A. B. Savel'ev

Generation of High-Energy Negative Hydrogen Ions upon the Interaction of Superintense Femtosecond Laser Radiation with a Solid Target

The formation of a high-energy (∼35 keV) beam of negative hydrogen ions was observed in the expanding femtosecond laser plasma produced at the surface of a solid target by radiation with an intensity of up to 2× 1016 W/cm2. The energy spectra of the H+ and H−-ions show a high degree of correlation.

Comparative study of amplified spontaneous emission and short pre-pulse impacts onto fast electron generation at sub-relativistic femtosecond laser-plasma interaction

This paper describes the study of hot electron generation under the action of intense (∼1018 W/cm2) femtosecond pulses onto the surface of a solid target, in the presence of a long pre-plasma, which varied with different spatial extents and densities. The corona was formed by pre-pulses with varied intensities and temporal profiles (amplified spontaneous emission (ASE) and short pre-pulses). The most efficient fast electron acceleration, to energies well beyond the ponderomotive potential, was observed if the ASE was able to form to the extent of ∼100 μm a slightly undercritical plasma. Energy of accelerated electrons underwent further growth if the laser pulse duration increased from ∼45 to ∼350 fs at constant energy fluence. The experimental results were supported by numerical simulations using 3D3V Mandor PIC code.

Prepulse controlled electron acceleration from solids by a femtosecond laser pulse in the slightly relativistic regime

We present results from the experimental and numerical study of electron heating and acceleration under the action of a 50 fs high contrast laser pulse [intensities ∼(1–4) × 1018 W/cm2] with a controlled preplasma that was created by a 6 ns laser “prepulse” with intensity ∼1012 W/cm2. A substantial increase both in the gamma yield and “temperature” was obtained by the proper adjustment of the time delay between the two pulses (0–5 ns), while the gamma yield dropped to almost zero values if the nanosecond pulse came 10–20 ns in advance of the femtosecond one. Comprehensive optical diagnostics (shadowgraphy, interferometry, and angular resolved self-emission measurements) data allowed us to estimate the electron density profile. The latter profile was used for making numerical Particle-in-cell simulations which describe the gamma yield enhancement well. We also illustrate how the observed drop in the gamma yield within a certain range of delays was due to ionization defocusing of the femtosecond beam in an ...

Interaction of High-Intensity Femtosecond Radiation With Gas Cluster Beam: Effect of Pulse Duration on Joint Terahertz and X-Ray Emission

This paper studies the phenomenon of joint generation of terahertz (THz) and X-ray radiation in the argon nanocluster jet under the action of high-power femtosecond laser pulse in both the single-color and dual-color regimes. It was discovered that in a gas cluster beam the pulse duration affects the properties of THz and X-ray emission differently. For the same given total energy of optical pulse in the dual-color excitation regime of cluster medium, more than a five times increase of THz radiation power was observed in comparison with the single-color regime, while the conversion efficiency to the argon X-ray K-line reached 7 × 10-6 and remained unchanged. The possibility of separation of contributions of different beam components into the THz signal was demonstrated experimentally, using contributions from clusters and nonclustered gas as an example. We suggest an interpretation of experimental results based on a theoretical model of cluster ionization that self-consistently predicts the level and dynamics of ionization and electron temperature in the clusters.

Four-wave parametric conversion of femtosecond laser pulse in a filament induced in a solid target

Nonlinear interaction of two femtosecond laser pulses in a filament, induced by one of them inside a fused silica plate, leads to generation of the new spectral components. These spectral components reach hundreds of nanometers bandwidth. Their spatial and spectral properties can be explained by four-wave parametric coupling in the filament. The energy measurements indicate the high efficiency of this process.

Self-channeling of femtosecond laser radiation in transparent two-component condensed medium

We have observed self-guiding of a single femtosecond visible laser pulse in the bulk of transparent nonlinear media (SiO2, KDP) and in the water. The dependence of filament length on laser pulse energy was measured. Continuous open-ended channels and frozen modifications of the matter were observed in transparent two-component condensed medium (thin glass plate placed in water).

Generation of Hermite–Gaussian modes of high-power femtosecond laser radiation using binary-phase diffractive optical elements

We present the results of experiments on generation of Hermite–Gaussian modes up to the third order inclusive using binary-phase diffractive optical elements (DOEs) illuminated by subterawatt femtosecond laser pulses. We perform a compariosn of the mode formation using DOEs designed by the kinoform method and the fractional coding technique, when the DOEs are illuminated by both femtosecond radiation and cw laser radiation at close wavelengths.

Acceleration of multiply charged ions by a high-contrast femtosecond laser pulse of relativistic intensity with the front surface of a solid target

It is shown that the acceleration efficiency of protons and multiply charged ions (and also the charge composition of the latter) accelerated backwards under irradiation of the front surface of thick solid targets by high-power femtosecond laser radiation with an intensity of 2 × 1018 W cm-2 is determined by the contrast of this radiation. Thus, highly ionised ions up to C6+, Si12+ and Mo14+ are recorded on polyethylene, silicon and molybdenum targets at a contrast of 10-8, the ions with charges up to C5+, Si10+ and Mo10+ possessing an energy of more than 100 keV per unit charge. In the case of a metal target, the acceleration efficiency of protons is significantly reduced, which indicates cleaning of the target surface by a pre-pulse. The measurements performed at a contrast increased by two-to-three orders of magnitude show the presence of fast protons (up to 300–700 keV) on all targets, and also a decrease in the energy and maximum charge of multiply charged ions.

Microjet formation and hard x-ray production from a liquid metal target irradiated by intense femtosecond laser pulses

By using a liquid metal as a target one may significantly enhance the yield of hard x-rays with a sequence of two intense femtosecond laser pulses. The influence of the time delay between the two pulses is studied experimentally and interpreted with numerical simulations. It was suggested that the first arbitrary weak pulse produces microjets from the target surface, while the second intense pulse provides an efficient electron heating and acceleration along the jet surface. These energetic electrons are the source of x-ray emission while striking the target surface. The microjet formation is explained based on the results given by both optical diagnostics and hydrodynamic modeling by a collision of shocks originated from two distinct zones of laser energy deposition.

Peculiarities of femtosecond laser radiation interaction with liquid metal targets

This study is devoted to investigation of femtosecond laser radiation parameters (intensity, contrast, polarization) influence on properties of hot electron population of dense plasma created at the surface of melted metal target.