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[en] On-line production facilities for radioactive isotopes nowadays heavily rely on resonance ionization laser ion sources due to their demonstrated unsurpassed efficiency and elemental selectivity. Powerful high repetition rate tunable pulsed dye or Ti:sapphire lasers can be used for this purpose. To counteract limitations of short pulse pump lasers, as needed for dye laser pumping, i.e. copper vapor lasers, which include high maintenance and nevertheless often only imperfect reliability, an all-solid-state Nd:YAG pumped Ti:sapphire laser system has been constructed. This could complement or even replace dye laser systems, eliminating their disadvantages but on the other hand introduce shortcomings on the side of the available wavelength range. Pros and cons of these developments will be discussed.
[en] Important figure of merit of high-intensity laser systems is the temporal and spatial quality of their pulses. Spatial filtering is a well known technique to improve the spatial quality by modulating the spatial components at the Fourier-plane, using a pinhole of appropriate size or recently by a nonlinear process. Modulation of the beam in the Fourier-plane allows however a simultaneous spatial and temporal filtering. By the use of a 'conjugate' pinhole arrangement before and after the nonlinear spatial selector, intensity dependent transmission is obtained: the low intensity part is efficiently suppressed. Numerical calculations predict practical operation for both amplitude and phase modulation at the Fourier-plane. In the preferred latter case the experimental observations are in good agreement with the theory, demonstrating >40% throughput. (authors)
[en] The problem of the operational stability of a KrF laser with an average output power of at least 600 W was investigated. An experimental study was made of the dependences of the rms deviation σ of the output energy on the charging voltage, on the pulse repetition rate, and on the operating time. The value of σ varied from 1.2% to 6.0%, depending on the experimental conditions. For an average power of ∼ 600 W, the deviation σ did not exceed 3.2%. (lasers and amplifiers)
[en] Complete text of publication follows. We are developing the science and technologies needed for a practical fusion energy source using high energy krypton fluoride (KrF) lasers. The physics basis for this work is a family of simulations that exploit the unique advantages of KrF lasers. KrF lasers provide uniform enough laser light to illuminate the capsule directly, greatly improving the laser-target coupling efficiency, as well as simplifying the target design. KrF's shorter wavelength allows higher ablation pressures and helps suppress laser-plasma instabilities. These advantages are being demonstrated on the NRL Nike KrF laser facility. A particularly promising approach is shock ignition, in which a high intensity laser pulse drives an intense shock at peak compression. Simulations with experimentally benchmarked codes predict a 1 MJ KrF laser can produce 200 MJ of pure fusion energy. We have similarly advanced the laser technology. We have developed a KrF laser, using technologies that scale to a reactor beamline, that fires 5 times per second for long duration runs and is projected be efficient enough for a reactor. The science and the technology for the key components are developed at the same time as part of a coherent system. A multi-institutional team from industry, national labs, and universities has developed credible solutions for these components. This includes methods to fabricate the spherical pellets on mass production basis, a means to repetitively inject the capsules into the chamber and precisely hit them with the laser, scaled tests to develop the laser optics, and designs for the reaction vessel. Based on these advances NRL and its collaborators have formulated a three stage plan that could lead to practical fusion energy on a much faster time scale than currently believed. Stage I develops full scale components: a laser beam line, target factory and injector, and chamber technologies. Stage II is the Fusion Test Facility (FTF). Simulations show a 500 Kj, 5 Hz KrF laser and could produce more than 100 MW of fusion power. It would optimize the target physics and demonstrate integration as well as be used to validate materials and sub modules in a fusion environment. It could be operating by 2025. Stage III would be a demonstration power plant based on the FTF, and would probably be led by industry. Acknowledgements. The work here was performed by over 60 researchers in the NRL Laser Plasma Branch and the US High Average Power Laser Program. Work supported by US Office of Naval Research and the US Department of Energy.
[en] A large anisotropy of the electron distribution function (EDF) was observed in high density plasma. The plasma was created by a 2ps TW KrF laser system in which a prepulse energy was well controlled with a saturable absorber. The observed electron density at the emission area with He-like F ions was 0.7-1.5x1022 cm-3. It is clarified with our experiments that, even with these high collisionality, the anisotropy of EDF was driven by the laser field. That means the anisotropy will become a parameter instead of temperature in the high density plasma which was far from the equilibrium states. However, the quantitative estimation of the anisotropy of EDF is not simple in the laser produced plasma because the upper states of the observed resonant line emission was created both with the excitation from the ground state and the deexcitation or the recombination from higher n states or higher-ionized ions. To overcome this difficulty, we selected the experimental condition carefully to separate these emission area in space and time. With this technique, the anisotropy of EDF was estimated with help of a cascade model for the recombining plasma and the usual polarization theory for ionizing plasmas. (author)
[en] Advances in high gain target designs for Inertial Fusion Energy (IFE), and the initiation of construction of large megajoule-class laser facilities in the U.S. (National Ignition Facility) and France (Laser-Megajoule) capable of testing the requirements for inertial fusion ignition and propagating burn, have improved the prospects for IFE. Accordingly, there have recently been modest increases in the US fusion research program related to the feasibility of IFE. These research areas include heavy-ion accelerators, Krypton-Fluoride (KrF) gas lasers, diode-pumped, solid-state (DPSSL) lasers, IFE target designs for higher gains, feasibility of low cost IFE target fabrication and accurate injection, and long-lasting IFE fusion chambers and final optics. Since several studies of conceptual IFE power plant and driver designs were completed in 1992-1996 [1-5], U.S. research in the IFE blanket, chamber, and target technology areas has focused on the critical issues relating to the feasibility of IFE concepts towards the goal of achieving economically-competitive and environmentally-attractive fusion energy. This paper discusses the critical issues in these areas, and the approaches taken to address these issues. The U.S. research in these areas, called IFE Chamber and Target Technologies, is coordinated through the Virtual Laboratory for Technology (VLT) formed by the Department of Energy in December 1998
[en] A KrF amplifier model verified against Nike laser data is used to design higher energy modules with segmented pumping. A 68 kJ nodule is designed, incorporating a new water line geometry and a combined switch/bushing. Two 68 kJ modules are combined in a 136 kJ multiplexed beamline incorporating incoherent spatial imaging that fits within a compact beam tunnel. A total of 16 such beamlines are arranged on four floors to deliver 64 beams to a target, the net energy being 2.0 MJ. The wallplug efficiency of this laser is 4%. Detailed calculations of prepulse ASE energy are given, and the levels are designed to be low enough not to initiate a prepulse plasma. The basic geometrical uniformity of target illumination is shown to be better than 0.3% for a 64 beam illumination geometry which has a high degree of symmetry. Amplification fidelity is verified. (author)
[en] Nike is a large angularly multiplexed Krypton-Fluoride (KrF) laser under development at the Naval Research Laboratory. It is designed to explore the technical and physics issues of direct drive laser fusion. When completed, Nike will deliver 2-3 kJ of 248 nm light in a 4 nsec pulse with intensities exceeding 2 x 10 14W/cm2 onto a planar target. Spatially and temporally incoherent light will be used to reduce the ablation pressure nonuniformities to less than 2% in the target focal plane. The Nike laser consists of a commercial oscillator/amplifier front end, an array of gas discharge amplifiers, two electron beam pumped amplifiers (one with a 20x20 cm2 aperture, the other with a 60x60 cm2 aperture) and the optics required to relay, encode, and decode the beam. Approximately 90% of the system is operational and currently undergoing tests: the system is complete through the 20 cm amplifier, the 60 cm amplifier has completed all the necessary electron beam/pulsed power tests, and is currently being developed into a laser amplifier, and most of the optics have been installed. It is anticipated that Nike will be fully operational in the fall of 1994
[en] An experimental investigation was made of the principal factors that determine the possibility to increase pulse repetition rates in high-power KrF lasers. Two prototypes of a compact industrial KrF laser were considered. The first one produces a maximum average output power of ∼620 W for a pulse repetition rate of 4 kHz; the second, a more compact prototype, yields a maximum pulse repetition rate of 5 kHz for an average output of 200 W. (lasers)