V. F. Tishkin

Nonequilibrium laser-produced plasma of volume-structured media and ICF applications

This paper is devoted to the investigation of powerful laser pulse interaction with regularly and statistically volume-structured media with near critical average density and properties of laser-produced plasma of such a media. The results of the latest experiments on laser pulse interaction with plane foam targets performed on Nd-laser facilities ABC in the ENEA-EURATOM Association (Frascati, Italy) and MISHEN in the Troitsk Institute of Innovation Thermonuclear Investigations (TRINITI, Troitsk Russia), and J-laser ISKRA-4 in the Russian Federal Nuclear Center, All-Russian Scientific Research Institute of Experimental Physics (RFNC-VNIIEF, Sarov, Russia) are presented and analyzed. High efficiency of the internal volume absorption of laser radiation in the foams of supercritical density was observed, and the dynamics of absorbing region formation and velocity of energy transfer process versus the parameters of porous matter are found. Some inertial confinement fusion (ICF) applications based on nonequilibrium properties of laser-produced plasma of a foam and regularly structured media such as the powerful neutron source with yield of 10 9 -10 11 DT-neutrons per 1 J of laser energy, laser-produced X-ray generation in high temperature supercritical plasma, and the compact ICF target absorbers providing effective smoothing and ablation are proposed.

Hydrodynamics of high-energy GARPUN KrF laser interaction with solid and thin-film targets in ambient air

Hydrodynamic regimes of KrF laser interaction with solid and thin-film targets in air at atmospheric and reduced pressures were investigated at the high-energy GARPUN facility. These experiments were performed with 100 J, 100 ns laser pulses in planar focusing geometry and compared with numerical simulations with the ATLANT code to verify the concept of the laser-driven shock tube (LST). Strong shock waves (SWs) are produced in the LST and the gas is accelerated to hypersonic velocity due to the deposition of laser energy. The laser beam is focused by a prism raster optical system that provides a very uniform intensity distribution at moderate laser intensities q ≤ 1 GW cm−2 over a square spot of ~1 cm size. Dynamics of laser-produced plasma and SWs in a surrounding gas were investigated by means of a high-speed photo-chronograph and streak camera in combination with shadow or schlieren techniques, and time and space resolved spectroscopy in the visible spectral range. Both experiments and simulations confirmed that target evaporation and blow-up of expanding plasma are the main mechanisms of UV laser–target interaction in the surrounding gas. Planar SWs with velocities up to 7 km s−1 towards the laser beam were observed in normal-density air and up to 30 km s−1 in rarefied air. Acceleration of thin CH films of 1–50 µm thickness was investigated both in free-expansion and plasma-confined regimes with the highest achieved velocities being up to 4 km s−1. The SW damping law in free space, independent of laser intensity and air pressure, could be approximated by a power law x ~ tn with power indices n1 = 0.85–0.95 at the initial stage and n2 = 0.5–0.6 later, when the distance of the SW front from the target became comparable with the size of the irradiated spot. Instability growth at contact interfaces between ablative plasma and accelerated film, as well as between plasma and compressed air were observed, and compared for various initial irradiation non-uniformities. These non-uniformities were introduced by a grid, which was placed in front of the film target.

Study of hydrodynamic instabilities of a Z-pinch during a high-current explosion of a thin wire

Results are presented from laboratory and numerical experiments on the influence of the core and associated hydrodynamic instabilities on the high-current implosion of a plasma of exploding metal wires. The experimental investigation of the discharge structure was carried out using the multiframe X-ray backlighting technique with high temporal and spatial resolution (<1 ns and 1 µm, respectively); X-pinches were used as small-sized radiation sources. The implosion of a dense Z-pinch was modeled by the free-point method with the use of a two-dimensional radiative MHD code. The onset of instabilities at the corona-core boundary was modeled by the NUTCY Eulerian code. The results show that hydrodynamic processes in the core are primarily responsible for the formation of small bright regions observed in X-rays. After the reflection of a shock wave from the axis, the rapid onset of hydrodynamic instabilities can occur at the corona-core boundary.

2D numerical simulation of the interaction of high-power laser pulses with plane targets using the “ATLANT-C” Lagrangian code

The “ATLANT-C” program developed for the two-dimensional (2D) simulation of problems of Lagrangian gas dynamics is described. The difference scheme of gas dynamics with an increased number of thermodynamic degrees of freedom used in the program preserves the plane, cylindrical, and spherical symmetry of the one-dimensional (1D) flows. The approach proposed provides an improved retention of the shape of the computational cell and the solution of problems incomputable with the conventional schemes. The efficiency of the algorithms applied is confirmed by the test calculations and results of simulation of actual physical experiments.

Efficiency of laser energy input into a hohlraum through a hole

Results are presented from the two-dimensional numerical simulations of laser energy input into a hohlraum through a hole. This problem is of interest for ICF research, specifically, for optimization of laser microtarget design. The optimum relations are found between the hole size and the effective laser spot radius under conditions close to those of present-day ICF experiments.

High-energy GARPUN KrF laser interaction with solid and thin-foil targets in ambient air

Hydrodynamic regimes of KrF laser interaction with solid and thin-film targets in atmospheric and reduced pressure air were investigated at high-energy GARPUN installation. These experiments were performed with 100-J, 100-ns laser pulses in planar focusing geometry and compared with numerical simulations with ATLANT code to verify the concept of laser-driven shock tube (LST), which could accelerate a gas to hypersonic velocity and produce strong shock waves (SW). Laser beam was focused by a prism raster optical system that provided very uniform intensity distribution at moderate laser intensities q ≤ 1 GW/cm2 over a square spot of ~ 1-cm size. Dynamics of laser-produced plasma and SW in a surrounding gas were investigated by means of high-speed photo-chronograph and streak camera in combination with shadow or schlieren techniques, time and space resolved spectroscopy in a visible spectral range. Both experiments and simulations confirmed that target evaporation and blow-up of expanding plasma are the main mechanisms of UV laser-target interaction in a surrounding gas. Planar shock waves with velocities up to 7 km/s towards the laser beam were observed in a normal density air and up to 30 km/s in a rarefied air. Acceleration of thin CH films of 1 to 50-μm thickness was investigated both in a free-expansion and plasma-confined regimes with the highest achieved velocities up to 4 km/s. The SW damping law in a free space independently on laser intensity and air pressure could be approximated by a power law x ~ tn with a power indexes n1 = 0.85 - 0.95 at the initial stage and n2 = 0.5 - 0.6 later, when a distance of the SW front from a target became comparable with a size of the irradiated spot. Instability growth at contact interfaces between ablative plasma and accelerated film, as well as between plasma and compressed air were observed and compared for various initial irradiation non-uniformities. They were introduced by a grid, which was set in front of the film target.

GARPUN KrF-laser-target experiments and numerical simulations on the concept of laser-driven shock tube

We have suggested a concept of laser-driven shock tube (LST) for generation of hypersonic shock waves (SW) in gases and compression waves in liquids. This novel laboratory technique might be applied to the studies of various fundamental hydrodynamic phenomena such as development of hydrodynamic instabilities at contact interfaces between different liquids and gases accelerated by shock waves, hypersonic gas flow around bodies, effects of strong shock wave refraction and cumulation in time scale of several microseconds and space scale of ten millimeters. These problems are of great importance in Inertial Confinement Fusion, comsology, astrophysics, and aerospace engineering. In this paper we present both numerical simulations and first experimental results to verify the laser-driven shock tube concept for studying of strong SW generation in the air.

Transverse structures in corona of nonuniformly irradiated solid targets

Formation of transverse inhomogeneities in the corona of a solid target irradiated by an inhomogeneous main laser pulse and a uniform background pulse was observed experimentally via side-on shadowgraphy. The experimental results were successfully interpreted using a two-dimensional hydrodynamics code. Our simulations identified the onset of sharp contact boundaries between plasma streams of different expansion velocities. The formation and the decay of the contact boundaries is investigated in detail. When the background pulse is used as a laser prepulse, a layer of coronal plasma is formed that enhances main pulse collisional absorption in underdense plasma and creates conditions for an efficient thermal smoothing of the transverse inhomogeneities.

On the Neutron Yield in the Two-Beam Scheme of Laser Heating and Compression of Spherical Shell Targets with a Low‐Density Coating

At the initial stage of development of large-scale multiple-beam laser facilities for the “ignition” of the fusion reaction, facilities with a small number of beams is expected to be created. Such facilities are characterized by poor symmetry of irradiation of spherical targets. We have shown that with appropriate design of the target and distribution of the radiation intensity in laser beams the attainment of a neutron yield of 1015-1016 is possible even in two‐side irradiation of the spherical targets.

Analysis and 2D numerical modeling of burn through of metallic foil experiments using power KrF and Nd lasers

The modeling of foil burn through effect with the help of traditional Lagrangian codes ( for example, “ATLANT”-code [4] has some difficulties. We are developing Euler 2D and 3D codes. It allows us to solve the problems of foil burn through, the input of a laser beam into a cavity [5] and other one. “NUT-CY”-code solves numerically the gas dynamic equations and electron heat conductivity in 2D cylindrical geometry ( r,z,t). It is assumed that the laser beam is propagating strictly along the axis ( 0Z ) and is absorbed due to bremsstrahlung mechanism. The laser flux which reaches the critical surface is absorbed in the cells. EOS is the ideal plasma.