Table of contents

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 $25\ {\text{MeV}}$, the possibility is demonstrated of transferring doses up to $10\ {\text{Gy}}$ 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.

Laser wakefield acceleration of electron bunches possessing various initial injection energies in capillary waveguides is studied in the conditions of an asymmetric input of laser radiation into a waveguide (the propagation direction of laser radiation deviates from the capillary axis or the laser spot is not symmetric). The factors determining the critical angle of the laser radiation input into the capillary, within which the wake acceleration of electrons is close to optimal, have been found. It is shown that in acceleration stages where electron energies are high, the requirements to angular concentricity of the capillary axis and laser radiation focusing are substantially weaker due to the relativistic 'weighting' of the electron mass.

An analytical model of a plasma bubble (a wake wave in the strongly nonlinear regime) in transversely inhomogeneous plasma is generalised to an arbitrary profile of an electron sheath at its boundary. Within the framework of this generalisation we have found the potential within the bubble and shown that its envelope is described by a second-order equation, similar to the equation of a less general theory. We have also determined the domain of parameters at which this equation is considerably simplified and no longer depends on the profile of the electron sheath.

We have carried out numerical simulations of oblique incidence of a laser pulse with an intensity of $\boldsymbol{I} = 1.33 \times 10^{23}\ {\text{W cm}}^{-2}$ on a planar plasma layer and found the plasma density and the angle of incidence of p-polarised laser pulses that correspond to the highest gamma-ray generation efficiency and high gamma-ray directivity. The shape of the plasma surface has been determined by simulation and conditions have been considered that lead to an increase in generation efficiency.

Quantum-electrodynamic cascade generation is numerically simulated for the case of the oblique collision of a beam of ultrarelativistic electrons with the field of two counterpropagating, focused, circularly polarised laser pulses. It is shown that although the 'collapse and revival' effect is observed at any value of the collision angle, the multiplicity of the cascade essentially depends on this angle and is maximal in the configuration, when the electron beam hits the focus perpendicularly to the optical axis of the laser pulses.

Quantum electrodynamic cascades in intense electromagnetic fields arise when the proper electron acceleration $\chi$, expressed in Compton units, can attain values greater than or on the order of unity. For times $\boldsymbol{t} \ll 1/ \omega$, where $\omega$ is the carrier frequency of the field, we have derived a general formula for $\chi$ of an initially resting electron in an arbitrary electromagnetic field. Using this formula, we have found an optimal configuration of colliding laser pulses, which provides a significant reduction in the threshold intensity of occurrence of cascades up to a level of $\sim 10^{23}\ {\text{W cm}}^{-2}$.

A coherent X-ray pulse of attosecond duration can be formed in the reflection of a counterpropagating laser pulse from a relativistic electron mirror. The reflection of a high-amplitude laser pulse from the relativistic electron mirror located in the field of an accelerating laser pulse is investigated by means of two-dimensional (2D) numerical simulation. It is shown that provided the amplitude of the counterpropagating laser pulse is several times greater than the amplitude of the accelerating laser pulse, the reflection process is highly nonlinear, which causes a significant change in the X-ray pulse shape and its shortening up to generation of quasi-unipolar pulses and single-cycle pulses. A physical mechanism responsible for this nonlinearity of the reflection process is explained, and the parameters of the reflected X-ray pulses are determined. It is shown that the duration of these pulses may constitute 50 – 60 as, while their amplitude may be sub-relativistic.

The problem of propagation of an ultraintense pulse in a gas medium is solved by using a 3D nonadiabatic model extended to the case of multiple ionisation of atoms in the interaction region. A complex space – time coupling of the pulse during its propagation and strong spatial and temporal changes in the field distribution are shown.

Surfaces covered with nanostructures, such as nanowire arrays, are shown to facilitate a significantly higher absorption of laser energy as compared to flat surfaces. Due to the efficient coupling of the laser energy, highly energetic electrons are produced, which in turn can emit intense ultrafast X-ray pulses. Full three dimensional PIC simulations are used to analyse the behaviour of arrays of carbon nanowires $400\ {\text{nm}}$ in diameter, irradiated by a $400\hbox{-}{\text{nm}}$ laser pulse of $60\hbox{-}{\text{fs}}$ duration at FWHM and a vector potential of $\boldsymbol{a}_{0} = 18$. We analyse the ionisation dynamics of the nanowires. The difference of the ionisation strength and structure between linearly and circularly polarised laser beam is investigated. The nanowires are found to be fully ionised after about 30 laser cycles. Circularly polarised light reveals a slightly stronger ionisation effect.

The electron density in the plasma channel of a femtosecond filament in air at pressures from 1 to $7\ {\text{atm}}$ is measured at different instants, starting from the ionisation onset and up to several hundreds of picoseconds after it. The initial electron density is found to increase sharply in the pressure range of $3 {-} 4\ {\text{atm}}$. The plasma channel diameter is found to decrease with an increase in pressure from 3 to $7\ {\text{atm}}$.

We have studied the nonlinear optical properties and supercontinuum spectrum of solutions of carbon quantum dots prepared by a hydrothermal process from chitin and then coated with titania. The titania coating has been shown to have an activating effect on the carbon quantum dots, enhancing supercontinuum generation in the blue-violet spectral region and enabling their nonlinear optical characteristics to be varied.

It is shown that the production of X-ray emission spectra in the interaction of high-intensity laser radiation with cluster targets may be affected by the bichromatic oscillating electric field arising from the generation of the second harmonic of laser radiation. A technique is proposed for diagnosing harmonic generation in laser – cluster interactions using the spectral line profiles of multiply charged helium ions. The efficiency of second harmonic generation at a laser intensity of $3 \times 10^{18}\ {\text{W cm}}^{-2}$ is shown to amount to about 2 %.

Different regimes of electron acceleration from solid foils and low-density targets are investigated using three-dimensional numerical simulations. The size of the plasma corona is shown to be the main parameter characterising the temperature and number of hot electrons, which determine the yield of X-ray radiation and its hardness. Also studied is the generation of X-ray radiation by laser-accelerated electrons, which bombard the converter target located behind the laser target.

We consider an X-ray optical system which permits obtaining laser plume images at a wavelength of $13.5\ {\text{nm}}$ with a resolution of up to $70\ {\text{nm}}$. The X-ray optical system comprises an X-ray Schwarzschild objective made up of two aspherical multilayer mirrors, a scintillator (YAG : Ce ceramics), which converts X-rays to the visible radiation, and a visible-optical system, which images the scintillator surface onto a CCD camera. The spatial resolution of the system is limited by the resolution of the optical system $(0.7\ \unicode{956}{\text{m}})$ and the magnification $(10^{\times})$ of the X-ray objective and is as high as $70\ {\text{nm}}$. The effect of Schwarzschild objective mirror shapes on the spatial resolution is analysed. The profile of concave mirror aspherisation is considered, which provides the attainment of the diffraction-limited quality of the objective. Data are given for the quantum efficiency of the system at a wavelength of $13.5\ {\text{nm}}$. We describe the experimental test bench intended for studying the developed X-ray optical system and outline the first experimental data which illustrate its efficiency. Owing to the natural division into the 'X-ray' and 'visible' parts, the optical system under discussion permits an easy change of the magnification and the field of view without realigning the X-ray optical elements. The wavelength may be varied in a range between 3 and $40\ {\text{nm}}$ by changing the multilayer mirrors.

We study theoretically the process of turning a laser pulse into a train of attosecond or even zeptosecond pulses due to high harmonic generation (HHG) upon backreflection of intense laser radiation from a plasma surface. It is shown that under appropriate conditions these attosecond pulses may have an amplitude that is several orders of magnitude larger than that of the laser pulse. We study this process in detail, especially the nanobunching of the plasma electron density. We derive the analytical expression that describes the electron density profile and obtain a good agreement with particle-in-cell simulations. We investigate the most efficient case of HHG at a moderate laser intensity (normalised vector potential $\boldsymbol{a}_{0} = 10$) on the overdense plasma slab with an exponential pre-plasma profile. Subsequently we calculate the spectra of single attosecond pulses from back radiation using our expression for density shape in combination with the equation for spectrum of nanobunch radiation.

We have studied spectra of above-threshold ionisation of atoms by a two-colour laser field with collinear linearly polarised components. We have found a sharp (gap-like) dependence of the length of the high-energy plateau in above-threshold ionisation spectra on the relative phase of the two-colour field at comparable intensities of the field components. Using the quasi-classical analysis we have shown that this effect results from the suppression of partial above-threshold ionisation amplitudes, associated with closed classical trajectories of an electron in the laser field, within a certain range of relative phase values.

Having solved numerically the time-dependent Schrödinger equation, we have analysed the dependence of the high harmonic generation yield on the ellipticity of an intense laser field. For the case of a zero angular momentum of an initial state, it has been shown that the ellipticity dependence of the HHG yield is affected by the harmonic number. The numerical results are interpreted in the framework of our recently developed quasi-classical analytical model for HHG. In the quasi-classical approximation, the difference in the ellipticity dependence of the HHG yield for different harmonics is shown to be caused by the interference effects of quantum orbits.

The energy of two orthogonally polarised pulses injected into an available multistage amplifier based on neodymium phosphate glass rods was increased from 300 to $500\ {\text{J}}$ (in both pulses). The second output pulse with an energy of $200\ {\text{J}}$ will be used to pump an additional parametric amplifier of a petawatt laser.

High (more than ten times) small-signal gain is demonstrated in an Yb : YAG single crystal thin rods with pumping by a fibre-coupled diode laser. A four-pass amplifier for a fibre master oscillator with an average output power exceeding $15\ {\text{W}}$ at a pulse repetition rate of $3\ {\text{MHz}}$ is fabricated based on this active element. The small-signal gain in the developed amplifier is $26\ {\text{dB}}$, which is comparable with the gain in regenerative amplifiers. The possibility of obtaining sub-millijoule pulse energy at a pulse repetition rate of tens of kilohertz is shown.

A colour-centre structure formed in a LiF crystal under filamentation of a femtosecond mid-IR laser pulse with a power slightly exceeding the critical power for self-focusing has been experimentally and theoretically investigated. Strictly periodic oscillations have been detected for the first time for the density of the colour centres induced in an isotropic LiF crystal under filamentation of a laser beam with a wavelength tuned in the range from 2600 to $3350\ {\text{nm}}$. The structure period is found to be about $30\ \unicode{956}{\text{m}}$. With an increase in the laser radiation wavelength, the period of the oscillations decreases and their amplitude increases. The maximum colour centre density, observed under filamentation of a $3100\hbox{-}{\text{nm}}$ beam, is related to the increased contribution of the direct generation of colour centres as a result of the absorption of an integer number of photons by the exciton band. It is numerically shown that the formation of a periodic colour-centre structure in LiF is due to the periodic change in the light field amplitude in the light bullet (1.5 optical periods long) formed under filamentation.

We have investigated a new fibre laser configuration for the generation of ultrashort pulses at a repetition rate far exceeding the fundamental cavity frequency. The laser configuration includes a nonlinear amplifying mirror as an artificial saturable absorber for mode locking and a spectral comb filter for pulse separation stabilisation. Generation of trains and sequences of ultrashort pulses at a repetition rate tunable in the range $8 {-} 200\ {\text{GHz}}$ has been demonstrated experimentally. The pulses generated by the laser have been shown to retain an ordered, equidistant structure on a nanosecond timescale.