R. Bahr

University of Rochester, Rochester, NY, United States

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Publications (61)84.67 Total impact

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    ABSTRACT: The path to successful inertial confinement fusion (ICF) requires to observe and control the micro balloon deformations. This will be achieved using X-ray microscope among other diagnostics. A high resolution, high energy X-ray microscope involving state-of-the-art toroidal mirrors and multilayer coatings is described. Years of experiments and experience have led to a small-scale X-ray plasma imager that proves the feasibility of all the features required for a LMJ diagnostic: spatial resolution of 5μm, broad bandwidth, millimetric field of view (FOV). Using the feedback given by this diagnostic, a prototype for the Laser MegaJoule (LMJ) experiments has been designed. The experimental results of the first diagnostic and the concepts of the second are discussed.
    Proc SPIE 10/2012;
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    ABSTRACT: The effect of laser--plasma interactions in the underdense coronal plasma on direct-drive target performance has been systematically studied on OMEGA. Room-temperature D2-filled, 27-μm-thick plastic shells were irradiated using triple-picket laser pulse shapes. The intensity on the main pulse is varied between 3.5 x 10^14 W/cm^2 and 1.1 x 10^15 W/cm^2, while picket energies are kept nominally the same to maintain similar shell adiabat in all designs. Time-resolved reflected light and its spectrum, neutron-rate histories, areal densities, ion temperatures, neutron yield, and time-resolved hard x-ray signals have been simultaneously measured on these implosions. At lower intensities below the two-plasmon-decay (TPD) threshold, only cross-beam transfer induced by laser--plasma interactions influences target performance, whereas both affect target performance at higher intensities. In particular, fast-electrons generated by TPD can potentially preheat the shell reducing compression at high intensities (˜1 x 10^15 W/cm^2). This work was supported by the U.S. Department of Energy under Cooperative Agreement Nos. DE-FC02-04ER54789 and DE-FC52-08NA28302.
    11/2011;
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    ABSTRACT: Time-resolved scattered-light spectroscopy from spherical-target implosions on OMEGA provides information about the time-dependent absorbed power and the spectral shift of the scattered light. Modeled spectra reproduce the major features in the observed spectral shifts but not the absolute magnitudes of the predicted spectral shifts and the total scattered light, suggesting that modeling overpredicts absorption (typically a difference of 10% to 15% of the total time-integrated laser-pulse power). Cross-beam energy transfer appears to explain the discrepancy and its effect on the absorbed power, scattered-light spectra, and bang-time and is examined for a series of implosions with varying intensity. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.
    11/2009;
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    ABSTRACT: Experiments show that application of laser smoothing schemes including smoothing by spectral dispersion and polarization smoothing effectively suppresses stimulated Raman scattering from a 2ω (527 nm) laser beam in a low average-gain plasma with a steep density gradient. Full-wave simulations reproduce the observed trends in the data and show that the scattering reduction is an indirect result of suppressing active filamentation.
    Physics of Plasmas 06/2009; 16(6):062704-062704-6. · 2.38 Impact Factor
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    ABSTRACT: A high-resolution x-ray imager (HRXI) devoted to laser-plasma experiments combines two state-of-the-art technologies developed in France: a high-resolution x-ray microscope and a high-speed x-ray streak camera. The resulting streaked imager achieves spatial and temporal resolutions of approximately 5 microm and approximately 10 ps, respectively. The HXRI has recorded enhanced spatial and temporal resolution radiographs of indirectly driven targets on OMEGA. This paper describes the main features of the instrument and details the activation process on OMEGA (particularly the alignment). Recent results obtained on joint CEA/LLE radiographic OMEGA experiments will also be presented.
    The Review of scientific instruments 11/2008; 79(10):10E904. · 1.52 Impact Factor
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    ABSTRACT: Many laser beams directly illuminate a spherical target in direct-drive inertial confinement fusion experiments, ionizing the outer surface and surrounding the target with a plasma containing an opaque critical surface. In the earliest stages of irradiation, before the plasma forms a critical-density surface, laser light can penetrate into the target. This “shinethrough” light can be sufficiently intense to undergo filamentation and damage the inside of the target, thereby seeding hydrodynamic instabilities. Laser shinethrough can be blocked by a thin coating of opaque material, such as, aluminum (Al). For cryogenic direct-drive targets, the shinethrough barrier material must also be compatible with cryogenic target fabrication procedures, which rules out Al layers since they would interfere with permeation filling and optical characterization of cryogenic targets. Silicon (Si) has been found to be a promising candidate for a direct-drive cryogenic target shinethrough barrier material. Several cryogenic targets have been coated with Si, successfully permeation filled with either deuterium (D2) or deuterium-tritium (DT), and subsequently layered and optically characterized. Various thicknesses of Si coatings have been applied to planar targets and tested under relevant irradiation conditions. Experiments have shown that 200 Å of Si is sufficient to protect targets from laser shinethrough.
    Physics of Plasmas 09/2008; 15(9):092704-092704-6. · 2.38 Impact Factor
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    ABSTRACT: Time-dependent and time-integrated absorption fractions are inferred from scattered-light measurements in room-temperature and cryogenic direct-drive-implosion experiments on OMEGA. The measurements agree reasonably well with hydrodynamic simulations that include nonlocal electron-heat transport. Discrepancies in the time-resolved scattered-light spectra between simulations and experiments remain for complex laser pulse shapes, indicating beam-to-beam energy transfer and commensurate coupling losses. Time-resolved scattered-light spectra near omega/2 and 3omega/2 as well as time-resolved hard-x-ray measurements indicate the presence of a strongly driven two-plasmon-decay (TPD) instability at high intensities that may influence the observed laser light absorption. Experiments indicate that energetic electron production due to the TPD instability can be mitigated with high-Z-doped plastic shells.
    Physics of Plasmas 01/2008; 15. · 2.38 Impact Factor
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    ABSTRACT: Analysis of time-resolved, scattered-light spectra from spherical target implosions on OMEGA reveals a discrepancy between the precision measurements and predictions based on hydrocode modeling. The total scattered light measured and the details of the spectral shifts in the scattered light suggest that less power is being absorbed by the target than predicted. Contradictorily, the model accurately predicts ``bang times'' suggesting that laser absorption is well modeled. Nonlinear LPI behavior that differs for portions of the beams penetrating to different depths in the corona may provide an explanation. Cross-beam power transfer and steepening of the density profile at the critical and/or quarter-critical surfaces are investigated in attempts to reconcile the observations. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.
    01/2008;
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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. · 1.51 Impact Factor
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    ABSTRACT: The time-dependent laser absorption during spherical direct-drive implosions on OMEGA is inferred from scattered-light spectroscopy. We compare measured spectral shifts for different pulse shapes with the shifts predicted using a hydrodynamic code. The predictions vary dramatically with the electron-heat-conduction model. A nonlocal transport model provides the best match to the measurements. The modeling calculates the ``blow-by'' signal from the beam opposite the detector, improving the measurements of total scattered light. Remaining spectral discrepancies suggest nonlinear energy exchange between crossed beams due to stimulated Brillouin scattering. Analogous planar experiments test this hypothesis. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement DE-FC52-92SF19460.
    01/2007;
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    ABSTRACT: The radiation temperature achieved inside a hohlraum, a high-Z cylindrical cavity heated by high-power lasers, is limited by plasma filling of ablated wall material. Recent work [ Dewald et al., Phys. Rev. Lett. 95, 215004 (2005) ] tested radiation temperature limits in a simple on-axis laser-hohlraum geometry and validated an analytic plasma-fill model. The experiments reported here use several cones of beams to heat a 600 μm diameter hohlraum. Thin-walled images show the time evolution: plasma stagnation followed by plasma filling of the hohlraum cavity. Features in the Raman backscatter spectra are correlated to the thin-walled images to measure a fill time. The quantity of hard x rays produced by hot electrons is proportional to the time left in the laser pulse after the fill time. Simulations using the radiation-hydrodynamic code LASNEX and the analytic plasma-fill model predict plasma filling consistent with the data. LASNEX predicts a much higher electron temperature than the analytic model.
    Physics of Plasmas 11/2006; 13(11):112701-112701-8. · 2.38 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.
    http://dx.doi.org/10.1051/jp4:2006133247. 11/2006; J. Phys. IV France(133):1205.
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    ABSTRACT: Imaging Thomson scattering has been implemented on the Omega Laser facility at the Laboratory for Laser Energetics, University of Rochester [J. M. Soures etal, Laser and Particle Beams 11, 317 (1993)]. Using a high-resolution spectrometer and an intensified charge-coupled device, the electron temperature profile, ranging from 1.5 to 3 keV , was measured along the axis of a CH gas-filled hohlraum. The system integrates over 200 ps a region 2 mm in length and has a spatial resolution of 50 μ m and a spectral resolution of 0.027 nm . Data obtained from this diagnostic are compared to hydrodynamic simulations.
    Review of Scientific Instruments 11/2006; · 1.60 Impact Factor
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    ABSTRACT: The laser light from a single beam transmitted through a plasma is collected by a 3ω transmitted beam diagnostic which is now operational on the Omega Laser Facility at the Laboratory for Laser Energetics, University of Rochester [ Soures et al., Laser. Part. Beams 11, 317 (1993) ]. Transmitted laser light from Beam 30 is collected by a focusing mirror and directed onto a diagnostic platform. The near field of the transmitted light is imaged; the system collects information from twice the original f cone of the beam. Two gated cameras capture the near field image of the transmitted light. The evolution of the beam spray is resolved temporally at 13 spatial positions around the near field. The forward stimulated Raman scattering and forward simulated Brillouin scattering are resolved spectrally and temporally at five independent locations within twice the original f cone. The total transmitted energy is measured in two spectral regions above and below 400 nm.
    Review of Scientific Instruments 09/2006; 77(10):10E507-10E507-3. · 1.60 Impact Factor
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    ABSTRACT: Laser-plasma interactions (LPI) have been studied experimentally in high-temperature, high-energy density plasmas. The studies have been performed using the Omega laser at the Laboratory for Laser Energetics (LLE), Rochester, NY. Up to 10 TW of power was incident upon reduced-scale hohlraums, distributed in three laser beam cones. The hot hohlraums fill quickly with plasma. Late in the laser pulse, most of the laser energy is deposited at the laser entrance hole, where most of the LPI takes place. Due to the high electron temperature, the stimulated Raman scattering (SRS) spectrum extends well beyond ω0/2, due to the Bohm-Gross shift. This high-temperature, high-energy density regime provides a unique opportunity to study LPI beyond inertial confinement fusion (ICF) conditions.
    Journal de Physique IV (Proceedings) 06/2006; 133(1):243-246. · 0.29 Impact Factor
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    ABSTRACT: We experimentally demonstrate that application of laser smoothing schemes including smoothing by spectral dispersion (SSD) and polarization smoothing (PS) increases the intensity range for efficient coupling of frequency doubled (527 nm) laser light to a long scalelength plasma with n{sub e}/n{sub cr} = 0.14 and T{sub e} = 2 keV.
    Journal de Physique IV (Proceedings) 06/2006; 133:321-324. · 0.29 Impact Factor
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    ABSTRACT: We have established the intensity limits for propagation of a frequency-doubled (2omega, 527 nm) high intensity interaction beam through an underdense large-scale-length plasma. We observe good beam transmission at laser intensities at or below 2x10(14) W/cm(2) and a strong reduction at intensities up to 10(15) W/cm(2) due to the onset of parametric scattering instabilities. We show that temporal beam smoothing by spectral dispersion allows a factor of 2 higher intensities while keeping the beam spray constant, which establishes frequency-doubled light as an option for ignition and burn in inertial confinement fusion experiments.
    Physical Review Letters 04/2005; 94(8):085005. · 7.73 Impact Factor
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    ABSTRACT: Summary form only given. We have implemented a complete set of optical diagnostics at the Omega laser to measure propagation and backscatter of a green (527 nm) high intensity laser beam (1015 W/cm2) in large-scale length, laser produced plasmas. The diagnostics include a transmitted beam diagnostic (TBD), a full aperture backscatter station (FABS) as well as a near backscatter imager (NBI) to simultaneously measure forward and backward scattered light with temporal, spectral and spatial resolution. Heating a mm-size gasbag or hohlraum target with up to 36 defocused heater beams, delivering a total energy of up to 16 kJ in a 1 ns pulse, creates a plasma at a density around 6times1020 cm-3 and temperatures of several keV. We observe a reduction of the beam transmission with increasing probe beam intensity, consistent with the onset of parametric scattering instabilities. Beam propagation can be significantly improved, by applying temporal- or polarization-beam smoothing. Our results indicate good beam propagation in ignition scale plasmas for inertial confinement fusion
    IEEE International Conference on Plasma Science 01/2005;
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    ABSTRACT: Summary form only given. Stimulated Raman scattering (SRS) instability has been experimentally investigated in small-scale halfraums irradiated by intense laser pulses at 10 TW power. The driver energy was delivered by the OMEGA laser (LLE, Rochester) in 1 ns pulses, and deposited in a hot, under-dense Au plasma. The laser energy deposition shifts at later times outside the laser entrance hole (LEH) due to a rapid plasma fill of the hohlraum. Most of the SRS light is scattered from this region and detected with time and spectral resolution. The high temperature of the scattering volume is reflected in the SRS spectrum that extends beyond two times the laser wavelength, due to the Bohm-Gross shift. Information about the plasma parameters inferred from the SRS spectra will be discussed
    IEEE International Conference on Plasma Science 01/2005;
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    ABSTRACT: The first experiments on the National Ignition Facility (NIF) have employed the first four beams to measure propagation and laser backscattering losses in large ignition-size plasmas. Gas-filled targets between 2 and 7 mm length have been heated from one side by overlapping the focal spots of the four beams from one quad operated at 351 nm (3ω) with a total intensity of 2 × 1015 W cm−2. The targets were filled with 1 atm of CO2 producing up to 7 mm long homogeneously heated plasmas with densities of ne = 6 × 1020 cm−3 and temperatures of Te = 2 keV. The high energy in an NIF quad of beams of 16 kJ, illuminating the target from one direction, creates unique conditions for the study of laser–plasma interactions at scale lengths not previously accessible. The propagation through the large-scale plasma was measured with a gated x-ray imager that was filtered for 3.5 keV x-rays. These data indicate that the beams interact with the full length of this ignition-scale plasma during the last ~1 ns of the experiment. During that time, the full aperture measurements of the stimulated Brillouin scattering and stimulated Raman scattering show scattering into the four focusing lenses of 3% for the smallest length (~2 mm), increasing to 10–12% for ~7 mm. These results demonstrate the NIF experimental capabilities and further provide a benchmark for three-dimensional modelling of the laser–plasma interactions at ignition-size scale lengths.
    Nuclear Fusion 11/2004; 44(12):S185. · 2.73 Impact Factor