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  • 1990-1994  (5)
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  • 1
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An amplified spontaneous emission (ASE) from Nd:glass has been introduced into the high power twelve beam Nd:glass laser system, Gekko XII for obtaining smooth intensity distribution of a focused beam. The angular dispersion of an ASE spectrum with large beam divergence was adopted for efficient beam smoothing without significant reduction of the harmonic conversion efficiency. Temporal evolution of the beam smoothing was evaluated as a function of the beam divergence by using a statistical model of speckle. In Gekko XII, the spectral width and beam divergence of ASE were controlled in a range of 0.4 to 0.6 nm and 6 to 22 times diffraction limited, respectively. Final output energy of 1.3 kJ/beam in a 2.2 ns duration was demonstrated without significant gain reduction and spectral narrowing. The doubling efficiency of 50% was obtained at a low intensity region of around 0.3 GW/cm2 by matching the angular dispersion of spectrum to that of phase matching condition of a frequency conversion crystal. The speckle structures in the focused beam pattern measured at the equivalent target plane were greatly smoothed out due to the introduced angular dispersion of spectrum and the large beam divergence. The standard deviation of the intensity distribution of the focused beam pattern was estimated to be 3.3%.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 74 (1993), S. 802-808 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Intractable low-mode nonuniformities caused by power imbalance or pointing error of beams are studied quasi-analytically based on Skupsky's axially symmetric model [S. Skupsky and K. Lee, J. Appl. Phys. 54, 3662 (1983)]. These nonuniformities can be improved by decreasing the imperfections σPΩ, or by increasing the number of laser beams NB: deteriorated irradiation uniformity is shown to be proportional to σPΩ/(square root of)NB. Criteria of these imperfections for high irradiation uniformity [≤1% root-mean square (rms)] are then presented in terms of NB. The model is further extended to treat general asymmetric beam patterns. Optimization of beam pattern is also a central issue.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Improvement of laser irradiation uniformity in the Gekko XII system [IEEE J. Quantum Electron. QE-17, 1639 (1981)], both single beam pattern and power balance, is discussed. Substantial reduction of laser absorption nonuniformity is obtained for spherical harmonic modes greater than 15 by introducing spectrally dispersed amplified spontaneous emission. No perturbation growth is observed in flat foils accelerated by spectrally dispersed amplified spontaneous emission. Dependences of laser absorption uniformity on beam pattern and power imbalance are investigated in detail. The design goal of the power imbalance in the precision Gekko XII system is 3% in peak to valley, and laser absorption nonuniformity is estimated to be a few percent. Growth of hydrodynamic instabilities is analyzed for high convergence ratio implosions in the precision Gekko XII system.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 417-422 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The kinetic effects of electron heat transport in an imploding plasma are investigated by using the one-dimensional hydrodynamics implosion code with the Fokker–Planck equation. The Fokker–Planck equation is solved in the implosion simulation by reducing it to the multigroup transport equations. The implosion time, mass ablation rates, and compressed core plasma parameters of the simulations are compared with those of the flux-limited Spitzer–Härm model simulations and the experimental results of high-density compression. As a result, it is shown that the Fokker–Planck simulation results agree well with the experiments, in which the random phase plate is employed for the uniform implosion.
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A series of laser fusion implosion experiments of plastic hollow shell targets was performed by using the Gekko XII glass laser in order to achieve the required fuel areal density for ignition. Introducing random phase plates to improve illumination uniformity, high-density compression of more than 600 times deuterium liquid density has been achieved. The implosion dynamics and symmetry were observed with a spatially resolved x-ray streak camera and an x-ray multiframing camera. The three-dimensional emission profile of the laser-heated plasma was reconstructed from the x-ray images by use of computed tomography and was compared with the laser illumination profiles. The areal density of the imploded core was measured by the neutron activation of a silicon tracer, the secondary reaction method, and the knock-on proton method. Although the measured density and areal density were consistent with those from 1-D hydrodynamic simulation, experimental neutron yields were significantly lower than those predicted by the simulation for convergence ratios larger than 20. This suggests that better implosion uniformity is required to create a hot spark.
    Type of Medium: Electronic Resource
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