A suppression of light-ion acceleration (from surface water contaminants) was observed when a moderate-intensity subpicosecond laser pulse was focused on a thick metal target. Simultaneously, an effective generation of high-energy multicharge ions of the target material (Fe) was experimentally observed. A numerical simulation based on the Boltzmann–Vlasov–Poisson model revealed that this is due to the very specific regime of cleaning contaminants from the target surface by the short weak prepulse preceding the main pulse by more than 10 ns and having an intensity below the surface breakdown threshold. Because this prepulse causes the contaminant layer to boil explosively, a low-density gap forms above the target surface. These conditions are consequently favorable for boosting the energy of heavy ions.

В подходе пробных частиц решена задача о прямом ускорении электронов коротким интенсивным лазерным импульсом радиальной поляризации для случая, когда размер фокального пятна может быть порядка длины волны лазера. Для описания полей остросфокусированного лазерного пучка использованы интегралы Стрэттона-Чу, позволяющие исследовать ускорение электронов в условиях, когда параксиальное приближение для лазерных полей неприменимо. Проанализирована динамика движения электронов в релятивистскисильном радиально поляризованном лазерном поле в зависимости от начального положения частиц в фокальной области лазерного пучка. Также исследованы свойства возникающих струй ускоренных электронов в зависимости от остроты фокусировки лазерного импульса. Обсуждаются преимущества возможного использования лазерных импульсов радиальной поляризации по сравнению со случаем линейной поляризации лазерного излучения для ускорения заряженных частиц.

We study ultrashort-laser-pulse absorption and plasma heating at a sharp plasma–vacuum interface using advanced models for all-range plasma permittivity and nonlocal heat transport. The electron response includes both collisional and collisionless dissipative effects in the plasma of an arbitrary ion charge. We show that nonlocal electron heat transport is important for correct determination of the value of electron temperature and spatial temperature profile. Nonlocal electron heat flux comes into play after electrons heat up to temperatures of the order of 1 keV and to temperatures several times less for low-density targets.

Relativistic self-focusing and channelling of intense laser pulses have been studied in underdense plasma using two-dimensional particle-in-cell
PIC simulations, for different laser powers and plasma densities. Analytical solutions for the stationary evacuated channels have been recovered in PIC simulations. It is shown that otherwise stable channels can accelerate electrons due to surface waves on the walls of the channels. Relativistic filaments with finite electron density are unstable to transverse modulations which lead in the nonlinear stage to the breakup of laser pulses into independent filaments. Different regimes of relativistic self-focusing and channelling, including electron heating, transverse instability, and break-up of the filaments, have been discussed and characterized using plasma density and laser power.

The problem of ultrashort laser pulse absorption at the sharp plasma-vacuum interface has been solved using a new expression for plasma permittivity. The electron response includes both collisional and collisionless dissipative effects in a plasma with arbitrary ion charge. The difference between known absorption models and our theoretical RFBR approach which accounts for localized plasma heating with nonlocal heat transport has been discussed.

A simple analytic model of light-ion acceleration in a double-layer foil target is proposed. It accounts for ion acceleration in the electrostatic sheath and Coulomb interaction between heavy and light ions. The model is used to study proton acceleration, and the conditions for a quasimonoenergetic proton beam formation are defined. Comparison with the hybrid and two-dimensional particle-in-cell kinetic simulations verifies the model results.

The problem of the acceleration of a test electron in the éeld of a tightly focused relativistic laser pulse is solved in the case when the focal spot size is of the order of the radiation wavelength and the exact solution of Maxwell\'s equations for the electromagnetic éeld should be used. The electron acceleration is studied depending on the initial position of the electron in the focal plane and is compared with the results corresponding to the paraxial approximation for the laser field. The maximum energy acquired by the electron in the laser focus is found. The dependences of the ejection angle of the electron on its initial position near the focus are analysed.

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.

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.

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 n*T^a=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.

Generation of relativistic electrons from the interaction of a laser pulse with a high density plasma foil, accompanied by an underdense preplasma in front of it, has been studied with two-dimensional particle-in-cell ~PIC! simulations for pulse durations comparable to a single cycle and for single-wavelength spot size. The electrons are accelerated predominantly in forward direction for a preplasma longer than the pulse length. Otherwise, both forward and backward electron accelerations occur. The primary mechanism responsible for electron acceleration is identified. Simulations show that the energy of the accelerated electrons has a maximum versus the pulse duration for relativistic laser intensities. The most effective electron acceleration takes place when the preplasma scale length is comparable to the pulse duration. Electron distribution functions have been found from PIC simulations. Their tails are well approximated by Maxwellian distributions with a hot temperature in the MeV range.