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Measurements of near forward scattered laser light in a large ICF plasma

Description: We describe an instrument which measures the angular spread and spectrum of near forward scattered laser light from a probe beam in a long scalelength laser-plasma. The instrument consists of a combination of time integrating and time resolving detectors which measure the scattered light amplitude over four orders of magnitude for a range of angles. These measurements allow us to study the beam spray resulting from various laser and plasma conditions and determine the density fluctuations associated with this beam spray.
Date: June 2, 1998
Creator: Moody, J. D.
Partner: UNT Libraries Government Documents Department

Imaging back scattered and near back scattered light in ignition scale plasmas

Description: Diagnostics have been developed and fielded at the Nova laser facility that image scattered light in the vicinity of the final laser focusing lens. The absolute calibration of optical components exposed to the target debris have been achieved by a combination of routine in situ calibration and maintenance. The scattering observed from plasmas relevant to ignition experiments indicates that light scattered just outside the lens can be larger than that collected by the lens, and is a significant factor in the energy balance when the f number is high.
Date: May 7, 1996
Creator: Kirkwood, R.K.; Back, C.A.; Glenzer, S.H. & Moody, J.D.
Partner: UNT Libraries Government Documents Department

Solid Deuterium-Tritium Surface Roughness In A Beryllium Inertial Confinement Fusion Shell

Description: Solid deuterium-tritium (D-T) fuel layers for inertial confinement fusion experiments were formed inside of a 2 mm diameter beryllium shell and were characterized using phase-contrast enhanced x-ray imaging. The solid D-T surface roughness is found to be 0.4 {micro}m for modes 7-128 at 1.5 K below the melting temperature. The layer roughness is found to increase with decreasing temperature, in agreement with previous visible light characterization studies. However, phase-contrast enhanced x-ray imaging provides a more robust surface roughness measurement than visible light methods. The new x-ray imaging results demonstrate clearly that the surface roughness decreases with time for solid D-T layers held at 1.5 K below the melting temperature.
Date: April 19, 2006
Creator: Kozioziemski, B J; Sater, J D; Moody, J D; Montgomery, D S & Gautier, C
Partner: UNT Libraries Government Documents Department

Angular Dependence of 3 Omega 0/2 Spectra from Laser-produced Plasmas

Description: Scattered light at three-halves of the incident laser frequency from solid targets is observed at five different angles. When the incident laser intensity is low enough, rescattering of two plasmon decay (TPD) instability electron plasma waves by ion acoustic waves is not significant. In this regime, Thomson scattering measurements of the electron temperature and the plasma flow velocity allow quantitative comparison of the angular dependence of the spectrum to theory.
Date: August 25, 1999
Creator: Young, P.E.; Moody, J.D. & Rhizomes, W.
Partner: UNT Libraries Government Documents Department

Comparison of Raman Scattering Measurements and Modeling in NIF Ignition Experiments

Description: Recent NIF indirect-drive experiments have shown significant Raman scattering from the inner beams. NIF data has motivated improvements to rad-hydro modeling, leading to the 'high flux model' [M. D. Rosen et al., HEDP 7, 180 (2011)]. Cross-beam energy transfer [P. A. Michel et al., Phys. Plasmas 17, 056305 (2010] in the laser entrance hole is an important tool for achieving round implosions, and is uniformly distributed across the laser spot in rad-hydro simulations (but not necessarily in experiments). We find the Raman linear gain spectra computed with these plasma conditions agree well in time-dependent peak wavelength with the measured data, especially when overlapping laser-beam intensities are used. More detailed, spatially non-uniform modeling of the cross-beam transfer has been performed. The resulting gains better follow the time history of the measured backscatter. We shall present the impact of spatially non-uniform energy transfer on SRS gain. This metric is valid when amplification is in a linear regime, and so we shall also present an assessment of whether electron trapping in Langmuir waves can play a role in these shots.
Date: November 4, 2011
Creator: Strozzi, D J; Hinkel, D E; Williams, E A; Town, R J; Michel, P A; Divol, L et al.
Partner: UNT Libraries Government Documents Department

Thermal Infrared Exposure of Cryogenic Indirect Drive ICF Targets

Description: Cryogenic inertial confinement fusion targets at the National Ignition Facility and the Laser Megajoule will be protected from thermal infrared radiation by a cold shroud. As the shroud is removed just before the laser pulse, infrared radiation will heat and possibly degrade the symmetry of the solid hydrogen fuel layer. A lumped component mathematical model has been constructed to calculate how long an indirect drive target can be exposed to thermal radiation before the fuel layer degrades. The allowed exposure time sets the maximum shroud removal time and therefore has important implications for the design of the cryogenic shroud systems. The model predicts that the maximum exposure time is approximately 0.18 s for plastic capsules inside hohlraums with transparent laser entrance holes. By covering the laser entrance holes with a partially reflective coating, the exposure time can be increased to approximately 1 s. The exposure time can be increased to about 2 s by using beryllium capsules. Several other design concepts could increase the exposure time even further. Lengthening of the allowed exposure time to 1 s or longer could allow a significant cost savings for the shroud system.
Date: July 8, 2005
Creator: London, R A; Moody, J D; Sanchez, J J; Sater, J D; Haid, B J & Bittner, D N
Partner: UNT Libraries Government Documents Department

Laser Machining For Fabrication Of Hohlraums And Capsules

Description: Laser machining technology has been used to demonstrate the ability to rapidly perform jobs on all aspects of ICF targets. Lasers are able to rapidly perform modifications and repairs to the gold metal parts on hohlraums, make cuts in the delicate polymer parts of the hohlraum, and drill holes in the capsules to enable them to be filled with fuel. Lasers investigated in this work include 193 nm ArF and 248 nm KrF excimers and 810 nm chirped-pulse amplification Ti:Sapphire lasers. The excimer lasers showed a definite advantage in drilling and machining of polymeric materials and the ultrashort infrared pulses of the Ti:Sapphire laser were far better for the gold structures.
Date: June 24, 2005
Creator: Shirk, M D; Kelly, B T; Haynes, S M; Stuart, B C; Sanchez, J J; Moody, J D et al.
Partner: UNT Libraries Government Documents Department

Laser Beam Propagation through Inertial Confinement Fusion Hohlraum Plasmas

Description: A study of the relevant laser-plasma interaction processes has been performed in long-scale length plasmas that emulate the plasma conditions in indirect drive inertial confinement fusion targets. Experiments in this high-temperature (T{sub e} = 3.5 keV), dense (n{sub e} = 0.5 - 1 x 10{sup -3}) hohlraum plasma have demonstrated that blue 351-nm laser beams produce less than 1% total backscatter resulting in transmission greater than 90% for ignition relevant laser intensities (I < 2 x 10{sup 15} W cm{sup -2}). The bulk plasma conditions have been independently characterized using Thomson scattering where the peak electron temperatures are shown to scale with the hohlraum heater beam energy in the range from 2 keV to 3.5 keV. This feature has allowed us to determine the thresholds for both backscattering and filamentation instabilities; the former measured with absolutely calibrated full aperture backscatter and near backscatter diagnostics and the latter with a transmitted beam diagnostics. Comparing the experimental results with detailed gain calculations for the onset of significant laser scattering processes shows that these results are relevant for the outer beams in ignition hohlraum experiments corresponding to a gain threshold for stimulated Brillouin scattering of 15. By increasing the gas fill density in these experiments further accesses inner beam ignition hohlraum conditions. In this case, stimulated Raman scattering dominates the backscattering processes. They show that scattering is small for gains smaller than 20, which can be achieved through proper choice of the laser beam intensity.
Date: October 26, 2006
Creator: Froula, D. H.; Divol, L.; Meezan, N. B.; DIxit, S.; Neumayer, P.; Moody, J. D. et al.
Partner: UNT Libraries Government Documents Department

Ideal Laser Beam Propagation through high temperature ignition hohlraum plasmas

Description: We demonstrate that a blue (3{omega}, 351 nm) laser beam with an intensity of 2 x 10{sup 15} W-cm{sup -2} propagates within the original beam cone through a 2-mm long, T{sub e}=3.5 keV high density (n{sub e} = 5 x 10{sup 20} cm{sup -3}) plasma. The beam produced less than 1% total backscatter; the resulting transmission is greater than 90%. Scaling of the electron temperature in the plasma shows that the plasma becomes transparent for uniform electron temperatures above 3 keV. These results are consistent with linear theory thresholds for both filamentation and backscatter instabilities inferred from detailed hydrodynamic simulations. This provides a strong justification for current inertial confinement fusion designs to remain below these thresholds.
Date: September 20, 2006
Creator: Froula, D H; Divol, L; Meezan, N; Dixit, S; Moody, J D; Pollock, B B et al.
Partner: UNT Libraries Government Documents Department

MULTI-KEV X-RAY YIELDS FROM HIGH-Z GAS TARGETS FIELDED AT OMEGA

Description: The authors report on modeling of x-ray yield from gas-filled targets shot at the OMEGA laser facility. The OMEGA targets were 1.8 mm long, 1.95 mm in diameter Be cans filled with either a 50:50 Ar:Xe mixture, pure Ar, pure Kr or pure Xe at {approx} 1 atm. The OMEGA experiments heated the gas with 20 kJ of 3{omega} ({approx} 350 nm) laser energy delivered in a 1 ns square pulse. the emitted x-ray flux was monitored with the x-ray diode based DANTE instruments in the sub-keV range. Two-dimensional x-ray images (for energies 3-5 keV) of the targets were recorded with gated x-ray detectors. The x-ray spectra were recorded with the HENWAY crystal spectrometer at OMEGA. Predictions are 2D r-z cylindrical with DCA NLTE atomic physics. Models generally: (1) underpredict the Xe L-shell yields; (2) overpredict the Ar K-shell yields; (3) correctly predict the Xe thermal yields; and (4) greatly underpredict the Ar thermal yields. However, there are spreads within the data, e.g. the DMX Ar K-shell yields are correctly predicted. The predicted thermal yields show strong angular dependence.
Date: November 4, 2010
Creator: Kane, J O; Fournier, K B; May, M J; Colvin, J D; Thomas, C A; Marrs, R E et al.
Partner: UNT Libraries Government Documents Department

Efficient Coupling of 527 nm Laser Beam Power to a Long Scalelength Plasma

Description: 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.
Date: August 24, 2005
Creator: Moody, J. D.; Divol, L.; Glenzer, S. H.; MacKinnon, A. J.; Froula, D. H.; Gregori, G. et al.
Partner: UNT Libraries Government Documents Department

0.351 micron Laser Beam propagation in High-temperature Plasmas

Description: A study of the laser-plasma interaction processes have been performed in plasmas that are created to emulate the plasma conditions in indirect drive inertial confinement fusion targets. The plasma emulator is produced in a gas-filled hohlraum; a blue 351-nm laser beam propagates along the axis of the hohlraum interacting with a high-temperature (T{sub e} = 3.5 keV), dense (n{sub e} = 5 x 10{sup 20}cm{sup -3}), long-scale length (L {approx} 2 mm) plasma. Experiments at these conditions have demonstrated that the interaction beam produces less than 1% total backscatter resulting in transmission greater than 90% for laser intensities less than I < 2 x 10{sup 15} W-cm{sup -2}. The bulk plasma conditions have been independently characterized using Thomson scattering where the peak electron temperatures are shown to scale with the hohlraum heater beam energy in the range from 2 keV to 3.5 keV. This feature has allowed us to determine the thresholds for both backscattering and filamentation instabilities; the former measured with absolutely calibrated full aperture backscatter and near backscatter diagnostics and the latter with a transmitted beam diagnostics. A plasma length scaling is also investigated extending our measurements to 4-mm long high-temperature plasmas. At intensities I < 5 x 10{sup 14} W-cm{sup -2}, greater than 80% of the energy in the laser is transmitted through a 5-mm long, high-temperature (T{sub e} > 2.5 keV) high-density (n{sub e} = 5 x 10{sup 20} w-cm{sup -3}) plasma. Comparing the experimental results with detailed gain calculations for the onset of significant laser scattering processes shows a stimulated Brillouin scattering threshold (R=10%) for a linear gain of 15; these high temperature, low density experiments produce plasma conditions comparable to those along the outer beams in ignition hohlraum designs. By increasing the gas fill density (n{sub e} = 10{sup 21} cm{sup -3}) in these ...
Date: December 10, 2007
Creator: Froula, D.; Divol, L.; Meezan, N.; Ross, J.; Berger, R. L.; Michel, P. et al.
Partner: UNT Libraries Government Documents Department

Status Of Cryogenic Layering For NIF Ignition Targets

Description: Recent advances in cryogenic layering include the development of a self-contained and self-filling cro hohlraum, application of phase contrast x-ray measurements for ice layer characterization, an ice layer achieved with beta-layering which meets the NIF specification for surface roughness at 1.5 K below the triple point. In addition, recent results on target integration in a hohlraum show effective layer control using heaters on the hohlraum.
Date: August 22, 2005
Creator: Moody, J D; Kozioziemski, B J; London, R L; Montgomery, D S; Sanchez, J J; Sater, J D et al.
Partner: UNT Libraries Government Documents Department