K. I. Popov

Laser triggered Coulomb explosion of nanoscale symmetric targets

Approximate kinetic solutions are found for Coulomb explosions of nanostructures in plane (thin foils), cylindrical (nanowires/tubes) and spherical (nanospheres/shells) geometries. The distribution function, mean velocity, density distribution, as well as the energy spectra for accelerated ions are derived. The proposed kinetic model describes physics of multiple flows that occur during Coulomb explosion. Comparison with particle-in-cell numerical simulations shows good agreement with analytical results.

Electron vacuum acceleration by a tightly focused laser pulse

Interaction of a tightly focused relativistic short laser pulse with electrons beyond the paraxial approximation is studied by using the test particle approach and the Stratton–Chu integrals for the laser fields. The investigation addresses the regime where the focal spot size may be comparable to the laser wavelength. The mapping of near focal positions of the electrons to energy space allows for the study of important features of electron vacuum acceleration by a tightly focused laser pulse and to choose optimal parameters for acceleration. Two acceleration mechanisms have been observed and discussed. Directional properties of the accelerated electrons have been analyzed.

Temperature scaling of hot electrons produced by a tightly focused relativistic-intensity laser at 0.5 kHz repetition rate

The energy spectrum of hot electrons emitted from the interaction of a relativistically intense laser with an Al plasma is measured at a repetition rate of 0.5 kHz by accumulating ∼103 highly reproducible laser shots. In the 1017–2×1018 W/cm2 range, the temperature of electrons escaping the plasma along the specular direction scales as (Iλ2)0.64±0.05 for p-polarized pulses incident at 45°. This scaling is in good agreement with three-dimensional particle-in-cell simulations and a simple model that estimates the hot-electron temperature by considering the balance between the deposited laser intensity and the energy carried away by those electrons.

Vacuum electron acceleration by tightly focused laser pulses with nanoscale targets

Electron acceleration using a tightly focused relativistic short laser pulse interacting with a spherical nanocluster, ultrathin foil or preformed mid-dense plasmas is studied by using three-dimensional particle-in-cell simulations with the Stratton–Chu integrals as the boundary conditions for the incident laser fields. The investigation is performed in the regime where the focal spot size is comparable with the laser wavelength. Generation of high-energy electron multibunch jets with quasimonoenergetic or waterbaglike spectra has been demonstrated. The physical process of acceleration and bunching of the electrons is discussed in detail, as well as particles energy and angular distributions for different laser intensities, focusing optics, target parameters, and laser incidence angles.

Ensemble of ultra-high intensity attosecond pulses from laser–plasma interaction

We report on the realistic scheme of intense X-rays and γ-radiation generation in a laser interaction with thin foils. It is based on the relativistic mirror concept, i.e., a flying thin plasma slab interacts with a counterpropagating laser pulse, reflecting part of it in the form of an intense ultra-short electromagnetic pulse having an up-shifted frequency. A series of relativistic mirrors is generated in the interaction of the intense laser with a thin foil target as the pulse tears off and accelerates thin electron layers. A counterpropagating pulse is reflected by these flying layers in the form of a swarm of ultra-short pulses resulting in a significant energy gain of the reflected radiation due to the momentum transfer from flying layers.

A detailed study of collisionless explosion of single- and two-ion-species spherical nanoplasmas

The collisionless adiabatic expansion into vacuum of spherical plasma targets (clusters) composed of cold single- or multispecies ions and hot electrons is studied kinetically by numerical solving of the nonrelativistic equations of motion of plasma particles in the self-consistent electrostatic field. The expansion dynamics for the whole range of electron temperatures from much less than to much higher than the cluster Coulomb energy is described for various initial plasma density profiles and cluster structures. The explosion of two ion species heterogeneous (layered) and homogeneously mixed targets is studied in detail for the wide range of light ion concentration and kinematic parameter.

Ion energy spectra directly measured in the interaction volume of intense laser pulses with clustered plasma

The use of gas cluster media as a target for an intense femtosecond laser pulses is considered to be uniquely convenient approach for the development of a compact versatile pulsed source of ionizing radiation. Also, one may consider cluster media as a nanolab to investigate fundamental issues of intense optical fields interaction with sub-wavelength scale structures. However, conventional diagnostic methods fail to register highly charged ion states from a cluster plasma because of strong recombination in the ambient gas. In the paper we introduce high-resolution X-ray spectroscopy method allowing to study energy spectra of highly charged ions created in the area of most intense laser radiation. The emission of CO2 clusters were analyzed in experiments with 60 fs 780 nm laser pulses of 1018 W/cm2 intensity. Theory and according X-ray spectra modeling allows to reveal the energy spectra and yield of highly charged oxygen ions. It was found that while the laser of fundamental frequency creates commonly expected monotonic ion energy spectrum, frequency doubled laser radiation initiates energy spectra featuring of distinctive quasi-monoenergetic peaks. The later would provide definite advantage in further development of laser-plasma based compact ion accelerators.

High-energy protons from submicron-sized targets

Improving of intensity contrast ratio of intense short laser pulses is making it possible to use submicron-sized targets, both spherical and plane, in the interest of proton acceleration for different applications. The way of improving of the ion beam quality is utilization of targets with two ion species - heavy ions (majority) and light ions, e.g. protons, (minority). Two different approaches, analytical theory and particle-in-cell simulations (PIC) are presented for studying the characteristics of laser-triggered ions due to the Coulomb-like mechanism of particle acceleration from submicron-sized targets. The comparative analysis of explosions of heterogeneous (layered) and homogeneously mixed targets for production of best quality ion bunches has been performed. We also found the regime of anisotropic proton acceleration from spherical targets with light and heavy ions relevant to the experiments with submicron-diameter droplets from water spray target irradiated by an ultrashort intense laser pulse.

Direct acceleration of electrons by radially polarized laser pulse

Direct electron acceleration by highly focused ultrahigh-power laser pulses of radial polarization in the ultrarelativistic mode was studied. The mode at which the focusing spot size appears of the same order as the laser radiation wavelength was considered. Electromagnetic fields were calculated using exact Stratton-Chu diffraction integrals. Calculations showed that, as for the case of linear polarization, too sharp focusing (in the diffraction limit) is not optimum for electron acceleration, despite the strong axial field namely in the case of a submicrometer laser spot. At the same time, the case of moderate focusing is more attractive for electron acceleration.