S. G. Bochkarev

Ion acceleration in expanding multispecies plasmas

The acceleration of light and heavy ions in an expanding plasma slab with hot electrons produced by an intense and short laser pulse is studied by using the hybrid Boltzmann–Vlasov–Poisson model. Spatial profiles, energy distributions, and maximum energies of accelerated ions are analyzed in function of the plasma and hot electron parameters. The crucial parameter for ion acceleration is found to be the ratio of the foil thickness to the hot electron Debye length. Special attention is paid to characterization of protons accelerated from a thin hydrogenated layer at the target surface. The evolution of the proton spectrum is studied for the cases of isothermal and cooling hot electron distributions. The obtained dependencies of the ion energy on the pulse duration and the target characteristics allow one to define the optimal conditions for the ion acceleration with lasers.

Ion acceleration in short-laser-pulse interaction with solid foils

We discuss the physical processes, which take place in a multi-component plasma set in expansion by a minority of energetic electrons. The expansion is in the form of a collisionless rarefaction wave associated with three types of electrostatic shocks. Each shock manifests itself in a potential jump and in the spatial separation of plasma species. The shock front associated with the proton–electron separation sets the maximum proton velocity. Two other shocks are due to the hot–cold electron separation and the light–heavy ion separation. They result in the light ion acceleration and their accumulation in the phase space. These structures open possibilities for control of the number and the energy spectrum of accelerated ions. Simple analytical models are confirmed in numerical simulations where the ions are described kinetically, and the electrons assume the Boltzmann distribution.

Investigation of ion acceleration in an expanding laser plasma by using a hybrid boltzmann-vlasov-poisson model

The acceleration of ions of different species from a plasma slab under the action of a charge-separation electric field driven by hot and cold electrons is studied by using a hybrid Boltzmann-Vlasov-Poisson model. The obtained spatial and energy distributions of light and heavy ions in different charge states demonstrate that the model can be efficiently used to study the ion composition in a multispecies expanding laser plasma. The regular features of the acceleration of ions of different species are investigated. The formation of compression and rarefaction waves in the halo of light ion impurity, as well as their effect on the energy spectrum of the accelerated ions, is analyzed. An approach is proposed that makes it possible to describe the production of fast ions by laser pulses of a given shape. It is shown that the energy of fast ions can be increased markedly by appropriately shaping the pulse. The effect of heating of the bulk of the cold target electrons on the ion acceleration is discussed.

Vacuum electron acceleration by a tightly focused, radially polarized, relativistically strong laser pulse

A test particle approach is used to solve the problem of direct electron acceleration by a short, intense, radially polarized laser pulse the focal spot diameter of which can be on the order of the laser wavelength. The fields of a tightly focused laser beam are described in terms of the Stratton-Chu integrals, with which to investigate electron acceleration when the paraxial approximation is inapplicable to laser fields. The dynamics of electron motion in a radially polarized, relativistically strong laser field is analyzed depending on the initial position of an electron in the focal region of the laser beam. The properties of the generated jets of accelerated electrons are investigated depending on the tightness of laser pulse focusing. Possible advantages of using radially polarized laser pulses for charged particle acceleration, as opposed to the use of linearly polarized ones, are discussed.

Nonequilibrium electron distribution functions and nonlinear thermal transport

Quasi-self-similar solutions to the stationary electron kinetic equation in diffusive approximation have been found in an inhomogeneous plasma. Electron density and temperature corresponding to these solutions satisfy a steady state plasma profile that is defined by nTa=const (a>1). The derived electron distribution functions describe particle transport, in particular thermal conduction and ambiporal electric field for the arbitrary amplitude of temperature perturbation in the wide range of particle collisionality. The quasi-self-similar solutions display enhanced algebraic tails in the isotropic part and the reduced number of energetic electrons in the anisotropic part of electron distribution functions. The quasi-self-similar theory of electron kinetics is applied to laser plasma heating and heat transport into the overdense region. Calculations of the linear Landau damping rate, growth rate of the return current instability, and dynamical form factor are presented.

Stochastic electron acceleration in plasma waves driven by a high-power subpicosecond laser pulse

The mechanism of stochastic electron acceleration and heating by a picosecond laser pulse in underdense plasma is studied using particle-in-cell simulations and theoretical models. The formation of wide electron energy spectra in the simultaneously acting laser and plasma fields is analyzed. It is shown that electron scattering by turbulent plasma fluctuations excited through stimulated forward Raman scattering plays a governing role in the formation of high-energy tails in the electron distribution function.

Nonlinear Thomson scattering of a relativistically strong tightly focused ultrashort laser pulse

The problem of nonlinear Thomson scattering of a relativistically strong linearly polarized ultrashort laser pulse tightly focused into a spot with a diameter of DF ≳ λ (where λ is the laser wavelength) is solved. The energy, spectral, and angular distributions of radiation generated due to Thomson scattering from test electrons located in the focal region are found. The characteristics of scattered radiation are studied as functions of the tightness of laser focusing and the initial position of test particles relative to the center of the focal region for a given laser pulse energy. It is demonstrated that the ultratight focusing is not optimal for obtaining the brightest and hardest source of secondary electromagnetic radiation. The hardest and shortest radiation pulse is generated when the beam waist diameter is ≃10λ.

On the theory of nonlinear Thomson scattering of a highly focused laser pulse

The problem of nonlinear Thomson scattering of an ultrashort laser pulse on free electrons from a sharp focus is solved within the test particle approach. To describe the laser beam fields, an exact solution to the Helmholtz equation is used, which allows simulation of the electron dynamics and emission when the paraxial approximation is certainly inapplicable. Characteristics of attosecond electromagnetic pulses of secondary emission from test electrons during their motion near the laser focus are studied.

Nonthermal tails of the electron distribution functions with nonlocal transport

Quasi-self-similar solutions to the stationary electron Fokker-Planck equation in diffusive approximation have been found in inhomogeneous plasma. These solutions describe reduction in the number of bulk electrons and formation of the suprathermal tail. The characteristics of the stationary electron distributions have been treated in terms of the collisionality parameter, the ratio of the electron stoping rage to the plasma gradient scale length. The dependencies of the electron distribution functions on density profile has been studied. Fokker-Plank simulations performed demonstrate good agreement with a theory.

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.