F. Weber

Lawrence Livermore National Laboratory, Livermore, California, United States

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Publications (63)120.56 Total impact

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    ABSTRACT: Processes during laser plasma experiments typically have time scales that are less than 100 ps. The measurement of these processes requires x-ray detectors with fast temporal resolution. We have measured the temporal responses and linearity of several different x-ray sensitive photoconductive detectors (PCDs). The active elements of the detectors investigated include both diamond (natural and synthetic) and GaAs crystals. The typical time responses of the GaAs PCDs are approximately 60 ps, respectively. Some characterizations using x-ray radiation from a synchrotron radiation source are presented.
    The Review of scientific instruments 11/2008; 79(10):10E304. · 1.58 Impact Factor
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    ABSTRACT: High temperature (“hot”) hohlraums are being developed to heat samples to high temperatures for opacity or other atomic physics studies. Hot hohlraums have been fielded at the National Ignition Facility [D.E. Hinkel, et al., Phys. Plasmas 12 (2005) 056305] and the OMEGA [M.B. Schneider, et al., Phys. Plasmas 13 (2006) 112701] lasers. They reach high radiation temperatures by coupling a maximum amount of laser energy (10 kJ) into small, i.e., 400–800 μm diameter, gold hohlraums in a 1 ns pulse causing the hohlraums to fill with gold plasma. Radiation temperatures of 370 eV have been measured in the laser entrance hole (LEH) region of these targets [D.E. Hinkel, et al., Phys. Rev. Lett. 96 (2006) 195001]. However, the LEH radiation is not the radiation drive of interest as the sample can neither be shielded from the non-thermal components of this radiation nor protected from the gold plasma. To mitigate these problems the source we are developing uses the radiation from the X-ray burnthrough of thin walls of a pair of hot hohlraums to heat a sample. We report on the measured radiation drive of this source and its use to heat a surrogate sample. We characterize the radiative heating of the sample by measuring its thermal expansion.
    High Energy Density Physics 05/2007; · 1.52 Impact Factor
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    ABSTRACT: A first set of shock timing, laser-plasma interaction, hohlraum energetics and hydrodynamic experiments have been performed using the first 4beams of the National Ignition Facility (NIF), in support of indirect drive Inertial Confinement Fusion (ICF) and High Energy Density Physics (HEDP). In parallel, a robust set of optical and X-ray spectrometers, interferometer, calorimeters and imagers have been activated. The experiments have been undertaken with laser powers and energies of up to 8TW and 17kJ in flattop and shaped 1–9ns pulses focused with various beam smoothing options. The experiments have demonstrated excellent agreement between measured and predicted laser-target coupling in foils and hohlraums, even when extended to a longer pulse regime unattainable at previous laser facilities, validated the predicted effects of beam smoothing on intense laser beam propagation in long scale-length plasmas and begun to test 3Dcodes by extending the study of laser driven hydrodynamic jets to 3Dgeometries.
    The European Physical Journal D 01/2007; 44(2):273-281. · 1.40 Impact Factor
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    ABSTRACT: An experimental campaign to maximize radiation drive in small-scale hohlraums has been carried out at the National Ignition Facility (NIF) at the Lawerence Livermore National Laboratory (Livermore, CA, USA) and at the OMEGA laser at the Laboratory for Laser Energetics (Rochester, NY, USA). The small-scale hohlraums, laser energy, laser pulse, and diagnostics were similar at both facilities but the geometries were very different. The NIF experiments used on-axis laser beams whereas the OMEGA experiments used 19 beams in three beam cones. In the cases when the lasers coupled well and produced similar radiation drive, images of x-ray burnthrough and laser deposition indicate the pattern of plasma filling is very different.
    Journal de Physique IV (Proceedings) 11/2006; J. Phys. IV France(133):1205. · 0.29 Impact Factor
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    ABSTRACT: Soft x-ray power diagnostics are essential for evaluating high temperature laser plasma experiments. The Dante soft x-ray spectrometer, a core diagnostic for radiation flux and temperature measurements of Hohlraums, installed on the Omega Laser Facility at the Laboratory for Laser Energetics has recently undergone a series of upgrades. Work performed at Brookhaven National Laboratory for the development of the National Ignition Facility (NIF) Dante spectrometer enables the Omega Dante to offer a total of 18 absolutely calibrated channels in the energy range from 50 eV to 20 keV . This feature provides Dante with the capability to measure higher, NIF relevant, radiation temperatures with increased accuracy including a differentiation of higher energy radiation such as the Au M and L bands. Diagnostic monitoring using experimental data from directly driven Au spherical shots is discussed.
    Review of Scientific Instruments 11/2006; · 1.58 Impact Factor
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    ABSTRACT: A first set of shock propagation, laser-plasma interaction, hohlraum energetics and hydrodynamic experiments have been performed using the first 4 beams of the National Ignition Facility (NIF), in support of indirect drive Inertial Confinement Fusion (ICF) and High Energy Density Physics.
    Journal de Physique IV (Proceedings) 06/2006; 133:43-45. · 0.29 Impact Factor
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    ABSTRACT: Experiments done at the National Ignition Facility laser [ J. A. Paisner, E. M. Campbell, and W. Hogan, Fusion Technol. 26, 755 (1994) ] using gas-filled hohlraums demonstrate a key ignition design feature, i.e., using plasma pressure from a gas fill to tamp the hohlraum-wall expansion for the duration of the laser pulse. Moreover, our understanding of hohlraum energetics and the ability to predict the hohlraum soft-x-ray drive has been validated in ignition-relevant conditions. Finally, the laser reflectivity from stimulated Raman scattering in the fill plasma, a key threat to hohlraum performance, is shown to be suppressed by choosing a design with a sufficiently high ratio of electron temperature to density.
    Physics of Plasmas 05/2006; 13(5). · 2.25 Impact Factor
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    ABSTRACT: Radiation drive diagnostics during the NIF early light campaign was supported by an 18 channel soft x-ray spectrometer (Dante). In order to achieve a measurement accuracy of 2% in radiation temperature absolute calibration of the individual channels was necessary and signal distortion through long transmission lines had to be compensated for as well. For fast signals the signal attenuation due to the long (50m) cables amounted to â 20% @ 100MHz, which was corrected by a cable compensation in the frequency domain. The varying effects of cable distortion for a variety of signals between 1ns and 9ns in length were evaluated and corrections were applied. Results of the thus calculated temperatures of the NEL campaign will be presented compared to LASNEX predictions, showing agreement in peak radiation temperature within less than 2%.
    01/2006;
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    ABSTRACT: The first hohlraum experiments on the National Ignition Facility (NIF) using the initial four laser beams tested radiation temperature limits imposed by plasma filling. For a variety of hohlraum sizes and pulse lengths, the measured x-ray flux shows signatures of filling that coincide with hard x-ray emission from plasma streaming out of the hohlraum. These observations agree with hydrodynamic simulations and with an analytical model that includes hydrodynamic and coronal radiative losses. The modeling predicts radiation temperature limits with full NIF (1.8 MJ), greater, and of longer duration than required for ignition hohlraums.
    Physical Review Letters 12/2005; 95(21):215004. · 7.73 Impact Factor
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    ABSTRACT: A platform for analysis of material properties under extreme conditions, where a sample is bathed in radiation with a high temperature, is under development. Depositing maximum laser energy into a small, high-Z enclosure produces this hot environment. Such targets were recently included in an experimental campaign using the first four of the 192 beams of the National Ignition Facility [ J. A. Paisner, E. M. Campbell, and W. J. Hogan, Fusion Technol. 26, 755 (1994) ], under construction at the University of California Lawrence Livermore National Laboratory. These targets demonstrate good laser coupling, reaching a radiation temperature of 340 eV. In addition, there is a unique wavelength dependence of the Raman backscattered light that is consistent with Brillouin backscatter of Raman forward scatter [ A. B. Langdon and D. E. Hinkel, Phys. Rev. Lett. 89, 015003 (2002) ]. Finally, novel diagnostic capabilities indicate that 20% of the direct backscatter from these reduced-scale targets is in the polarization orthogonal to that of the incident light.
    Physics of Plasmas 04/2005; 12(5):056305-056305-8. · 2.25 Impact Factor
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    ABSTRACT: A pump-probe scheme measures the rise time of ultrafast x-ray pulses. Conventional high speed x-ray diagnostics (x-ray streak cameras, PIN diodes, diamond PCD devices) do not provide sufficient time resolution to resolve rise times of x-ray pulses on the order of 50 fs or less as they are being produced by modern fast x-ray sources. Here, we are describing a pump-probe technique that can be employed to measure events where detector resolution is insufficient to resolve the event. The scheme utilizes a diamond plate as an x-ray transducer and a p-polarized probe beam.
    01/2005;
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    ABSTRACT: The Dante soft x-ray spectrometer, installed on the Omega laser facility at the Laboratory for Laser Energetics, University of Rochester, is a 12-channel filter-edge defined soft x-ray power diagnostic. It is used to measure the spectrally resolved, absolute flux from direct drive, indirect drive (hohlraums) and other plasma sources. Dante component calibration efforts using two beam lines, U3C (50 eV–1 keV) and X8A (1–6 keV) at the National Synchrotron Light Source have been implemented to improve the accuracy of these measurements. We have calibrated metallic vacuum x-ray diodes, mirrors and filters.
    Review of Scientific Instruments 11/2004; · 1.58 Impact Factor
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    ABSTRACT: The National Ignition Facility is operational at Lawrence Livermore National Laboratory (LLNL). The inertial confinement fusion and HED programs at LLNL have formed diagnostic research and development groups to institute improvements outside the charter of core diagnostics. We will present data from instrumentation being developed. A major portion of our work is improvements to detectors and readout systems. We have efforts related to charge-coupled device (CCD) development. Work has been done in collaboration with the University of Arizona to back thin a large format CCD device. We have developed in collaboration with a commercial vendor a large format, compact CCD system. We have coupled large format CCD systems to our optical and x-ray streak cameras leading to improvements in resolution and dynamic range. We will discuss gate width and uniformity improvements to microchannel plate-based framing cameras. We will present data from single shot data link work and discuss technology aimed at improvements of dynamic range for high-speed transient measurements from remote locations.
    Review of Scientific Instruments 10/2004; 75(10):4200-4203. · 1.58 Impact Factor
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    ABSTRACT: High energy (>10 keV) x-ray self-emission imaging and radiography will be essential components of many NIF high energy density physics experiments. In preparation for such experiments, we have evaluated the pros and cons of various static [x-ray film, bare charge-coupled device (CCD), and scintillator + CCD] and time-resolved (streaked and gated) 10-1000 keV detectors.
    Review of Scientific Instruments 10/2004; 75(10):4037-4039. · 1.58 Impact Factor
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    ABSTRACT: We present measurements of the absolute albedos of hohlraums made from gold or from high-Z mixtures. The measurements are performed over the range of radiation temperatures (70-100 eV) expected during the foot of an indirect-drive temporally shaped ignition laser pulse, where accurate knowledge of the wall albedo (i.e., soft x-ray wall reemission) is most critical for determining capsule radiation symmetry. We find that the gold albedo agrees well with calculations using the supertransition array opacity model, potentially providing additional margin for inertial confinement fusion ignition.
    Physical Review Letters 09/2004; 93(6):065002. · 7.73 Impact Factor
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    ABSTRACT: One key parameter in the design and optimization of ICF indirect drive experiments is the measurement of the absolute radiation drive flux inside the hohlraum. This has been accomplished on both Nova and Omega with the Dante x-ray spectrometer, DMX, and diamond PCDs. With the ability to measure the absolute soft x-ray flux for photon energies up to 3keV we can determine material albedo, laser to x-ray conversion efficiency and the characteristic blackbody temperature. The Dante x-ray spectrometer is the core diagnostic for absolute flux measurement on Omega and a new version capable of measuring photon energies up to 10 keV to cover the predicted fluxes will serve the same function on NIF. Each channel has a characteristic spectral response depending on the transmission of the filters, the reflectivity of the mirrors and the response of the x-ray diodes. Periodic calibrations on these three elements from 60 eV up to 6 keV have been performed for the last two years at the National Synchrotron Light Source at Brookhaven National Lab and compared to previous measurements. This allows us to update the Dante analysis with the latest measured response functions. The Dante measured peak radiation temperatures for various Nova and Omega hohlraum experiments have been compared using the Marshak scaling method and agree well with each other for the expected laser conversion efficiencies of up to 90% for Au hohlraums. This method will be used starting with the early NIF Dante to bridge the experimental data between facilities.
    10/2003;
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    ABSTRACT: The inner‐shell photo‐ionization (ISPI) scheme requires photon energies at least high enough to photo‐ionize the K‐shell, ∼286 eV, in the case of carbon. As a consequence of the higher cross‐section, the inner‐shell are “selectively” knocked out, leaving a hole state 1s2s22p2 in the singly charged carbon ion. This generates a population inversion to the radiatively connected state 1s22s22p in C+, leading to gain on the 1s‐2p transition at 45 Å. The resonant character of the lasing transition in the single ionization state intrinsically allows much higher quantum efficiency compared to other schemes. Competing processes that deplete the population inversion include auto‐ionization, Auger decay, and in particular collisional ionization of the outer‐shell electrons by electrons generated during photo‐ionization. These competing processes rapidly quench the gain. Consequently, the pump method must be capable of populating the inversion at a rate faster than the competing processes. This can be achieved by an ultra‐fast, high intensity laser that is able to generate an ultra‐fast, bright x‐ray source. With current advances in the development of high‐power, ultra‐short pulse lasers it is possible to realize fast x‐ray sources based that can deliver powerful pulses of light in the multiple hundred terawatt regime and beyond. We will discuss in greater detail concept, target design and a series of x‐ray spectroscopy investigations we have conducted in order to optimize the absorber/x‐ray converter ‐ filter package. © 2002 American Institute of Physics
    AIP Conference Proceedings. 11/2002; 641(1):342-348.
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    ABSTRACT: This report summarizes the major accomplishments of this three-year Laboratory Directed Research and Development (LDRD) Lab Wide (LW) project entitled, ''An Inner-Shell Photo-Ionized X-Ray Laser at 45 {angstrom}'', tracking code 99-LW-042. The most significant accomplishments of this project include the design of a suitable x-ray laser target, the invention of a measurement technique for the determination of rise times of x-ray pulses on the order of 50 femtoseconds, and a novel setup for generating a traveling wave with an ultrashort optical laser pulse. The pump probe technique for rise time measurement will allow us to detect ultrashort x-ray pulses, whose generation by means of a variety of 4th generation light sources is currently under planning elsewhere.
    01/2002;
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    ABSTRACT: Initial laser-driven equation of state (EOS) experiments on liquid deuterium employed x-ray radiography to track the shock and particle speeds in the shock compressed sample. With the high pressures available with laser drivers we found that it is also possible to track the shock front directly with a velocity interferometer system for any reflector (VISAR) because the shock front reflects light across the visible spectrum with reflectance around 50% for shocks stronger than 50 GPa in liquid deuterium. We have observed similar reflectances in other dielectric samples, such as diamond, LiF, and water. The pressure required to produce a reflecting shock varies with each material. This phenomenon allows us to design impedance-matched EOS experiments using velocity interferometry to measure the propagation speed in the transparent shocked materials, and step breakout measurements to determine the speed in the pusher. In a different kind of experiment we have observed double shock compression in liquid deuterium by impacting a shock in liquid deuterium at a LiF anvil placed in the liquid sample. VISAR can be used to track the shock in the deuterium as well as the motion of the deuterium-LiF interface subsequent to impact. This allows us to diagnose double-shock states using the VISAR technique. As a final example VISAR can be used to track shock overtake events such as produced by shaped pulse compression or shock reverberation effects in the accelerating pusher. This capability is directly applicable to shock timing experiments needed to tune the drive pulse for inertial confinement fusion capsules on the National Ignition Facility.
    Review of Scientific Instruments 01/2001; 72(1):1038-1038. · 1.58 Impact Factor
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    ABSTRACT: A variety of plasma physics experiments require the recording of continuous time history of x-ray emission. Many laboratories have developed x-ray streak camera technology in order to time resolve x-ray spectra or images produced by laser-driven plasma experiments. These cameras record x rays by converting photons to electrons, which in turn are focused and swept across an electron sensitive area detector as a function of time. X-ray photons impinging on a transmission type photocathode generate photoelectrons which are accelerated to energies between 10 and 20 keV and focused onto a phosphor screen. The light from the phosphor image may be intensified using a microchannel plate, and is usually optically coupled directly onto film or an optical charge coupled device. We have designed and built an x-ray sensitive streak camera readout where we replaced the microchannel plate based intensifier and film package with a modified charge coupled device area detector to directly absorb accelerated photoelectrons emitted from the cathode. This system has been integrated into the streak tube arrangement. We will present a set of system performance data, which have been obtained from both bench top experiments on a dc source and dynamic measurements at the Nova laser facility. X-ray images at various exposure times show better spatial resolution, improved signal to noise ratio, and higher dynamic range. Other advantages include instantaneous data readout, which enables fast postprocessing, and no increase in overall cost for an engineered system.
    Review of Scientific Instruments 01/2001; 72(1):651-651. · 1.58 Impact Factor