Valerii G. Bakaev

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.

Amplified spontaneous emission and nanosecond pulse amplification in large-aperture electron-beam-pumped KrF amplifiers

The experiments have been performed at 100-J-class GARPUN KrF laser installation, which consists of 20-ns discharge-pumped master oscillator and two stages of e-beam-pumped amplifiers with gain volumes of 10x10x100 cm and 16x18x100 cm. Gain and absorption coefficients were measured in a single-pass scheme, while intrinsic efficiency of about 12% was demonstrated in the saturated double-pass amplification. They were compared with numerical simulations. A numerical code solved a set os simultaneous self-consistent kinetic equations together with amplification of laser radiation and spontaneous emission in large-aperture KrF lasers. Being verfied with the experimental data the code was used to forecast the parameters of IFE-scale KrF amplifiers.

Optics of powerful e-beam-pumped KrF-lasers

The comprehensive results are presented on the behavior of high purity synthetic CaF2, MgF2, quartz glass and Al2O3 under the action of intensive ionizing radiation (x-rays and energetic electrons) and UV laser radiation with 248-nm wavelength. They are concerned to the application of e-beam-pumped large-size KrF-laser as a driver in the Inertial Fusion Energy.

Development and evaluation of fluorine-resistant coatings for windows of high-energy KrF laser

Fluorine-resistant coatings for fused silica windows of high-energy KrF lasers with damage thresholds as high as 20 J/cm2 have been developed using two comparative deposition techniques: ion-assisted e-beam evaporation and magnetron ion sputtering. They were evaluated by means of atomic-force microscopy, Fourier spectroscopy, and laser-damage testing in large irradiated spots ~ 1 cm at powerful GARPUN KrF laser.

E-beam pumping and energy extraction from large-aperture KrF amplifiers

Experiments were performed at 100-J-class GARPUN KrF laser installation on optimization of e-beam pumping and amplification of 20-ns pulses in e-beam-pumped amplifiers with gain volumes of 10 × 10 × 100 cm3 and 16 × 18 × 100 cm3. Amplified spontaneous emission (ASE) was measured in the near field close to the output window and in the far field along the amplifier axis. Suppression of transverse ASE by amplification of a laser signal was also investigated. The experimental data were compared with numerical simulations of e-beam transport using Monte Carlo code and 3-D numerical simulations of large-aperture single-pass and double-pass KrF laser amplifiers. Finally, the verified numerical codes were applied for optimization studies of large-scale KrF amplifiers with output energy up to 10 kJ being developed for Inertial Fusion Energy application.

Characterization of bremsstrahlung x-ray emission as a reason for optics degradation in rep-rate e-beam-pumped KrF laser driver in IFE application

Time-resolved scintillation technique and absolutely calibrated thermo-luminescence dosimeters have been applied to characterize bremsstrahlung x-ray emission from a large-aperture GARPUN KrF laser module pumped in a transverse geometry by double-sided e-beams stabilized by pulsed magnetic field of 0.1 T. It was produced by electrons with ~300 keV kinetic energy, 50 A/cm2 current density, and 100 ns pulse duration when they passed from vacuum diodes through foil windows into laser chamber and were decelerated in a working gas. Regularization algorithm was developed to reconstruct the bremsstrahlung spectra using experimental data on x-ray transmission through different absorbers. The energy fluence of ionizing radiation escaped onto laser windows has been measured. It was shown that a long-term degradation of optical transmission due to bremsstrahlung x-ray irradiation should be taken into account for KrF laser application in Inertial Fusion Energy.

GARPUN laser experiments on e-beam transportation through Al-Be foil and optical window stability addressed to the NRL rep-rate e-beam-pumped KrF facility

Two key engineering issues in the development of a KrF laser driver for Inertial Fusion Energy are high transmitting and long life e-beam window and optical laser windows. We have performed experiments with e-beam-pumped KrF laser installation GARPUN on the transport of relativistic e-beams through Al-Be and Ti foils and compared them with Monte Carlo numerical calculations. It was shown that both 50-?m thick Al-Be foils and 20-?m thick Ti foils had equal transmittance of about 75% for ~3OO keV, 50 A/cm2, 100 ns e-beams. However in both cases the observed transmission was lower than calculated one. In contrast to Ti foil, whose surface was strongly etched by fluorine, no surface modification or fatal damages were observed for Al-Be foil after ~1000 laser shots and protracted fluorine exposure. It was shown that applied magnetic field of ~1kG significantly reduced electron scattering both across and along laser cell at typical pumping conditions with 1 .5-atm pressure working gas. Without magnetic field irradiation of optical windows by scattered electrons resulteu in additional transient and residual induced absorption of laser radiation. The analysis of different optical materials for KrF laser windows and coatings has also been done.

Electron beam transport and foil stability in high-energy e-beam- pumped KrF lasers

There are several key engineering and physical issues for the development of Krypton Fluoride driver for Inertial Fusion Energy Program. In the frame of this program we have performed experiments with e-beam-pumped KrF laser installation GARPUN on the transportation of relativistic e- beams through aluminum-beryllium and titanium foils and compared them with Monte Carlo numerical calculations. It was shown that 50-micrometers thickness Al-Be and 20-micrometers Ti foils had equal transmittance of 75% for approximately 300 keV, 50 kA/cm2, 100 ns e-beams, being lower than calculated one. In contrast to Ti foil, which surface was strongly etched by fluorine, no surface modification and no fatal damages were observed for Al-Be foils after approximately 1000 laser shots and protracted fluorine exposure. It was shown that applied magnetic field of approximately 1 kGs significantly reduced electron scattering both across and along laser cell at typical pumping conditions with 1.5-atm pressure working gas. The energy fluence of scattered electrons fell down from approximately 100 mJ/cm2 to approximately 1 mJ/cm2 per pulse at 8.5-cm distance from the boundary of injected e-beams. Without magnetic field the scattered electrons were spread up to 20 cm, thus strongly irradiating optical windows and being a cause of additional induced absorption of laser radiation.

Optical materials for output windows of large-scale e-beam-pumped KrF lasers

Summary form only given. E-beam-pumped KrF lasers are very promising drivers for inertial confinement fusion due to their high efficiency, short radiation wavelength and a large bandwidth, as well as to principle feasibility to operate with repetition rate of a few Hz at multi-kilojoule energy level. One of the problems to be developed for reliable rep-rate KrF laser operation is a stability of output laser windows. In addition to intense UV laser light they would be exposed to fluorine etching (it is contained in a working gas mixture), and to irradiation by bremsstrahlung X-rays and energetic scattered electrons. A number of optical materials such as fused silica (SiO/sub 2/), calcium fluoride (CaF/sub 2/), magnesium fluoride (MgF/sub 2/), and synthetic leucosapphire (Al/sub 2/O/sub 3/) have been comparatively examined in regard of chemical resistance to fluorine, nonlinear and ionizing-radiatian-induced absorption at KrF laser radiation wavelength of 248 nm.