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.

2D PIC code simulations have been performed for the optimization of gas jet target parameters to achieve a maximal energy and efficiency of charged particle acceleration in planned experiments at the 20 TW picosecond SOKOL-P laser. These calculations specify an opportunity to obtain energy up to Ee ∼ 200 MeV and efficiency ηe ∼ 10% for accelerated electrons and Ep ∼ 30 − 50 MeV and ηp ∼ 5% for accelerated protons in these experiments at laser intensity I ∼ 5 ⋅ 1019 W/cm2. They show the necessity of providing a formation of hydrogen jets with diameter ∼ 1mm, a gas molecule concentration ∼ 2 ⋅ 1019 cm−3 and steep density gradients ∼ 200 μm at the edge of the gas jet target for achieving these parameters of laser accelerated particle beams.

The LegoLPI code was used for simulation of laser-driven proton acceleration with the purpose to find an optimal conditions for the generation of proton beams with parameters required for hadron therapy. The 2D PIC simulations were done for various type of targets (double-layer foils, polyethylene foils, hydrogen jets) and target irradiation conditions (different intensities and polarizations, laser pulse wavelengths and durations, focal spot sizes). It is shown that efficiency of proton generation would be high enough for the optimal condition of target irradiation, but the spectrum of accelerated protons is rather broad for both linearly and circularly polarized laser pulses because of the two-dimensional effects arising when the proton acceleration length is comparable with the focal spot. As follows from the 2D LegoLPI calculations, the efficiency of formation of proton spectrum in energy range of 200–250 MeV and normalized to laser energy may reach values (0.5–1)•107 protons•(MeV•J)−1 for the considered types of laser – targets systems. The performed calculations specify an opportunity of generation proton beams whose parameters meet proton therapy requirements with the use of a laser that would have a peak power of about 1 PW and average power up to 1 kW. The powerful ultra-short pulse lasers, which are under construction in some countries around the world, are very close to the facilities necessary for this purpose.

The PM2D code is used to simulate relativistic electron-positron plasma generated in the target irradiated by laser light of relativistic intensities. The paper discusses results of PM2D simulations performed with the aim to increase mean energy and brightness of positron jets that could be formed near the rear surface of the target. The calculations demonstrate the possibility to significantly improve positron jet acceleration by target reconfiguring.

The parameters of a plasma produced upon the interaction of ultrashort laser pulses with cluster targets are measured by the methods of X-ray spectroscopy. The dependence of the plasma parameters on the initial properties of a cluster target (the design of a supersonic nozzle, the average size of clusters, the spatial inhomogeneity) and the laser pulse properties (its duration and contrast) is studied. The plasma diagnostics is performed using the model of formation of emission spectra, which was proposed earlier and includes a number of fitting parameters, which provide good agreement with experimental spectra. The systematic experimental studies performed by us showed that our model of cluster heating by ultrashort pulses is indeed a physical model, and the fitting parameters represent the average values of plasma parameters in the corresponding space-time regions.

Line emission spectrum of a laser plasma produced in an argon cluster jet target was measured on the n 1 P 1−1 1 S 0 ( n =5–9) transitions of the helium-like Ar XVII ion for a pulse duration varying from 45 fs to 1.1 ps and a constant fluence of ∼10 5 J/cm 2 . The independent modeling of the relative intensities of the transitions from the n =5,..., 10 levels, as well as of the 2 1 P 1 − 1 2 S 0 and 2 3 P 1 −1 2 S 0 lines and dielectronic satellites indicates that the electron temperature is anomalously low and that the electron density in emitting plasma increases with shortening the laser pulse. The excitation from the ground state by a small fraction of hot electrons is expected to be the main channel of populating the Ar XVII levels.

The paper considers designs of moderators where fast positron stopping medium consists of very fine tungsten strips separated by vacuum gaps and the strips are arranged into Venetian blinds- or honeycomb-type structures. Moderator efficiency is evaluated through Monte-Carlo simulations. According to the maximal estimate, the efficiency of conversion of fast positrons into slow ones in the Venetian blinds and honeycomb-type moderators is ∼5×10−3 for the reasonable thickness of the tungsten foil. If such moderator is used, the intensity of slow positron source on the hard synchrotron of SPring-8 storage ring can reach the level of ∼5×1010e+/s.

The paper provides results of numeric simulations of in-target positron production process, processes of moderation, thermalization, diffusion, and reemission of positrons in high-efficiency multi-wire moderator made of tungsten monocrystalline wire with regular wire spacing. The paper looks into dynamics of slow positrons in the moderator's vacuum gaps taking into account of external fields. The feasibility of using multi-wire moderator with non-regular structure - multi-layer “wire felt” moderator is discussed. According to maximal estimate the multi-wire moderators can reach very high efficiency of fast-slow positron transformation ∼10−2. Using such moderator the intensity of slow positron source on hard synchrotron radiation of SPring-8 can reach the level of ∼1011e+/s.