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A concept of the front-end system of the XCELS (eXawatt Center for Extreme Light Studies) facility is presented. Its design is aimed at achieving high stability of laser radiation parameters and possibility of their control in a wide range. Optically synchronised chirped signal (wavelength 910 nm, bandwidth more than 100 nm, and duration ∼3 ns) and pump (wavelength 1054 nm, bandwidth ∼1 nm, and duration ∼4 ns) pulses for XCELS parametric amplifiers will be implemented at the output of the front-end system. Chirped femtosecond pulses with energies above 100 mJ [no more than 15 fs long after compression, with carrier-envelope phase (CEP) stabilisation] will have a repetition rate up to 100 Hz, which will allow one to implement active energy stabilisation and to minimise the angular jitter of the emitted beam at the XCELS output. The application of picosecond pumping in the parametric amplifier of the front-end system should provide a high contrast of femtosecond pulses. The pump pulse will be linearly frequency-modulated; this approach will not affect the parametric amplification efficiency but make it possible to use spectral methods to control the pump pulse shape in order to form a pulse of specified shape at the output of power amplifiers, even under conditions of their strong saturation.

It is shown theoretically and experimentally that free space propagation in vacuum provides self-filtering of beam fluence fluctuations (spatial small-scale noise). The linear interference of the noise and the primary smooth beam during the propagation results in temporal filtering invoked by the noise pulse delay with respect to the primary beam pulse and in spatial filtering caused by the noise spatial walk-off from the aperture of the primary beam.

The results of the operation of a wavefront correction system based on a deformable bimorph mirror of the PEARL subpetawatt laser facility are presented. An improvement in the quality of focusing of laser radiation, which led to an increase in the Strehl ratio from 0.3 to 0.6, is demonstrated. The features of the compensation for phase distortions of the wavefront in the case of a low pulse repetition rate, as well as the correct allowance for the noise of the CCD camera when calculating the Strehl ratio are investigated.

The physical reason for the difference between the small-scale self-focusing of nanosecond and femtosecond pulses is that the typical intensity of the latter is three orders of magnitude higher, i.e., TW/cm2 versus GW/cm2. This causes a significant shift of the growth-rate maximum of the Bespalov–Talanov instability to the region of high spatial frequencies. During free propagation, a decrease in the spectral density of noise and the self-filtering of the beam lead to the noise-density decrease in the region of the maximum growth rate and, therefore, slowing of the self-focusing development. This is shown to shift the restriction on using the transmissive optical elements in the superpower lasers towards high powers.

Two-stage compression of laser pulses with a power of 250 TW is experimentally realised by broadening their spectrum during self-phase modulation in fused silica and subsequent dispersion compensation upon reflection from chirping mirrors. A five-fold decrease in the duration is demonstrated, from 75 to 15 fs, with a B-integral value of about 5 at each stage. It is possible to avoid small-scale self-focusing due to self-filtering of the laser beam during free propagation in vacuum. With optimal parameters of the dispersive mirror, the pulse can be compressed to a duration of less than 5 fs.

We present the results of studying experimentally the expansion of laser plasma in a strong external magnetic field (with a magnetic flux density of 13.5 T) at various sizes of the region of plasma formation on the surface of a solid-state target. It is shown that when the size of the plasma formation region is smaller than the classical plasma braking radius, a nearly identical topology of plasma flows is observed, which is characterized by the formation of a thin plasma sheet directed along the external magnetic field. If the width of the plasma formation region is comparable with the classical plasma braking radius, an additional plasma sheet starts to be formed.

The 18J, 60fs laser pulse was compressed to 10fs after passing through KDP crystal and reflecting from chirping mirrors. The experiments were performed at В-integral up to 19 without visible damages of the optical elements.

The output pulse of the PEARL was compressed to 10 fs by reflecting from chirped mirrors after propagation through Kerr nonlinear medium. The power was more than 1.5 PW. It is the shortest pulse for all petawatt lasers.

Analytical expressions for the spatial spectrum of fluence fluctuations of a laser pulse propagating in a medium with Kerr nonlinearity have been obtained. It is shown that inhomogeneities with a spatial scale much larger than the critical scale of self-focusing grow insignificantly even at large values of the B-integral. Experiments using BK7 glass and a KDP crystal as a nonlinear medium confirm the obtained theoretical results. This may be interesting for pulse post-compression, frequency doubling, and other experiments using transmission optical elements in ultra-high intensity lasers.

The use of the post-compression technique ensures gain in laser pulse peak power but at the same time degrades beam focusability due to the nonlinear wavefront distortions caused by a spatially nonuniform beam profile. In this paper a substantial focusability improvement of a post-compressed laser pulse by means of adaptive optics was demonstrated experimentally. The Strehl ratio increase from 0.16 to 0.43 was measured. Simulations showed that the peak intensity in this case reaches 0.52 of the theoretical limit.

The nonlinear nature of laser pulse post-compression inevitably leads to wavefront distortions. These distortions are nonstationary, i.e., they vary during the pulse, which significantly complicates the use of adaptive optics. We propose four approaches to compensation of wavefront distortions by means of standard adaptive mirrors, three of which can be implemented experimentally. Numerical simulations show that these approaches allow achieving a focal spot intensity close to the case of a perfectly flat wavefront.

Aims. EXor-type objects are protostars that display powerful UV-optical outbursts caused by intermittent and powerful events of magnetospheric accretion. These objects are not yet well investigated and are quite difficult to characterize. Several parameters, such as plasma stream velocities, characteristic densities, and temperatures, can be retrieved from present observations. As of yet, however, there is no information about the magnetic field values and the exact underlying accretion scenario is also under discussion. Methods. We use laboratory plasmas, created by a high power laser impacting a solid target or by a plasma gun injector, and make these plasmas propagate perpendicularly to a strong external magnetic field. The propagating plasmas are found to be well scaled to the presently inferred parameters of EXor-type accretion event, thus allowing us to study the behaviour of such episodic accretion processes in scaled conditions. Results. We propose a scenario of additional matter accretion in the equatorial plane, which claims to explain the increased accretion rates of the EXor objects, supported by the experimental demonstration of effective plasma propagation across the magnetic field. In particular, our laboratory investigation allows us to determine that the field strength in the accretion stream of EXor objects, in a position intermediate between the truncation radius and the stellar surface, should be of the order of 100 G. This, in turn, suggests a field strength of a few kilogausses on the stellar surface, which is similar to values inferred from observations of classical T Tauri stars.

Optics express 29(18), 28297 - (2021). doi:10.1364/OE.434216, The PEARL laser output pulse with a duration of 60-70 fs was compressed to 11 fs after passing through a 5-mm thick silica plate and reflecting from two chirping mirrors with a total dispersion of -250 fs2. The experiments were carried out for the B-integral values up to 19 without damage of the optical elements, which indicates that small-scale self-focusing was suppressed. The results obtained show the possibility of further nonlinear compression scaling to multipetawatt power in pulses with duration commensurate with the field period., Published by Soc., Washington, DC

The input pulse of the laser PEARL with energy of 18 J and pulse duration of about 60 fs was compressed to 10 fs after passage through a 4-mm-thick KDP crystal and reflection at two chirped mirrors with sum dispersion of −200 fs2. The experiments were performed for the В-integral values from 5 to 19 without visible damage to the optical elements, which indicates that small-scale self-focusing is not a significant issue. It was shown that, by virtue of the low dispersion of the group velocity, the KDP crystal has some advantages over silica: a larger pulse compression coefficient, especially at a small value of the В-integral (B = 5, …, 9), lower absolute values of chirped mirror dispersion, and also a possibility to control the magnitude of nonlinearity and dispersion by changing crystal orientation.

The pulse spectrum at the laser output was broadened due to self-phase modulation in fused silica and then the pulse was compressed by chirped mirrors. It was demonstrated that with an optimal choice of mirror dispersion a pulse with an energy of 17 J can be compressed from 70 to 14 fs. This compression after compressor approach has undoubted merits: simplicity, low cost, negligible pulse energy losses, and applicability to any high-power laser.

We demonstrated two-stage laser pulse compression via self-phase modulation and dispersion compensation by chirped mirrors. 250-TW-pulse was compressed from 70fs to 14fs. This technique may be applied to any high power laser.

A superbright X-ray source with a radiation temperature of ~1.2 keV making it possible to create a solid-state plasma whose kinetics is determined by the radiative processes has been implemented under the impact of a 170-TW pulse of the PEARL femtosecond laser facility on an aluminum target with submicron thickness. The diagnostics of the created plasma is performed by X-ray spectral methods using spectral transitions in hollow multicharged ions.

An original experimental stand is presented, aimed at studying the impact of high-energy protons, produced by the laser-plasma interaction at a petawatt power level, on biological objects. In the course of pilot experiments with the energy of laser-accelerated protons up to 25 MeV, the possibility is demonstrated of transferring doses up to 10 Gy to the object of study in a single shot with the magnetic separation of protons from parasitic X-ray radiation and fast electrons. The technique of irradiating the cell culture HeLa Kyoto and measuring the fraction of survived cells is developed. The ways of optimising the parameters of proton beams and the suitable methods of their separation with respect to energy and transporting to the studied living objects are discussed. The construction of the stand is intended for the improvement of laser technologies for hadron therapy of malignant neoplasms.

By extending the concept of thin film compression [Mourou et al., Eur. Phys. J. Spec. Top. 223(6), 1181 (2014)] to a thin plate, nonlinear post-compression from 24 fs to 13 fs of sub-petawatt laser pulses is demonstrated experimentally using a 1 mm-thick silica plate and chirped mirrors with a total anomalous dispersion of −50 fs2. The measurements were implemented with a specially designed dispersionless vacuum frequency-resolved optical gating, which is based on second harmonic generation of tested pulses in a 10 μm β-barium borate crystal glued on a 1 mm fused silica substrate. The used compression scheme is implemented in a geometry compatible with high power on-target experiment realization.

Spectral broadening of high-power transform-limited laser pulses under self-phase modulation in a medium with cubic nonlinearity is widely used to reduce pulse duration and to increase its power. It is shown that the cubic spectral phase of the initial pulse leads to a qualitatively different broadening of its spectrum: the spectrum has narrow peaks and broadening decreases. However, the use of chirped mirrors allows such pulses to be as effectively compressed as transform-limited pulses.

Using of Thin Film Compressor for shortening of PW laser pulses was investigated in experiments. Cascade quadratic nonlinearity in a single KDP crystal can be applied for control temporal parameters of the laser pulses.

Heating efficiently solid-density, or even compressed, matter has been a long-sought goal in order to allow investigation of the properties of such state of matter of interest for various domains, e.g. astrophysics. High-power lasers, pinches, and more recently Free-Electron-Lasers (FELs) have been used in this respect. Here we show that by using the high-power, high-contrast “PEARL” laser (Institute of Applied Physics-Russian Academy of Science, Nizhny Novgorod, Russia) delivering 7.5 J in a 60 fs laser pulse, such coupling can be efficiently obtained, resulting in heating of a slab of solid-density Al of 0.8 µm thickness at a temperature of 300 eV, and with minimal density gradients. The characterization of the target heating is achieved combining X-ray spectrometry and measurement of the protons accelerated from the Al slab. The measured heating conditions are consistent with a three-temperatures model that simulates resistive and collisional heating of the bulk induced by the hot electrons. Such effective laser energy deposition is achieved owing to the intrinsic high contrast of the laser which results from the Optical Parametric Chirped Pulse Amplification technology it is based on, allowing to attain high target temperatures in a very compact manner, e.g. in comparison with large-scale FEL facilities.

We report a method for ultrashort pulse reconstruction based only on the pulse spectrum and two self-phase modulated (SPM) spectra measured after pulse propagation through thin media with a Kerr nonlinearity. The advantage of this method is that it is a simple and very effective tool for characterization of complex signals. We have developed a new retrieval algorithm that was verified by reconstructing numerically generated fields, such as a complex electric field of double pulses and few-cycle pulses with noises, pedestals and dips down to zero spectral intensity, which is challenging for commonly used techniques. We have also demonstrated a single-shot implementation of the technique for the reconstruction of experimentally obtained pulses. This method can be used for high power laser systems operating in a single-shot mode in the optical, near- and mid-IR spectral ranges. The method is robust, low cost, stable to noise, does not require a priori information and has no ambiguity related to time direction.

Spectrum broadening by self-phase modulation (SPM) may be used for increasing PW-class laser pulse power. The influence of high-order spectral phase on SPM and compression of intensity pulses will be discussed. The results of compression of sub-PW pulses of the PEARL laser will be presented.

A theoretical model of second-harmonic generation (SHG) under strong influence of cubic nonlinearity was verified in experiment. Effective energy conversion in thin potassium dihydrogen phosphate crystals at peak intensity up to 5 TW/cm2 (B-integral equaled 6.4) was demonstrated and no crystal damage was observed. Comparative analysis of SHG of radiation at the fundamental wavelengths of 910 and 800 nm showed the major advantages of the first one. The double-pass geometry of SHG in an ultrathin crystal on a substrate is discussed in detail. Additional correction of parabolic spectral phase of the SH radiation allows pulse duration to be shortened from 20 to 9 fs for 910 nm fundamental wavelength and from 20 to 12 fs for 800 nm.

A highly efficient (73%) second-harmonic generation of femtosecond pulses in a 1-mm-thick KDP crystal at a fundamentalharmonic peak intensity of 2 TW cm{sup -2} has been demonstrated experimentally. In a 0.5-mm-thick KDP crystal, a 50% efficiency has been reached at a peak intensity of 3.5 TW cm{sup -2}. We examine the key factors that limit the conversion efficiency and present numerical simulation results on further temporal compression of second-harmonic pulses.

We use a PW-class PEARL facility to study fast electron beam generation during high intensity laser pulse interaction with a supersonic gas jet. We show that electron beams with several hundreds of MeV and relatively large charges, of hundreds of pC and more, can be effectively produced without any guiding structures. PIC simulations also confirm the obtained experimental data and provide optimized conditions of laser–plasma interaction for high-charged beam production.

Linearised equations for the amplitudes of harmonic perturbations of strong waves in quadratic and cubic nonlinear media are obtained within the model of plane monochromatic waves. The dependences of the gains of noise components of the first and second harmonic waves upon frequency doubling on the B integral are found. The maximally admissible noise level in the fundamental radiation beam is calculated by the example of a 0.5-mm-long KDP crystal at the peak intensity of 4.5 TW cm-2.

The paper is devoted to investigation of adaptive optical delay lines, intended for femtosecond pulse shaping. Experiments were performed in the sub PW laser system, based on optical parametrical chirped pulse amplification. The experimental device has revealed high spectral resolution (0.6 nm at 1250 nm) high efficiency, (70% over a 120 nm spectral width at 1250 nm) and flexibility of seed pulse amplitude control. Proper shaping of the transmission function of adaptive optical delay line allows to double the femtosecond pulse spectrum band width.

A third-order correlator with a single-shot time window and a full dynamic range of 108 is developed. The time contrast of radiation from the front-end system of a petawatt femtosecond laser complex measured with the correlator within time windows ±1 ps and ±100 ps was 104 and more than 108, respectively. Based on the theoretical analysis of the cross-correlator operation, a number of requirements providing the optimal functioning of the cross-correlator are found. The reasons restricting the technical parameters of the correlator are discussed.

560 TW peak power has been achieved experimentally using a Cr:forsterite master oscillator at 1250 nm, a stretcher, three optical parametrical amplifiers based on KD*P crystals providing 38 J energy in the chirped pulse at 910 nm central wavelength, and a vacuum compressor providing 43 fs pulse duration. To our knowledge, it is a world-record OPCPA system and one of the five most powerful laser systems currently available.

The ultra-broadband phase matching was experimentally observed in a DKDP crystal upon parametric amplification of signal radiation with a wavelength of 911 nm in a pump field with a wavelength of 527 nm. The original scheme was used to excite the first parametric amplification stage by chirped pulses of idler radiation with a wavelength of 1250 nm. The saturated gain of a three-stage parametric amplifier was equal to 108.

In experiments on the parametrical amplification of femtosecond pulses in wide-aperture DKDP crystals, a power of more than 100 TW has been reached, which is much higher than the record level achieved in such lasers. The energy efficiency obtained for the parametric amplifier is equal to 27%. The energy of a 72-fs pulse is equal to 10 J.

The results of laser wakefield acceleration experimental series carried out at PEARL (PEtawatt pArametrical Laser) system are discussed in the paper. The electron beams with energies up to 300 MeV were observed.

Method of small-scale self-focusing suppression at intense laser beams (1÷4TW/cm2) was developed and verified in experiments successfully. The theoretical model of plane wave instability in mediums with quadratic and cubic nonlinearity was created.

High intensity contrast ratio (ICR) is required for majority of experiments with strong-field radiation. The demands to temporal intensity profile are determined by task of application of super strong radiation. As a rule, the experiments require ICR as higher as possible. SHG process of powerful femtosecond radiation is known to be an excellent approach to significant enhancement of a temporal intensity profile. The peak power of output radiation of modern laser complexes exceeds the petawatt level (Aoyama, Yamakawa et al. 2003; Lozhkarev, Freidman et al. 2007; Liang, Leng et al. 2007 ; Yanovsky, Chvykov et al. 2008 ) and power density in the radiation is in a range of 10TW/cm2. At such high intensities cubic polarization of frequency doubling media is needed to be taken into consideration. The polarization leads to nonlinear phase accumulation of interacted waves (Razumikhina, Telegin et al. 1984; Choe, Banerjee et al. 1991; Chien, Korn et al. 1995; Ditmire, Rubenchik et al. 1996; Mironov, Lozhkarev et al. 2009) and small-scale self-focusing (SSSF) appearance (Bespalov and Talanov 1966; Rozanov and Smirnov 1980; Lowdermilk and Milam 1981; Kochetkova, Martyanov et al. 2009; Poteomkin, Martyanov et al. 2009). The present research is devoted to demonstration of a possibility of SHG process implementation for ICR enhancement. In the chapter, we present theoretical and experimental results of SHG of output radiation of petawatt level femtosecond laser (Lozhkarev, Freidman et al. 2007). The influence of cubic polarization and dispersion effects are taken into account in the theoretical model. The coincidence of experimental results and the observed theoretical model are thoroughly discussed in section 2. Numerical simulation of SHG process has made it possible to demonstrate the possibility of ICR enhancement. We suggested a technique of additional pulse compression after SHG process. Results of the theoretical explorations are presented in section 3. The model of linear stage of plane wave instability in media with quadratic and cubic nonlinearity is developed. The model is the generalization of the classical theory of beam

The femtosecond pulse quality in a laser system is determined by a non-compensated high-order dispersion and spectral deformation of the amplifier. A dispersive device that modifies the spectral amplitude and phase of the femtosecond pulse should be used to improve the duration of the compressed pulse and suppress the prepulses. The application of acousto-optical dispersive delay lines is considered. Different modifications of delay lines are compared and the types of devices used to shape femtosecond pulses are discussed. Several experimental dispersive delay lines were fabricated and investigated.

In this paper we discuss laser wake field acceleration experiments made at the PEtawatt pARametric Laser (PEARL). Using free 2mm and 5 mm gas jet without preplasma we generated electron beam with energy up to 260 MeV. The charge of quasimonoenergetic beams achieved 300 pC, angular divergence 0.2 degree. The special attention is paid for diagnostics which is adapted for low repetition rate systems with low output parameters stability.

Theoretical investigation of second harmonic generation (SHG) of super intense femtosecond radiation demonstrated that temporal Intensity Contrast Ratio (ICR) of output radiation of petawatt level laser complexes can be significantly increased. The cubic polarization effects in the process give possibility of additional pulse compression. We present experimental results of SHG of radiation with average intensity 2TW/cm 2 in 0.6mm KDP crystal. Theoretical model of linear regime of plane wave instability in mediums with quadratic and cubic nonlinearity is developed and thoroughly discussed. Analysis of small-scale self-focusing suppression methodic is presented. The influence of surface dust to spatial noises generation in SHG process is pointed out.

The created theory of second harmonic generation under strong influence of cubic polarization was experimentally verified. The 35% energy conversion efficiency of second harmonic generation in 0.6 mm KDP crystal at input average intensity 2TW/cm2 was experimentally achieved. No optical breakdown was observed in 1 mm KDP crystal at peak intensity 5TW/cm2 (B‐integral is 6) close to output intensity of multi‐petawatt lasers including Extreme Light Infrastructure (ELI) project. The comparable analyze of second harmonic generation of radiations with fundamental wave lengths 910 nm and 800 nm evidently shows the major advantage of the first one. The double‐pass scheme of second harmonic generation may be useful especially for 800 nm fundamental wave length.

200 TW peak power has been achieved experimentally using a Cr:forsterite master oscillator at 1250 nm, a stretcher, three optical parametrical amplifiers based on KD*P (DKDP) crystals providing 14.5 J energy in the chirped pulse at 910 nm central wavelength, and a vacuum compressor. The final parametrical amplifier and the compressor are described in detail. Scaling of such architecture to multipetawatt power is discussed.

Experiments on electron and ion acceleration using powerful laser radiation have been successfully carried out lately. A possibility to get high electron energies amounting hundreds of MeV and even GeV, the electron energy spread being small and the angular spectrum being narrow, makes laser-driven accelerators interesting and promising for many applications in atomic physics, medicine, etc.

In spite of the fact that the petawatt level has been attained quite a time ago [ 1 ], only a few petawatt facilities are available worldwide. One of them, a 056 PW OPCPA facility, was created at the Institute of Applied Physics in 2006 [ 2 ]. The main OPCPA merit is a possibility of power scaling. In the current work we report construction of still another Russian OPCPA-based laser complex.