the presence of multiple crystal domains inside Ti:sapphire slabs with ≈10 c m diameter. The results of our numerical calculations show that crystalline c-axis orientation inhomogeneity caused by these crystal domains can lead to the generation of prepulses with a relative contrast of >10 -10 within several picoseconds before the main pulse. In a multiple-slab amplifier head configuration sometimes used in high-repetition-rate systems, the misalignment of the crystalline c-axes of the amplifier slab with respect to each other can lead to the generation of prepulses with relative contrast as high as 10 -6 , depending on the magnitude of misalignment.

Vacuum chambers are frequently used in high-energy, high-peak-power laser systems to prevent deleterious nonlinear effects, which can result from propagation in air. In the vacuum sections of the Allegra laser system at ELI-Beamlines, we observed degradation of several optical elements due to laser-induced contamination (LIC). This contamination is present on surfaces with laser intensity above 30 G W / c m 2 with wavelengths of 515, 800, and 1030 nm. It can lead to undesired absorption on diffraction gratings, mirrors, and crystals and ultimately to degradation of the laser beam profile. Because the Allegra laser is intended to be a high-uptime source for users, such progressive degradation is unacceptable for operation. Here, we evaluate three methods of removing LIC from optics in vacuum. One of them, the radio-frequency-generated plasma cleaning, appears to be a suitable solution from the perspective of operating a reliable, on-demand source for users.

Stable picosecond supercontinuum generated in long crystals is an excellent means of seeding broadband, high-energy CPA systems. The generated output energy and spectrum can be almost three times as stable as the pump for a wide range of input pulse parameters. In this work, we show this is an intrinsic property for crystals longer than the filament and for a range of input energy values. We present a description of the stability mechanism in both the visible and infrared regions together with experimental data that support the theoretical explanation.

We compare for the first time the influence of different Yb:YAG gain media on the performance of a large-area, high average-power laser system with an output energy of up to 6 J. Monocrystalline slabs grown by a new technique without central growth defect are compared with ceramics. Small signal gain, maximum output energy and thermal lensing are compared for ceramic slabs with co-sintered amplified spontaneous emission (ASE) absorber cladding, monocrystalline slab with and without optically bonded ASE absorber cladding, and surface structured monocrystalline slabs. We show that these large monocrystals with optically bonded absorber cladding have similar performance to cladded ceramics, so far the only material for high-energy Yb:YAG lasers.

To optimize optical coating materials, designs, and technologies for high damage resistance, understanding the growth of laser damage is of paramount importance. In this Letter, we show the evolution of femtosecond laser damage in a hafnia-silica (HfO 2 /SiO 2 ) multilayer dielectric mirror coating. Depending on various spatial features of damaged sites, we identified several regimes of the laser-material interaction with varying laser fluence and incident number of pulses. A change in surface roughness has been observed only for a small number of pulses, and interestingly, a threshold number of pulses is found for nanocrack formation. We report the polarization-dependent orientation of nanocracks and their growth with an increasing number of pulses. The presented results demonstrate that the laser damage originates from the nanobumps and surface roughening, which then leads to the formation of nanocracks. The presented experimental results acknowledge the existing theoretical models in bulk dielectrics to explain the formation of nanostructures by interference of the incident laser with the scattering radiation from laser-induced inhomogeneities and growth of the field enhancement due to nanoplasma.

We present a broadband optical parametric chirped pulse amplification (OPCPA) system delivering 4 J pulses at a repetition rate of 5 Hz. It will serve as a frontend for the 1.5 kJ, <150  fs, 10 PW laser beamline currently under development by a consortium of National Energetics and Ekspla. The spectrum of the OPCPA system is precisely controlled by arbitrarily generated waveforms of the pump lasers. To fully exploit the high flexibility of the frontend, we have developed a 1D model of the system and an optimization algorithm that predicts suitable pump waveform settings for a desired output spectrum. The OPCPA system is shown to have high efficiency, a high-quality top-hat beam profile, and an output spectrum demonstrated to be shaped consistently with the theoretical model.

A robust and simple method is presented for ensuring constant energy and pointing of a high average power solid state laser on a target. In addition to providing long-term stability, this scheme also eliminates any drifts in energy or pointing resulting from the initial warm-up after a cold start. This is achieved using two separate feedback loops: one loop stabilizes the pointing of the beam external to the amplifier cavity and the other locks the cavity mode to have optimum overlap with the pump spot on the active medium. The key idea of the cavity mode stabilization is to monitor the overlap of the cavity mode and the gain medium with a camera and control it with an actively controlled, intra-cavity mirror. While this method is demonstrated on a thin-disk regenerative amplifier, it can also be applied to a wide variety of solid state laser amplifiers.

We present a stable supercontinuum (SC) generated in a bulk YAG crystal, pumped by 3 ps chirped pulses at 1030 nm. The SC is generated in a loose focus geometry in a 13 cm long YAG crystal, allowing for stable and robust single-filament generation. The SC energy stability exceeds that of the pump laser by almost a factor of 3. Additionally, we show that the SC spectrum has long-term stability and that the SC is coherent and compressible by compressing the portions of SC spectra close to the corresponding Fourier limit. This makes the picosecond-pulse-driven SC a suitable stable seed for OPCPA amplifiers.

We report on the design and performance of a fiber-based, multi-channel laser amplifier seed pulse distribution system. The device is designed to condition and distribute low energy laser pulses from a mode-locked oscillator to multiple, highly synchronized, high energy amplifiers integrated into a laser beamline. Critical functions such as temporal pulse stretching well beyond 100 ps/nm, pulse picking, and fine control over the pulse delay up to 300 ps are all performed in fiber eliminating the need for bulky and expensive grating stretchers, Pockels cells, and delay lines. These functions are characterized and the system as a whole is demonstrated by seeding two high energy amplifiers in the laser beamline. The design of this system allows for complete computer control of all functions, including tuning of dispersion, and is entirely hands-free. The performance of this device and its subsystems will be relevant to those developing lasers where reliability, size, and cost are key concerns in addition to performance; this includes those developing large-scale laser systems similar to ours and also those developing table-top experiments and commercial systems.

We derived a formula for calculation of the spectral phase of ultrashort pulses propagating through aberrated stretchers. Our approach is based on Seidel aberration theory. The dependence of spectral phase dispersion terms and residual angular dispersion on the individual Seidel aberration coefficients is found. As an example, the spectral phase deviation and the residual angular dispersion of an ultrashort pulse for the Martinez/Banks stretcher using spherical optics is calculated.

We report on a broadband OPCPA system, pumped at 515 nm by frequency doubled Yb:YAG thin disk lasers. The system delivers 11.3 mJ pulses at a central wavelength of 800 nm with a spatial beam quality of M 2 = 1.25 and > 25% pump-to-signal conversion efficiency. The broadband pulses were demonstrated to be compressible to 12 fs using a chirped mirror compressor.

We report on a frequency-doubled picosecond Yb:YAG thin disk regenerative amplifier, developed as a pump laser for a kilohertz repetition rate OPCPA. At a repetition rate of 1 kHz, the compressed output of the regenerative amplifier has a pulse duration of 1.2 ps and pulse energy of 90 mJ with energy stability of σ < 0.8% and M 2 < 1.2. The pulses are frequency doubled in an LBO crystal yielding 42 mJ at 515 nm.

A simple and compact scheme for synchronization of the pump and signal pulses for short-pulse OPCPA is demonstrated. Relative timing jitter of 17 fs RMS is achieved (1% of the pump pulse duration) and the system remains locked for hours. The scheme uses a balanced optical cross correlator to detect relative delays between the pump and signal pulses and can be operated with just 10's of μJ of pump energy and pJ-level signal energies.

An overview of the last experimental campaigns on laser-driven ion acceleration performed at the PALS facility in Prague is given. Both the 2 TW, sub-nanosecond iodine laser system and the 20 TW, femtosecond Ti:sapphire laser, recently installed at PALS, are used along our experiments performed in the intensity range 10(16)-10(19) W∕cm(2). The main goal of our studies was to generate high energy, high current ion streams at relatively low laser intensities. The discussed experimental investigations show promising results in terms of maximum ion energy and current density, which make the laser-accelerated ion beams a candidate for new-generation ion sources to be employed in medicine, nuclear physics, matter physics, and industry.