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Improved beam smoothing with SSD using generalized phase modulation

Description: The smoothing of the spatial illumination of an inertial confinement fusion target is examined by its spatial frequency content. It is found that the smoothing by spectral dispersion method, although efficient for glass lasers, can yield poor smoothing at low spatial frequency. The dependence of the smoothed spatial spectrum on the characteristics of phase modulation and dispersion is examined for both sinusoidal and more general phase modulation. It is shown that smoothing with non-sinusoidal phase modulation can result in spatial spectra which are substantially identical to that obtained with the induced spatial incoherence or similar method where random phase plates are present in both methods and identical beam divergence is assumed.
Date: January 1, 1997
Creator: Rothenberg, J.E.
Partner: UNT Libraries Government Documents Department

Electro-optical deflectors as a method of beam smoothing for Inertial Confinement Fusion

Description: The electro-optic deflector is analyzed and compared to smoothing by spectral dispersion for efficacy as a beam smoothing method for ICF. It is found that the electro-optic deflector is inherently somewhat less efficient when compared either on the basis of equal peak phase modulation or equal generated bandwidth.
Date: January 1, 1997
Creator: Rothenberg, J.E.
Partner: UNT Libraries Government Documents Department

Two dimensional beam smoothing by spectral dispersion for direct drive inertial confinement fusion

Description: Two dimensional smoothing by spectral dispersion is analyzed by using diffraction theory calculations. It is shown that by using standard frequency modulated light one can obtain bandwidth limited smoothing over integration times relevant to inertial confinement fusion (about 1 nsec) with modest induced beam divergence. At longer integration times one can obtain bandwidth limited smoothing by increasing the divergence and/or by using more advanced phase modulation methods.
Date: July 11, 1995
Creator: Rothenberg, J.E.
Partner: UNT Libraries Government Documents Department

Smoothing by spectral dispersion using random phase modulation for inertial confinement fusion

Description: Numerical simulations of beam smoothing using random phase modulation and grating dispersion are presented. Spatial spectra of the target illumination show that significantly improved smoothing at low spatial frequency is achieved while maintaining uniform intensity in the laser amplifier.
Date: November 15, 1995
Creator: Rothenberg, J.E.
Partner: UNT Libraries Government Documents Department

Impact of beam smoothing method on direct drive target performance for the NIF

Description: The impact of smoothing method on the performance of a direct drive target is modeled and examined in terms of its l-mode spectrum. In particular, two classes of smoothing methods are compared, smoothing by spectral dispersion (SSD) and the induced spatial incoherence (ISI) method. It is found that SSD using sinusoidal phase modulation (FM) results in poor smoothing at low l-modes and therefore inferior target performance at both peak velocity and ignition. Modeling of the hydrodynamic nonlinearity shows that saturation tends to reduce the difference between target performance for the smoothing methods considered. However, using SSD with more generalized phase modulation results in a smoothed spatial spectrum, and therefore target performance, which is identical to that obtained with the ISI or similar method where random phase plates are present in both methods and identical beam divergence is assumed.
Date: November 1, 1996
Creator: Rothenberg, J.E. & Weber, S.V.
Partner: UNT Libraries Government Documents Department

Impact of beam smoothing method on direct drive target performance for the NIF

Description: The impact of smoothing method on the performance of a direct drive target is modeled and examined in terms of its 1-mode spectrum. In particular, two classes of smoothing methods are compared, smoothing by spectral dispersion (SSD) and the induced spatial incoherence (ISI) method. It is found that SSD using sinusoidal phase modulation (FM) results in poor smoothing at low 1-modes and therefore inferior target performance at both peak velocity and ignition. This disparity is most notable if the effective imprinting integration time of the target is small. However, using SSD with more generalized phase modulation can result in smoothing at low l-modes which is identical to that obtained with ISI. For either smoothing method, the calculations indicate that at peak velocity the surface perturbations are about 100 times larger than that which leads to nonlinear hydrodynamics. Modeling of the hydrodynamic nonlinearity shows that saturation can reduce the amplified nonuniformities to the level required to achieve ignition for either smoothing method. The low l- mode behavior at ignition is found to be strongly dependent on the induced divergence of the smoothing method. For the NIF parameters the target performance asymptotes for smoothing divergence larger than {approximately}100 {mu}rad.
Date: January 1, 1997
Creator: Rothenberg, J.E. & Weber, S.V.
Partner: UNT Libraries Government Documents Department

Performance of smoothing by spectral dispersion (SSD) on Beamlet

Description: The performance of the Beamlet laser with 1D SSD implemented is investigated. Simulations indicate that the critical issue for laser performance is the amount of additional divergence owing to SSD in comparison to the size of the spatial filter pinholes. At the current {plus_minus}200 {mu}rad pinholes used on Beamlet, simulations indicate that the levels of SSD divergence anticipated for the National Ignition Facility (NIF) results in a very slight degradation to the near field beam quality. Experiments performed with the Beamlet front end show no degradation to the near field beam with up to 100 {mu}rad of SSD divergence. Measurements of the smoothing of a far field speckle pattern generated by a phase plate show the expected improvement in contrast with increasing amounts of SSD divergence.
Date: January 1, 1997
Creator: Rothenberg, J.E.; Moran, B.; Henesian, M. & Van Wonterghem, B.
Partner: UNT Libraries Government Documents Department

Performance of smoothing by spectral dispersion (SSD) with frequency conversion on the Beamlet Laser for the National Ignition Facility

Description: Simulations and ongoing measurements indicate that SSD results in small degradation to the near field beam quality. The measured effect of SSD bandwidth on conversion to the third harmonic and smoothing of the target illumination will also be described.
Date: November 4, 1997
Creator: Rothenberg, J.E.; Morgan, B.; Wegner, P. & Weiland, T.W.
Partner: UNT Libraries Government Documents Department

Illumination uniformity requirements for direct drive inertial confinement fusion

Description: The requirements for laser uniformity are discussed in terms of the {ell}-mode spectrum. It is shown that the choice of smoothing methods can significantly alter this spectrum and that this choice should be made in the context of the target physics. Although two dimensional smoothing by spectral dispersion yields a high quality near field beam profile, it results in poor smoothing for low spatial frequency. The partially coherent light method (fiber smoothing) leads to superior smoothing at low spatial frequencies, but has very poor near field beam quality. As a result, it may be desirable to use partially coherent light during the driver pulse foot (at low intensity and when minimizing the laser imprint is critical) and smoothing by spectral dispersion during the main pulse.
Date: July 11, 1995
Creator: Rothenberg, J. E.; Eimerl, D.; Key, M. H. & Weber, S. V.
Partner: UNT Libraries Government Documents Department

Laser imprint and implications for direct drive ignition with the National Ignition Facility

Description: For direct drive ICF, nonuniformities in laser illumination can seed ripples at the ablation front in a process called imprint. Such nonuniformities will grow during the capsule implosion and can penetrate the capsule shell impede ignition, or degrade burn. We have simulated imprint for a number of experiments on tile Nova laser. Results are in generally good agreement with experimental data. We leave also simulated imprint upon National Ignition Facility (NIF) direct drive ignition capsules. Imprint modulation amplitude comparable to the intrinsic surface finish of {approximately}40 nm is predicted for a laser bandwidth of 0.5 THz. Ablation front modulations experience growth factors up to several thousand, carrying modulation well into the nonlinear regime. Saturation modeling predicts that the shell should remain intact at the time of peak velocity, but penetration at earlier times appears more marginal.
Date: July 9, 1996
Creator: Weber, S.V.; Glendinning, S.G.; Kalantar, D.H.; Remington, B.A. & Rothenberg, J.E.
Partner: UNT Libraries Government Documents Department

The issue of FM to AM conversion on the National Ignition Facility

Description: The National Ignition Facility (NIF) baseline configuration for inertial confinement fusion requires phase modulation for two purposes. First, ~ 1Å of frequency modulation (FM) bandwidth at low modulation frequency is required to suppress buildup of Stimulated Brioullin Scattering (SBS) in the large aperture laser optics. Also ~ 3 Å or more bandwidth at high modulation frequency is required for smoothing of the speckle pattern illuminating the target by the smoothing by spectral dispersion method (SSD). Ideally, imposition of bandwidth by pure phase modulation does not affect the beam intensity. However, as a result of a large number of effects, the FM converts to amplitude modulation (AM). In general this adversely affects the laser performance, e.g. by reducing the margin against damage to the optics. In particular, very large conversion of FM to AM has been observed in the NIF all-fiber master oscillator and distribution systems. The various mechanisms leading to AM are analyzed and approaches to minimizing their effects are discussed.
Date: August 13, 1998
Creator: Browning, D F; Rothenberg, J E & Wilcox, R B
Partner: UNT Libraries Government Documents Department

Laser techniques for extreme-ultraviolet spectroscopy

Description: In this paper we describe several techniques for using lasers to study core-excited energy levels in the spectral region between 10 eV and 100 eV. We are particularly interested in levels that are metastable against autoionization and, in some cases, against both autoionization and radiation.
Date: June 1, 1983
Creator: Harris, S.E.; Young, J.F.; Caro, R.G.; Falcone, R.W.; Holmgren, D.E.; Walker, D.J. et al.
Partner: UNT Libraries Government Documents Department

Implementation of smoothing by spectral dispersion on Beamlet and NIF

Description: The performance of the Beamlet laser with one dimensional smoothing by spectral dispersion (1D SSD) implemented is investigated. Measurements of the near field beam quality, nonlinear breakup, and transmission through spatial filter pinholes show a modest effect only at large SSD divergence. No measurable effect was found at the divergence level planned for indirect drive ignition experiments. The efficiency of conversion to the third harmonic was also measured with SSD present and found to be somewhat larger than expected from an ideal plane wave model.
Date: June 9, 1999
Creator: Auerbach, J M; Moran, B D; Murray, J E; Rothenberg, J E; Wegner, P J & Weiland, T L
Partner: UNT Libraries Government Documents Department

Implementation and performance of beam smoothing on 10 beams of the Nova Laser

Description: Recent simulations and experiments on Nova indicate that some level of smoothing may be required to suppress filamentation in plasmas on the National Ignition Facility (NIF), resulting in the addition of 1-D smoothing capability to the current baseline design. Control of stimulated Brillouin scattering (SBS) and filamentation is considered essential to the success of laser fusion because they affect the amount and location of laser energy delivered to the x-ray conversion region (hohlraum wall) for indirect drive and to the absorptive region for direct drive, Smoothing by spectral dispersion (SSD)[1], reduces these instabilities by reducing nonuniformities in the focal irradiance when averaged over a finite time interval. We have installed SSD on Nova to produce beam smoothing on all 10 beam lines. A single dispersion grating is located in a position common to all 10 beam lines early in the preamplifier chain. This location limits the 1{omega} bandwidth to 2.2 {angstrom} with sufficient dispersion to displace the speckle field of each frequency component at the target plane by one half speckle diameter. Several beam lines were modified to allow orientation of the dispersion on each arm relative to the hohlraum wall. After conversion to the third harmonic the beam passes through a kinoform phase plate (KPP) designed to produce an elliptical spot at best focus. The KPPs produce a focal spot having an elliptical flat-top envelope with a superimposed speckle pattern. Over 93% of the energy is contained in the central 400 km. Calculations indicate a 16% rms. intensity variance will be reached after 330 ps for a single beam.
Date: March 11, 1997
Creator: Pennington, D. M.; Dixit, S. N.; Weiland, T. L.; Ehrlich, R. & Rothenberg, J. E.
Partner: UNT Libraries Government Documents Department

Integrated operations of the National Ignition Facility (NIF) optical pulse generation development system

Description: We describe the Optical Pulse Generation (OPG) testbed, which is the integration of the MOR and Preamplifier Development Laboratories. We use this OPG testbed to develop and demonstrate the overall capabilites of the NIF laser system front end. We will present the measured energy and power output, temporal and spatial pulse shaping capability, FM bandwidth and dispersion for beam smoothing, and measurements of the pulse-to-pulse power variation of the OPG system and compare these results with the required system performance specifications. We will discuss the models that are used to predict the system performance and how the OPG output requirements flowdown to the subordinate subsystems within the OPG system.
Date: July 31, 1998
Creator: Browning, D.; Crane, J. K.; Dane, C. B.; Hackel; Henesian, M.; Hopps, N. W. et al.
Partner: UNT Libraries Government Documents Department

Simulations of laser imprint for Nova experiments and for ignition capsules

Description: In direct drive ICF, nonuniformities in laser illumination seed ripples at the ablation front in a process called imprint. These non nonuniformities grow during the capsule implosion and, if initially large enough, can penetrate the capsule shell, impede ignition, or degrade burn. Imprint has been simulated for recent experiments performed on the Nova laser at LLNL examining a variety of beam smoothing conditions. Most used laser intensities similar to the early part of an ignition capsule pulse shape, I=10X13 W/cm3. The simulations matched most of the measurements of imprint modulation. The effect of imprint upon National Ignition Facility (NIF) direct drive ignition capsules has also been simulated. Imprint is predicted to give modulation comparable to an intrinsic surface finish of 10 nm RMS. Modulation growth was examined using the Haan model, with linear growth as a function of spherical harmonic mode number obtained from an analytic dispersion relation. Ablation front amplitudes are predicted to become substantially nonlinear, so that saturation corrections are large. Direct numerical simulations of two- dimensional multimode growth were also performed. The capsule shell is predicted to remain intact, which gives a basis for believing that ignition can be achieved.
Date: November 8, 1996
Creator: Weber, S.V.; Glendinning, S.G.; Kalantar, D.H.; Key, M.H.; Remington, B.A.; Rothenberg, J.E. et al.
Partner: UNT Libraries Government Documents Department

Mercury and Beyond: Diode-Pumped Solid-State Lasers for Inertial Fusion Energy

Description: We have begun building the ''Mercury'' laser system as the first in a series of new generation diode-pumped solid-state lasers for inertial fusion research. Mercury will integrate three key technologies: diodes, crystals, and gas cooling, within a unique laser architecture that is scalable to kilojoule and megajoule energy levels for fusion energy applications. The primary near-term performance goals include 10% electrical efficiencies at 10 Hz and 1005 with a 2-10 ns pulse length at 1.047 {micro}m wavelength. When completed, Mercury will allow rep-rated target experiments with multiple chambers for high energy density physics research.
Date: December 1, 1999
Creator: Bibeau, C.; Bayramian, A.; Beach, R.J.; Chanteloup, J.C.; Ebbers, C.A.; Emanuel, M.A. et al.
Partner: UNT Libraries Government Documents Department

Optical pulse generation system for the National Ignition Facility (NIF)

Description: We describe the Optical Pulse Generation (OPG) system for the National Ignition Facility ( NIF ). The OPG system begins with the Master Oscillator Room ( MOR ) where the initial, seed pulse for the entire laser system is produced and properly formatted to enhance ignition in the target. The formatting consists of temporally shaping the pulse and adding additional bandwidth to increase the coupling of the laser generated x-rays to the high density target plasma. The pulse produced in the MOR fans out to 48 identical preamplifier modules where it is amplified by a factor of ten billion and spatially shaped for injection into the 192 main amplifier chai
Date: June 18, 1998
Creator: Penko, F; Braucht,; Browning, D; Crane, J K; Dane, B; Deadrick, F et al.
Partner: UNT Libraries Government Documents Department

Laser beam smoothing and backscatter saturation processes in plasmas relevant to National Ignition Facility hohlraums

Description: We have used gas-filled targets irradiated at the Nova laser to simulate National Ignition Facility (NlF) hohlraum plasmas and to study the dependence of Stimulated Raman (SRS) and Brillouin (SBS) Scattering on beam smoothing at a range of laser intensities (3{omega}, 2 - 4 10{sup 15}Wcm{sup -2}) and plasma conditions. We have demonstrated the effectiveness of polarization smoothing as a potential upgrade to the NIF. Experiments with higher intensities and higher densities characteristic of 350eV hohlraum designs indicate that with appropriate beam smoothing the backscatter from such hohlraums may be tolerable.
Date: October 1, 1998
Creator: Berger, R L; Cohen, B I; Decker, C D; Dixit, S; Glenzer, S H; Hinkel, D E et al.
Partner: UNT Libraries Government Documents Department

Recent laser-plasma interaction studies at LLNL

Description: Recent analysis and modeling of Nova experiments which address our understanding of stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) will be presented. Forward and backward SBS levels have been investigated with SSD of varying bandwidth and with polarization smoothing using an f/8 focusing geometry to emulate NIF conditions. The interpretation of the experiments is aided using F3D, a fluid code which includes modeling of SBS and SRS; a parallel version of F3D is being developed which will allow the modeling of a plasma approaching the size of an entire NIF laser beam. Cryogenic targets have recently been employed to investigate the dependence of SRS saturation levels on ion wave damping via the Langmuir decay instability.
Date: September 1, 1999
Creator: Berger, R L; Decker, C; Divol, L; Geddes, C; Glenzer, S H; Hinkel, D E et al.
Partner: UNT Libraries Government Documents Department

Mercury and Beyond: Diode-Pumped Solid-State Lasers for Inertial Fusion Energy

Description: We have begun building the ''Mercury'' laser system as the first in a series of new generation diode-pumped solid-state lasers for inertial fusion research. Mercury will integrate three key technologies: diodes, crystals, and gas cooling, within a unique laser architecture that is scalable to kilojoule energy levels for fusion energy applications. The primary performance goals include 10% electrical efficiencies at 10 Hz and 100 J with a 2-10 ns pulse length at 1.047 pm wavelength. When completed, Mercury will allow rep-rated target experiments with multiple target chambers for high energy density physics research.
Date: October 19, 1999
Creator: Bibeau, C.; Beach, R.J.; Bayramian, A.; Chanteloup, J.C.; Ebbers, C.A.; Emanuel, M.A. et al.
Partner: UNT Libraries Government Documents Department