Paul B. Sergeev

E-beam-induced absorption in various grades of quartz

The comprehensive results are presented on the behaviour of high purity synthetic quartz glasses 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 for the Inertial Fusion Energy.

High-purity CaF2 as a material for windows of powerful excimer lasers

The behavior of high-purity fluorite crystals under the action of ionising radiation (fast electrons and roentgen) and also intensive KrF- and ArF-laser radiation has been investigated. On the basis of these experimental results and literature data the benefits of high-purity fluorite for manufacturing of windows and other optical elements for excimer lasers were demonstrated.

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.

Radiation resistance of optical materials for excimer laser windows

The results of test for radiation resistance of high purity crystals (MgF2, CaF2, BaF2Al2O3, SiO2) and quartz glasses (QU-1, KS-4V, Corning 7940) under action of e-beam are submitted. Energy density of e-beam for a pulse duration 80 ns was within the limits of 0,2-2 J/cm2. More than 104 pulses has been made on samples within one year. Changes of transmission spectra of samples in a range of 100-1000 nm were traced during an irradiation.

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.

Nonlinear mechanisms of UV laser radiation absorption in CaF2

The numerical model for description of interaction of ionizing and UV laser radiation with CaF2 is submitted. Is shown, that the appearance of short-lived electronic excitations in a crystal strongly affects on nonlinear absorption of laser radiation at pulse duration more than 0.1 ns. The consequences of this absorption for structure of crystal are analyzed.

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.

Nonlinear absorption and laser-induced damage of BaF2, CaF2, and Al2O3 at 248 nm

Transmission dependencies of optical materials samples BaF2, Al2O3 and CaF2 (the infrared range sample) from KrF-laser radiation intensity with pulse duration of 85 ns are experimentally received. From these dependencies, two-photon absorption coefficients at laser wavelength 248 nm are found which for the appropriate materials are equal to 0.5 +/- 0.2, 2 +/- 1 and 30 cm/GW. Laser damage thresholds of the surface and volume of the investigated samples at 248 nm are also determined.

Simulation of mechanisms causing a nonlinear absorption of UV laser radiation in ionic crystals

In the work two models describing interaction of UV laser radiation (LR) with ionic crystals are considered. In the first version the wavelength of LR lies in absorption region of F centers, in the second--it is in a short-wave area of H centers absorption wing. Is shown, that the photodissociation in crystals of F2- complexes on two holes is the effective mechanism of defects formation in a crystalline structure. The account of these and other electronic excitation relaxation processes in models allows to explain many experimental results of LR action on ionic crystals.