K.A. Ivanov

Femtosecond laser-plasma interaction with prepulse-generated liquid metal microjets

Ultrashort laser pulse interaction with a microstructured surface of a melted metal is a promising source of hard x-ray radiation. Microstructuring is achieved by a weak prepulse that produces narrow high-density microjets. As an x-ray source, the interaction of the main laser pulse with such jets is shown to be nearly two orders of magnitude more efficient than the interaction with ordinary metal targets. This paper presents the results of optical and x-ray studies of laser-plasma interaction physics under such conditions supported by numerical simulations of microjet formation and fast-electron generation.

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.

Enhanced relativistic laser–plasma coupling utilizing laser-induced micromodified target

The interaction of slighly relativistic femtosecond laser radiation with microstructured Si targets was studied. The microstructuring was performed by nanosecond pulse laser ablation with additional chemical etching of the target material. An analysis was made of the optical damage under the action of femtosecond radiation near the ablation threshold. It was experimentally demonstrated that the hot electron temperature increases appreciably in the laser-driven plasma (from ~370 to almost 500 keV) as well as radiation yield in the MeV range at the interaction of a high power femtosecond laser pulse with a microstructured surface in comparison with a flat surface. Numerical simulations using 3D3V PIC code Mandor revealed that the charged particle energy growth is caused by the stochastic motion of electrons in the complex field formed by the laser field and the quasistatic field at the sharp tips of micromodifications.

Acceleration of Heavy Multicharged Ions in the Interaction of a Subrelativistic Femtosecond Laser Pulse with a Melted Metal Surface

Results are presented from experimental studies of ion acceleration under the action of femtosecond laser pulses with an intensity of 1017 W/cm2, incident onto the free surfaces of melted gallium and indium. The effect of the polarization direction of a linearly polarized laser pulse and the amplitude of a short prepulse, which precedes the main pulse by several nanoseconds, on the parameters of accelerated ions is investigated. It is found that, even for such a moderate laser intensity, the characteristic velocity of fast ions ejected along the reflected beam is a factor of 1.5 higher than that of ions ejected along the normal to the target surface. It is shown that, as the prepulse energy increases, the hard X-ray yield and the mean energy of hot electrons increase substantially, whereas the velocity of both fast and slow ions decreases appreciably regard-less of laser polarization.

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.

Investigation of the reaction D(γ, n)H near the threshold by means of powerful femtosecond laser radiation

The possibility of studying photonuclear reactions near the threshold by means of powerful femtosecond lasers is explored by considering the example of deuteron photodisintegration. The respective experiment was performed by employing the terawatt femtosecond laser facility of the International Laser Center at Moscow State University. The radiation from this facility is characterized by a pulse energy of up to 50 mJ, a duration of 50 fs, a repetition rate of 10 Hz, and a wavelength of 805 nm. This provides a power above 1018 W/cm2. Intense relativistic-electron and photon beams of energy up to 10 MeV were obtained after the optimization of relevant experimental parameters, including the focus of the laser beam, its time structure, and the choice of target. The use of these beams made it possible to study neutron generation in heavy water, to measure the time of neutron moderation, and to determine the detection efficiency. The experimental data obtained in this way are in qualitative agreement with the results of simulations based on the GEANT-4 and LOENТ code packages and indicate that it is possible to create a neutron source on the basis of the aforementioned laser. The cross section measured for deuteron photodisintegration complies with theoretical estimates available in the literature.

Laser-plasma sources of ionizing radiation for simulation of radiation effects in microelectronic materials and components

The characteristics of the X-ray and gamma pulses as well as pulsed proton fluxes induced by the action of femotsecond laser pulses (800 nm, 1018 W/cm2) on the surface of liquid gallium and solid molybdenum targets are measured. Estimated calculations show the possibility in principle of availability of these ionizing radiation sources for experimental simulation of single-event effects in advanced microelectronic components with a high degree of integration.

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.