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  • 1
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: In this article, we present how ion acoustic waves can be unambiguously identified and can be used to definitely demonstrate the existence of the Langmuir decay instability (LDI) of electron plasma waves associated with stimulated Raman scattering. This observation is very important in the context of inertial confinement fusion because the LDI is a mechanism that may influence the stimulated Raman scattering saturation and growth. The experimental identification of those waves relies on Thomson scattering of a short wavelength probe beam. The Thomson scattered light was analyzed by two sets of spectrometer and streak camera providing information on the density fluctuations associated with the electron plasma waves and ion acoustic waves with temporal and spectral resolutions. Selection of a narrow range of angles (4° of aperture) inside the probe and scattered light apertures provided necessary resolution in wave vector. The method used for the design of the Thomson scattering diagnostic is described as well as the technique used to clearly identify the observed ion acoustic waves as one of the Langmuir decay instability products. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A simpler and faster method for determining the total radiated power emitted from a tokamak plasma in real-time has been developed. This quantity is normally calculated after the discharge by a deconvolution of line integrals from a bolometer camera. This time-consuming algorithm assumes constant emissivity on closed flux surfaces and therefore needs the exact magnetic equilibrium information. Thus, it is highly desirable to have a different, simpler way to determine the total radiated power in real-time without additional magnetic equilibrium information. The real-time calculation of the total radiated power is done by a summation over ten or 18 lines of sight selected out of a bolometer camera with 40 channels. The number of channels is restricted by the summation hardware. A new selection scheme, which uses a singular value decomposition, has been developed to select the required subset of line integrals from the camera. With this subset, a linear regression analysis was done against the radiated power calculated by the conventional algorithm. The selected channels are finally used with the regression coefficients as weighting factors to determine an estimation of the radiated power for subsequent discharges. This selection and the corresponding weighting factors can only be applied to discharges with a similar plasma shape, e.g., in our case the typical ASDEX upgrade elliptical divertor plasma. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    Journal of Mathematical Physics 40 (1999), S. 408-426 
    ISSN: 1089-7658
    Source: AIP Digital Archive
    Topics: Mathematics , Physics
    Notes: We show that a left-rigid monoidal C* category with irreducible monoidal unit is also a sovereign and spherical category. Defining a Frobenius–Schur-type indicator we obtain selection rules for the fusion coefficients of irreducible objects. As a main result we prove S4 invariance of 6j symbols in such a category. © 1999 American Institute of Physics.
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  • 4
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A model for stimulated Brillouin (SBS) and Raman (SRS) backscattering of a spatially smoothed laser beam interacting with a collisional, inhomogeneous, expanding plasma is presented. It is based on the independent hot spots description [H. A. Rose and D. F. DuBois, Phys. Rev. Lett. 72, 2883 (1994)], in which the overall plasma reflectivity is assumed to be a sum of the individual speckle reflectivities. Self-focusing is taken into account in the computation of the speckle intensity profile and reflectivities. Two additions have been made to previous similar theories: (i) the thermal effects are retained along with the ponderomotive force for what concerns speckle self-focusing, and (ii) SRS (convective and absolute) is accounted for in calculations of the speckle reflectivity. The model is benchmarked against recent laser–plasma experiments at Laboratoire pour l'Utilisation des Lasers Intenses, at École Polytechnique, France, with well-characterized interaction conditions. A good agreement is found between the experimental SBS levels and the model calculations using the measured plasma parameters. This agreement applies for two types of beam smoothing techniques, random phase plates, and polarization smoothing, various plasma densities, and laser energies. Self-focusing itself, and thermal effects in it, play both a fundamental role in defining the level of plasma backscattering. The absolute Raman instability in speckles dominates the SRS response. The model predictions for the SRS reflectivity are less satisfactory, although they demonstrate the same trends as the experimental data. It follows from model calculations and experimental data that the polarization smoothing technique provides an efficient method of control of parametric instabilities allowing a reduction of several times in the level of SBS and SRS reflectivities. © 2001 American Institute of Physics.
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  • 5
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Propagation of a high-contrast frequency-doubled subpicosecond (300 fs) relativistic (Iλ2 up to 5×1018 W⋅cm−2⋅μm2) laser pulse through thin and initially solid foils is studied. Transmission values up to 10% are measured through targets with initial near solid densities. The strong intensity threshold observed for the transmitted energy is correlated with clear modifications of the transmitted and reflected spectra, electron generation, and beam imaging. Two-dimensional Cartesian particle-in-cell (PIC) simulations that qualitatively reproduce the experimental results suggest specific rapid heating of the thin targets by fast electrons, plasma expansion, and density decrease to relativistically transmissive conditions during the pulse. © 1999 American Institute of Physics.
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  • 6
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 2563-2568 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Propagation of a high-contrast subpicosecond (400–600 fs) relativistic (Iλ2 up to 2×1019 W⋅cm−2⋅μm2) laser pulse through long preformed overdense plasmas is experimentally studied. Transmission values up to 10% are measured in plasmas with initial peak densities above 10×nc. We compare these results with one-dimensional (1D) analytical calculations of the hole boring effect. It is shown that the ponderomotive force of the laser beam can push forward the overdense plasma, create a channel, and lead to transmission levels similar to the experimental ones. © 1999 American Institute of Physics.
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    Journal of Mathematical Physics 32 (1991), S. 1703-1708 
    ISSN: 1089-7658
    Source: AIP Digital Archive
    Topics: Mathematics , Physics
    Notes: A similarity analysis of the nonlinear nonstationary real MHD equations is presented. For several cases of real MHD the complete Lie algebra of infinitesimal symmetries is calculated. Using subalgebras of this Lie algebra, reduced systems for similarity solutions can be obtained. Some similarity solutions of the ideal MHD equations are discussed.
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  • 8
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Experiments have been conducted at the LULI (Laboratoire pour l'Utilisation des Lasers Intenses) multibeam laser facility to study in detail stimulated Brillouin (SBS) and Raman (SRS) scattering from an intense (mean average intensity up to 1014 W/cm2) long (600 ps full width at half-maximum) laser beam interacting with thin exploded plastic foils. The plasmas are well characterized and the vacuum laser intensity distribution is well known due to using either random phase plates or polarization smoothing. Direct and simultaneous Thomson scattering measurements of the associated plasma waves allow us to obtain detailed information about the SBS and SRS temporal evolution and spatial localization. These data are being used to benchmark a statistical model of SBS and SRS from self-focused speckles. The results of this comparison will be presented in a companion paper. The analysis shows that both SBS and SRS are originated from self-focused speckles and reveals that plasma heating has an important effect on speckle self-focusing. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
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