Nikolai N. Demchenko

Hard x‐ray emission from intense short pulse laser plasmas

Hard x‐ray emission in the range of 100 keV has been measured from plasmas produced by irradiation of solid targets with ps laser pulses up to 7×1017 W/cm2. The experimental data obtained for oblique incidence of p‐polarized laser light at different illumination angles are compared to computer calculations, which include the processes of resonance absorption and vacuum heating. The scaling of hard x‐ray emission with varying laser flux is consistent with the theoretical model of bremsstrahlung emission of hot electrons. From this, together with an absolute radiation dose measured with calibrated detectors, a transfer of up to 50% of the incident laser energy to suprathermal (∼10...100 keV) electrons is estimated.

Laser-driven ablation through fast electrons in PALS-experiment at the laser radiation intensity of 1–50 PW/cm 2

The paper is directed to the study of high-temperature plasma and ablation plasma formation as well as efficiency of the laser energy transfer to solid targets irradiated by laser pulses with intensities of 1–50 PW/cm and duration of 200–300 ps, i.e., at conditions corresponding to the characteristics of the laser spike designed to generate the igniting shock wave in the shock ignition concept. The experiments have been performed at Prague Asterix Laser System. The iodine laser delivered 250 ps (full width at half maximum) pulses with the energy in the range of 100–600 J at the first (λ1= 1.315 μm) and third (λ3= 0.438 μm) harmonic frequencies. The focal spot radius of the laser beam on the surface of Al or Cu targets made was gradually decreased from 160 to 40 μm. The diagnostic data collected using three-frame interferometry, X-ray spectroscopy, and crater replica technique were interpreted by two-dimensional numerical and analytical modeling which included generation and transport of fast electrons. The coupling parameter Iλ was varied in the range of 1 × 10−8 × 10 Wμm/cm covering the regimes of weak to intense fast electron generation. The dominant contribution of fast electron energy transfer into the ablation process and shock wave generation was found when using the first harmonic laser radiation, the focal spot radius of 40–100 μm, and the laser energy of 300–600 J.

Space-time resolved measurements of spontaneous magnetic fields in laser-produced plasma

The first space-time resolved spontaneous magnetic field (SMF) measurements realized on Prague Asterix Laser System are presented. The SMF was generated as a result of single laser beam (1.315 μm) interaction with massive planar targets made of materials with various atomic numbers (plastic and Cu). Measured SMF confirmed azimuthal geometry and their maximum amplitude reached the value of 10 MG at the laser energy of 250 J for both target materials. It was demonstrated that spatial distributions of these fields are associated with the character of the ablative plasma expansion which clearly depends on the target material. To measure the SMF, the Faraday effect was employed causing rotation of the vector of polarization of the linearly polarized diagnostic beam. The rotation angle was determined together with the phase shift using a novel design of a two-channel polaro-interferometer. To obtain sufficiently high temporal resolution, the polaro-interferometer was irradiated by Ti:Sa laser pulse with the wavelength of 808 nm and the pulse duration of 40 fs. The results of measurements were compared with theoretical analysis.

Interaction of a laser-produced copper plasma jet with ambient plastic plasma

This work is aimed at the investigation of mutual interaction of plastic (CH) and copper axially symmetric plasmas, and the dependence on the ratio of their ablated masses. For that purpose we irradiated a plastic target with a Cu cylindrical insert of diameter 400 µm with a laser beam of radius exceeding that of the insert. In such an arrangement we were able to control the volumes and masses of interacting CH and Cu plasmas by varying the laser beam diameter. The experiment was carried out at the Prague Asterix Laser System (PALS) iodine laser. The laser provided a 250 ps pulse with an energy of 130 J at the third harmonic frequency (i.e. a wavelength of λ3 = 0.438 µm). The interaction of the laser-driven plasma jet with ambient plasma was studied by means of a three-frame interferometric system and a four-frame x-ray pinhole camera. For simulation of the experiment we used the two-dimensional hydrodynamic code ATLANT-HE. The reported results demonstrated that there is an optimal ratio of Cu and plastic plasma masses, at which stable extended Cu plasma jets can be created. Even a relatively thin plastic plasma envelope can compress the Cu plasma and control the jet formation. Thicker plastic plasma layers, however, lead to complex hydrodynamic motion accompanied by generation of transverse shock waves, which can completely suppress the jet formation. The pressure of the plastic plasma was found to be 1.35 times higher than that of the copper plasma. A theoretical analysis of the experimental results allows us to conclude that this difference in pressures follows from the essential differences in expansion dynamics of plasmas with low and high atomic numbers.

Ablation of metals by ultrashort laser pulses: Theoretical modeling and computer simulations

Laser ablation of metals by femto- and picosecond pulses is analytically and numerically studied within the framework of different models for the ablated material. Within the plasma model ablation is initiated by high-power thermal and hydrodynamic waves which propagate into the irradiated material. Analytical expressions for the thermal ablation and for the ablation by the shock wave are obtained. Numerical simulations with the computer code RAPID are in a good agreement with analytical results.

Simulation of hydrodynamic phenomena caused by pre-pulse in picosecond laser-plasma interaction

We consider a physical model of the interaction of high-power laser pulses with plasma created upon irradiation of condensed targets. The model is based on the equations of single-fluid, two-temperature hydrodynamics taking into account the ponderomotive force and the Maxwell equations for laser radiation at oblique incidence in the cases of s- and p-polarizations. The model takes into account the generation of fast electrons in the conditions of plasma resonance at the critical surface, and their transport with consideration for the friction force, caused by the ionization losses. For a number of experiments we have performed the numerical modeling of the laser picosecond pulse interaction with targets. We present the interpretation of the experiment on the basic harmonic shift depending on the pre-pulse energy. It has been shown that, if under the irradiation of a deuterated target the pre-pulse energy grows, the neutron yield of DD-reactions diminishes, since the produced plasma prevents the heating of the dense part of the target. It has been also shown that the growth of the pre-pulse energy can provoke, due to the induced scattering, the losses in the main pulse radiation. We give interpretation of the experimental data on the picosecond pulse absorption by plasma at the flux density of 1016-1019 W/cm2.

Direct drive targets for the megajoule facility UFL-2M

Development of direct drive target schemes for the megajoule facility is a topical problem of up-to-date inertial fusion physics. The choice of possible schemes and solutions depends essentially on the irradiation conditions. The installations both running (NIF) and under construction (LMJ) are destined to the 3ω irradiation in PDD (polar direct drive) configuration. The UFL-2M installation that is under construction is based on 2ω irradiation and a symmetrical scheme of direct drive target irradiation. Under these conditions possible schemes for direct drive targets demonstrating the ignition and the achievement of gain G=10÷20 are considered in this report. At the same time, the possibilities are analyzed for the target compression and ignition with a reliability reserve at the conditions that can deviate from the standard ones, and if our understanding of the physics of the processes is not completely adequate to the physics of the real processes.

Plastic plasma interaction with plasmas with growing atomic number

This paper describes the investigation of the influence of target material atomic number (Z) on the laser-produced plasma pressure. For this reason, several target materials representing a wide range of atomic numbers (Z = 3.5 - 73), i.e. plastic (CH), Al, Cu, Ag, and Ta, were used. The results presented show that the plasma pressure decreases with growing atomic number but in a limited range of Z only. For higher Z, starting approximately from Z = 47 (Ag), the plasma pressure becomes constant, as confirmed by interferometric measurements and x-ray plasma imaging.

Experimental and theoretical studies of the crater formation process on PALS experiments

Experimental, theoretical, and numerical results of investigations of the iodine laser interaction with single and double targets (consisting of an Al foil placed in front a massive Al slab and separated by a certain gap) were employed. Experiments were performed with the use of the first harmonic of laser radiation with parameters as follows: laser energy in the range of 100 ÷ 400 J, pulse duration of 0.4 ns and focal spot diameter of 250 μm. Velocities of accelerated foils and electron density distributions of plasma streams were determined by means of 3-frame interferometry. Shapes and volumes of craters were obtained employing crater replica technology and microscopy measurement. To interpret the experimental results the theoretical model of plasma ablation and shock wave formation as well as the two-dimensional Lagrangian hydrodynamics code were used.

Threshold characteristics of short and ultrashort laser pulse ablation of metals

The results of theoretical studies are reported for threshold characteristics of a metal ablation by picosecond and femtosecond laser pulse. Two possible mechanisms of the laser ablation at laser fluence F ≤ Fth are considered: thermal mechanism of ablation connected with a kinetics of a metal-vacuum surface evaporation and the mechanism of ablation connected with a hydrodynamics of a dense matter. The analysis has been made within the framework of a two-temperature model of metals for femtosecond and picosecond region of laser pulse duration and the extended of a two-temperature model of the metal in the case when the surface temperature Ti more than the critical temperature of metals. Analytical expressions for the ablation-threshold fluency Fth as well as the threshold values of the lattice temperature and the characteristic time of lattice temperature decay td(Fth) are obtained. This analytical description is in satisfactory agreement with particular numerical calculations.