Monitoring the internal temperature of active elements (AE) of high-power lasers is necessary for their safe operation. The article describes a method and device for monitoring the internal temperature of the AE of lasers. The measurements utilize pulsed ultrasonic (US) probing and the temperature dependence of the sound speed in the AE material. A change in the sound speed leads to a change in the phase of the ultrasonic signal passing through the object, which is recorded by the described device. The results of monitoring the AE temperature using ultrasonic probing during operation of an laser device are presented.

Контроль внутренней температуры активных элементов (АЭ) мощных лазеров необходим для их безопасной работы. В статье описана методика и устройство для мониторинга внутренней температуры АЭ лазеров. Измерения основаны на импульсном ультразвуковом (УЗ) зондировании и зависимости от температуры скорости звука в материале АЭ. Изменение скорости звука приводит к изменению фазы УЗ сигнала, прошедшего через объект, которое регистрируется описываемым устройством. Представлены результаты мониторинга температуры АЭ с помощью ультразвукового зондирования в процессе работы действующей лазерной установки.

The goal of the XCELS (eXawatt Center for Extreme Light Studies) project is to establish a large-scale scientific infrastructure based on laser sources with gigantic peak power. A brief description of the laser system and the radiation focusing system from different channels, taking into account their synchronization, is presented. The program of possible experiments at the XCELS facility is discussed, including experiments to test strong-field quantum electrodynamics, particle acceleration, radiation generation in less accessible regions of the electromagnetic spectrum, laboratory astrophysics, high-energy-density processes, extreme light diagnostics, and other applications.

We present the first experimental results obtained with a setup created on the basis of the PEARL laser facility for studying the processes of generating terahertz radiation from laser wake fields which are formed during the propagation of a high-power femtosecond laser pulse in a rarefied plasma. In particular, the occurrence of terahertz generation in the case where the laser–plasma interaction region is located between a pair of dielectric prisms of total internal reflection is demonstrated. The dependence of the terahertz radiation energy on the energy of a femtosecond laser pulse and on the plasma density is studied.

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.

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.

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.

Laser wakefield acceleration of electrons is studied experimentally in the strongly mismatched regime. Up to 20 J, 60 fs laser pulses are focussed at the input of a gas cell with thef/46 focussing system. The resulting 44 mu m focal spot at the cell entrance is about two times greater than the matched spot size. Experimental results are supported by 3D particle-in-cell (PIC) simulations of the laser-plasma dynamics as well as by numerical simulations of the gas distribution inside the gas cell. Special attention is paid to the accuracy of spectra reconstruction affected by low pointing stability of the accelerated electron beam. It is demonstrated experimentally that the electron energy in the mismatched regime can be higher than that in the matched one and can reach GeV level for the used laser parameters.

Compression of a 12-J laser pulse from 63 to 21 fs is experimentally demonstrated for a beam 18 cm in diameter. The compression is implemented for a pulse freely propagating in glass under conditions of self-phase modulation and subsequent compensation for dispersion under reflection front dispersion mirrors. This simple and inexpensive technique for increasing multiply the pulse power is characterised by almost 100% energy efficiency and can be used at the output of ally ultra-high-power laser, without any changes in its optical scheme.

We propose a new method for compensating the thermally induced depolarization of laser radiation, which is based on using spatial light modulators. It is shown that using one modulator, the integral depolarization degree can be reduced by several times, whereas two modulators make it possible to almost completely rule out depolarization, regardless of its nature. The phase front of the laser beam is analyzed and it is shown that this phase front remains qualitatively unchanged when depolarization is compensated by one modulator, while in the case of compensation by two modulators the phase nonuniformity decreases, which leads to an increase in the far-field intensity.

Generation of a second giant pulse in a longitudinal mode neighbouring to the longitudinal mode possessing minimal losses is theoretically and experimentally studied in actively Q-switched lasers. A mathematical model is suggested for explaining the giant pulse generation in a laser with multiple longitudinal modes. The model makes allowance for not only a standing, but also a running wave for each cavity mode. Results of numerical simulation and data of experiments with a Nd : YLF laser explain the effect of second giant pulse generation in a neighbouring longitudinal mode. After a giant pulse in the mode with minimal losses is generated, the threshold for the neighbouring longitudinal mode is still exceeded due to the effect of burning holes in the population inversion spatial distribution.

We measured the dependence of the weak signal gain of 100 mm diameter rod amplifiers on pump energy and transverse coordinates for four neodymium glass grades. The highest gain was obtained in N31-05 (China), and the highest radial gain uniformity in KGSS-0180 glass (Russia). The data obtained enable the optimal glass grade to be chosen for each specific problem by finding a compromise between maximum output energy and minimum distortion of beam profile.

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.

The energy of two orthogonally polarised pulses injected into an available multistage amplifier based on neodymium phosphate glass rods was increased from 300 to (in both pulses). The second output pulse with an energy of will be used to pump an additional parametric amplifier of a petawatt laser.

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 , the possibility is demonstrated of transferring doses up to 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.

A unique large-aperture neodymium glass rod amplifier is experimentally studied. The small-signal gain distribution is measured at different pump energies. The aperture-averaged gain is found to be 2.3. The stored energy (500 J), the maximum possible pump pulse repetition rate, and the depolarisation in a single pulse and in a series of pulses with a repetition rate of one pulse per five minutes are calculated based on the investigations performed. It is shown that the use of this amplifier at the exit of the existing laser can increase the output pulse energy from 300 to 600 J.

We report possible employment of short spatial filters based on spherical lenses in a pulsed laser source (neodymium glass, 300 J, 1 ns). The influence of the spherical aberration on the quality of output radiation and coefficient of conversion to the second harmonics is studied. The ultra-short aberration spatial filter of length 1.9 m with an aperture of 122 mm is experimentally tested. A considerable shortening of multi-cascade pump lasers for modern petawatt laser systems is demonstrated by the employment of short spatial filters without expensive aspherical optics.

Amplifiers based on neodymium phosphate glass rods 60 - 100 mm in diameter are experimentally studied. The amplifiers are pumped by INP-16/250 tubular flash lamps placed in a universal pump cavity with a two-section mirror reflector. A compact high-voltage capacitive energy storage with a preionisation circuit was developed to supply the lamps.