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  • 11
    Electronic Resource
    Electronic Resource
    Measurement of areal density modulation of laser-imploded shells through K-edge imaging (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 3727-3735 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A new method for studying the modulations in the ρΔR of imploded shells is introduced: using a recently developed pinhole-array x-ray spectrometer, core images are obtained at energies below and above the K-edge energy of a titanium dopant. The ratio of these images reflects the nonuniformity of the cold shell around the time of peak compression, independently of modulation in core emission. The two-dimensional images of ρΔR modulations are of interest in laser fusion because they show the final result of unstable implosion evolution. The measured average ρΔR of the cold shell and the amplitude and spectrum of its modulations are in agreement with the results of two-dimensional code simulations that include imprinting of laser nonuniformity on the target. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1287916
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  • 12
    Electronic Resource
    Electronic Resource
    A high-resolution x-ray microscope for laser-driven planar-foil experiments (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 1118-1124 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A soft x-ray microscope (E(approximately-less-than)3 keV) with high spatial resolution (∼3 μm) has been characterized at the University of Rochester's Laboratory for Laser Energetics and used for initial experiments on the Omega laser system [Boehly et al., Opt. Commun. 133, 495 (1997)] to study the hydrodynamic stability of directly driven planar foils. The microscope, which is an optimized Kirkpatrick–Baez-type design, is used to obtain four x-ray radiographs of laser-driven foils. Time-resolved images are obtained with either custom-built framing cameras (time resolution ∼80 ps) or by using short-pulse backlighter beams (Δt(approximately-less-than)200 ps). In the former case, a spatial resolution of ∼7 μm was obtained (limited by the framing camera), while in the latter case a resolution of ∼3 μm was obtained. This paper details the testing, calibration, and initial use of this microscope in the laboratory and on Omega. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872642
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  • 13
    Electronic Resource
    Electronic Resource
    Measurements of core and pusher conditions in surrogate capsule implosions on the OMEGA laser system : The 39th annual meeting of division of plasma physics of APS (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 1870-1879 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Experiments have been carried out on the 60-beam, 30 kJ OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] as part of an integrated program to diagnose all phases of direct-drive capsule implosions. Laser-imprint levels and Rayleigh–Taylor growth rates associated with the spherical implosions have been inferred from planar-foil radiography experiments. In spherical targets, measurements of the combined effects of imprint and unstable growth at the ablation surface have been carried out using the burnthrough technique [J. Delettrez et al., Phys. Plasmas 1, 2342 (1994)]. Target behavior during the deceleration phase has been investigated using a series of surrogate cryogenic capsules in which the main fuel layer is represented by a Ti-doped CH shell and the hot spot is represented by an Ar-doped deuterium fill gas. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872858
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  • 14
    Electronic Resource
    Electronic Resource
    Compressed-shell integrity measurements in spherical implosion experiments (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 2872-2882 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The shell integrity near peak compression of spherical implosions using the 60-beam, 30-kJ UV OMEGA laser system [Opt. Commun. 133, 495 (1997)] has been measured. Hot core emission backlights a shell with a thin titanium-doped layer that is imaged at x-ray photon energies above and below the titanium K edge. The x-ray intensity ratio between the two images is related to perturbations in the cold, or absorbing, part of the shell. The measured cold-shell areal-density modulations, integrated over the time of peak compression, are of the order of 25% to 50% with nonuniformity spectra peaked at spatial wavelengths of 30 to 50 μm and with the smallest detectable nonuniformity features extending down to spatial wavelengths of 12 to 15 μm. Hot-shell areal-density modulations of the emitting part of the shell (inner edge) are of the order of 13% to 20%. The measured shell modulations are in agreement with the results of two-dimensional simulations that include initial shell perturbations, imprinted shell modulations due to nonuniformities in a single laser beam, and a beam-to-beam energy imbalance in the laser drive. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1368141
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  • 15
    Electronic Resource
    Electronic Resource
    Core performance and mix in direct-drive spherical implosions with high uniformity : The 42nd annual meeting of the division of plasma physics of the American Physical Society and the 10th international congress on plasma physics (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 2251-2256 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The performance of gas-filled, plastic-shell implosions has significantly improved with advances in on-target uniformity on the 60-beam OMEGA laser system [T. R. Boehly, D. L. Brown, R. S. Craxton et al., Opt. Commun. 133, 495 (1997)]. Polarization smoothing (PS) with birefringent wedges and 1-THz-bandwidth smoothing by spectral dispersion (SSD) have been installed on OMEGA. The beam-to-beam power imbalance is ≤5% rms. Implosions of 20-μm-thick CH shells (15 atm fill) using full beam smoothing (1-THz SSD and PS) have primary neutron yields and fuel areal densities that are ∼70% larger than those driven with 0.35-THz SSD without PS. They also produce ∼35% of the predicted one-dimensional neutron yield. The results described here suggest that individual-beam nonuniformity is no longer the primary cause of nonideal target performance. A highly constrained model of the core conditions and fuel–shell mix has been developed. It suggests that there is a "clean" fuel region, surrounded by a mixed region, that accounts for half of the fuel areal density. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1350964
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  • 16
    Electronic Resource
    Electronic Resource
    Signatures of target performance and mixing in titanium-doped, laser-driven target implosions (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 3021-3030 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Absorption-spectroscopy measurements using an embedded titanium layer have been performed on laser implosions of spherical shells. Predicted absorption features formed by the titanium layer were observed: absorption lines due to 1s–2p transitions in titanium ions of incomplete L shell, as well as K-edge absorption in cold titanium. Also observed were oscillations due to EXAFS (Extended X-Ray Absorption Fine Structure) above the Ti K edge. The core temperature and shell areal density and temperature are derived from the measurements and are found to fall short of the values predicted by one-dimensional code simulations. This shortfall is attributed to the lack of irradiation symmetry, which leads to hydrodynamic instability and mixing (smoothing by phase plates was not used in this experiment). Signatures of mixing include the emission of the He-α line of Ti+20 due to titanium migrating to the core, as well as the EXAFS spectrum indicating cold titanium close to peak compression.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872443
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  • 17
    Electronic Resource
    Electronic Resource
    Direct-drive high-convergence-ratio implosion studies on the OMEGA laser system : The 41st annual meeting of the division of plasma physics of the american physical society (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 2108-2113 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A series of direct-drive implosion experiments, using room-temperature, gas-filled CH targets, are performed on the University of Rochester's OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. The target performance at stagnation and its dependence on beam smoothing and pulse shaping is investigated. Compressed core conditions are diagnosed using x-ray and neutron spectroscopy, and x-ray imaging. The individual beams of OMEGA are smoothed by spectral dispersion in two dimensions (2D SSD) with laser bandwidths up to ∼0.3 THz, with 1 ns square to 2.5 ns shaped pulses. A clear dependence of target performance on pulse shape and beam smoothing is seen, with the target performance (yield, areal density, and shell integrity) improving as SSD bandwidth is applied.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.874032
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  • 18
    Electronic Resource
    Electronic Resource
    First results from cryogenic target implosions on OMEGA : Review,Tutorial and Invited Papers from the 43rd Annual Meeting of the APS Division of Plasma Physics (2002)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 9 (2002), S. 2195-2201 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Initial results from direct-drive spherical cryogenic target implosions on the 60-beam OMEGA laser system [T. R. Boehly, D. L. Brown, R. S. Craxton et al., Opt. Commun. 133, 495 (1997)] are presented. These experiments are part of the scientific base leading to direct-drive ignition implosions planned for the National Ignition Facility (NIF) [W. J. Hogan, E. I. Moses, B. E. Warner et al., Nucl. Fusion 41, 567 (2001)]. Polymer shells (1-mm diam with walls 〈3 μm) are filled with up to 1000 atm of D2 to provide 100-μm-thick ice layers. The ice layers are smoothed by IR heating with 3.16-μm laser light and are characterized using shadowgraphy. The targets are imploded by a 1-ns square pulse with up to ∼24 kJ of 351-nm laser light at a beam-to-beam rms energy balance of 〈3% and full-beam smoothing. Results shown include neutron yield, secondary neutron and proton yields, the time of peak neutron emission, and both time-integrated and time-resolved x-ray images of the imploding core. The experimental values are compared with 1-D numerical simulations. The target with an ice-layer nonuniformity of σrms=9 μm showed 30% of the 1-D predicted neutron yield. These initial results are encouraging for future cryogenic implosions on OMEGA and the NIF. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1458586
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  • 19
    Electronic Resource
    Electronic Resource
    Direct-drive, hollow-shell implosion studies on the 60-beam, UV OMEGA laser system (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 1006-1013 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Direct-drive implosion experiments have been performed on the University of Rochester's 60-beam, 30 kJ, UV (351 nm) OMEGA [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] laser system to investigate the conditions at maximum compression of polymer–shell targets with zero- or low-pressure (≤3 atm) gas fills. By using deuterium-bearing shells (CD), the imploded-core conditions have been diagnosed with both x-ray and neutron spectral measurements. The core electron temperature (kTe) and shell areal density (ρRshell) are determined from the emergent x-ray spectrum, while independent inferences of ρRshell are obtained from the measured primary (DD) and secondary (DT) neutron yields. Target performance was investigated for a range of beam-smoothing conditions [none to 0.25 THz-bandwidth smoothing by spectral dispersion along two dimensions (2-D SSD)] and a set of pulse shapes (1 ns square pulse to a 2.5 ns pulse with a 1:40 foot-to-main-pulse power ratio). The results have conclusively demonstrated the ability to compress targets to shell areal densities in the range of ∼60 to 130 mg/cm2. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873900
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  • 20
    Electronic Resource
    Electronic Resource
    Calibration system for electron detectors in the energy range from 10 eV to 50 keV (1986)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 57 (1986), S. 229-235 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A system has been developed to provide accurate calibration of electron detectors measuring in the energy range from approximately 10 eV to 50 keV. The principal component of the system is a large area (730 cm2), monoenergetic electron beam which is tunable in energy. This beam is produced by illuminating, in vacuum, a thin gold film deposited onto a quartz flat with ultraviolet light, such that photoelectrons are ejected from the gold surface. With the gold film floated at an adjustable negative voltage, the photoelectrons ejected from the surface are accelerated towards an electrically grounded screen which is positioned in front of the quartz flat. All high voltages in the beam system are guarded so as to eliminate any significant leakage current. The current leaving the gold surface which determines the beam intensity is measured using a specially designed picoammeter. Electron beam intensities of ∼106 electrons/cm2 s are produced by illuminating the gold photocathode with light from a 2-W mercury vapor lamp which has 90% of its output at 2537 A(ring). The resulting beam has an energy spread of 0.41 eV. Electron beam intensities of ∼107 electrons/cm2 s are produced with a 200 W medium pressure mercury arc lamp. For this case, the beam has a width in energy of 0.55 eV. Detectors to be calibrated are mounted within a set of computer-controlled rotational and translational tables. The tables allow the look direction of the detector to be oriented with respect to the beam in two orthogonal angles and two orthogonal directions. The entire calibration system operates within a set of Helmholtz coils that allow the Earth's magnetic field to be canceled. The voltage on the gold surface, the angular and spatial positioning of the detector, and the accumulation and display of the data are all controlled by a minicomputer.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1138975
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