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Backlit optical shadowgraphy is the primary diagnostic for hydrogenic ice-layer characterization in cryogenic targets at the Laboratory for Laser Energetics (LLE). Reflection and refraction of ligh...

The characterization of OMEGA cryogenic targets is based on shadowgraphs obtained from multiple angular views taken with the target in the layering sphere. The D 2 ice has been observed to re-layer during slow rotations, leading to procedural changes that avoid re-layering thus ensuring high-quality, spherical-harmonic, 3-D ice layer reconstructions. Shadowgrams taken inside the target chamber within 20 ms of shot time have verified that the ice layers remain preserved during the transport.

Backlit optical shadowgraphy is the primary diagnostic for D 2 ice layer characterization of cryogenic targets for the OMEGA Laser System at the Laboratory for Laser Energetics (LLE). Reflection and refraction of light passing through the ice layer produce characteristic rings. The position of the most prominent of the shadowgraph rings, known as the bright ring, can be resolved to ∼0.1-pixel rms, corresponding to about 0.12 μm for typical LLE target shadowgraphs. Measurement of the bright ring position in conjunction with ray-trace model predictions determines the ice layer thickness and the Fourier-mode spectrum of the ice roughness for that view. The LLE target characterization stations use two camera angles and target rotation to record target shadowgraphs from many different views. Combining these views allows construction of a 3-D ice layer representation, an estimation of the global surface roughness, and a determination of a Legendre-mode spectrum suitable for implosion modeling. The standard operating procedure is to construct a 3-D ice layer representation using the analysis of 48 separate shadowgraphic views. The 3-D ice surface is then decomposed in terms of spherical harmonics, allowing the determination of low-mode number (l ≤ 8 to 10) elements of a Legendre-mode power spectrum. Higher-mode number elements of the Legendre power spectrum are determined by mapping the Fourier-mode power spectrum averaged over all views.

The OMEGA cryogenic target handling system provides deuterium-filled cryogenic targets for direct-drive implosion experiments. The targets are 0.9 mm in diameter with a 3-{mu}m-thick outer plastic ablator and an inner ice layer that ranges from 80 to 100 {mu}m thick. The smoothest ice layer possessed an average root-mean-square (rms) roughness of 1.2 {mu}m, although values ranging from 2 to 4 {mu}m are more typical. Implosion experiments achieved a maximum yield of 2.11 x 10{sup 11} primary neutrons (70% of the clean one-dimensional yield) with an average areal density of 50 mg/cm{sup 2} with a 1-ns square, high-adiabat ({alpha} = 25) laser pulse. Lower yields (1 x 10{sup 10} primary neutrons) and higher areal densities (88 mg/cm{sup 2}) were obtained using a lower-adiabat ({alpha} = 4) laser pulse. Better performance is expected once smoother ice layers (better than 2-{mu}m average rms roughness) are positioned within 10 {mu}m of where the laser beams are pointed. Currently, the offset between the target's location and where the laser beams are pointing at the moment of implosion is 14 to 60 {mu}m.

Layered and characterized cryogenic D2 capsules have been imploded using both low- and high-adiabat (α, the ratio of the electron pressure to the Fermi-degenerate pressure) pulse shapes on the 60-beam OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] at the Laboratory for Laser Energetics (LLE). These experiments measure the sensitivity of the direct-drive implosion performance to parameters such as the inner-ice-surface roughness, the adiabat of the cryogenic fuel during the implosion, the laser power balance, and the single-beam nonuniformity. The goal of the direct-drive program at LLE is to demonstrate a high neutron-averaged fuel ρR at a significant fraction of the predicted one-dimensional (1-D) neutron yield using an energy-scaled, low-adiabat (α∼3) ignition pulse shape driving a hydrodynamically scaled deuterium–tritium ignition capsule. New results are reported from implosions of ∼920-μm-diam, thin (∼5 μm) polymer shells containing 100 μm D2-ice layers with characterized inne...

The University of Rochester's Laboratory for Laser Energetics has been imploding thick cryogenic targets for six years. Improvements in the Cryogenic Target Handling System and the ability to accurately design laser pulse shapes that properly time shocks and minimize electron preheat, produced high fuel areal densities in deuterium cryogenic targets (202±7 mg/cm2). The areal density was inferred from the energy loss of secondary protons in the fuel (D2) shell. Targets were driven on a low final adiabat (α = 2) employing techniques to radially grade the adiabat (the highest adiabat at the ablation surface). The ice layer meets the target-design toughness specification for DT ice of 1-μm rms (all modes), while D2 ice layers average 3.0-μm-rms roughness. The implosion experiments and the improvements in the quality and understanding of cryogenic targets are presented.

Ignition target designs for inertial confinement fusion on the National Ignition Facility (NIF) [W. J. Hogan et al., Nucl. Fusion 41, 567 (2001)] are based on a spherical ablator containing a solid, cryogenic-fuel layer of deuterium and tritium. The need for solid-fuel layers was recognized more than 30 years ago and considerable effort has resulted in the production of cryogenic targets that meet most of the critical fabrication tolerances for ignition on the NIF. At the University of Rochester’s Laboratory for Laser Energetics (LLE), the inner-ice surface of cryogenic DT capsules formed using β-layering meets the surface-smoothness requirement for ignition (

More than 100 spherical deuterium ice layers have been formed to make cryogenic targets for direct-drive ICF implosion experiments on OMEGA. These ice layers have an inner surface roughness that ranges from 1.3to6μm root-mean-square (rms), with the best layers having a value less than 2μm rms. These surface roughness values are averaged two-dimensional roughness measurements that cover the entire surface and includes all of the Fourier cosine modes. The ice thickness variation within the layer is predominately in the low spectral modes (mode 5 and lower) and is caused by the support used to hold the target. Changing the design of this support to minimize the thermal effect is constrained by the necessity of having a dynamically stable target for the implosion. We have demonstrated that it is possible to form crystalline ice layers that are facet-free and transparent by slowing the solidification rate of the liquid. Faster freezing rates form layers comprised of polycrystalline ice with a greater roughness (1to2μm greater). Cooling an ice layer 0.5K below the triple point temperature does not affect the roughness of the layer. Cooling the layer a further 1K to achieve the desired internal gas pressure sometimes induces additional ice roughness; this roughness is manifest over low- to mid-spectral modes. Removing the thermal shrouds from around the target causes the ice to melt and the internal gas pressure to increase. Using the behavior of a cryogenic deuterium target as a reference, calculations of the response of the more interesting National Ignition Facility-scale deuterium and tritium targets show that exposing the target for 0.8s to ambient radiation will cause ∼10% of the ice to melt and partially slump whereas the gas pressure will increase by 15%.

Direct-drive spherical implosions of cryogenic, D2-filled capsules are performed on the 60-beam OMEGA laser system [T. R. Boehly, D. L. Brown, R. S. Craxton, R. L. Keck, J. P. Knauer, J. H. Kelly, T. J. Kessler, S. A. Kumpan, S. J. Loucks, S. A. Letzring, F. J. Marshall, R. L. McCrory, S. F. B. Morse, W. Seka, J. M. Soures, and C. P. Verdon, Opt. Commun. 133, 495 (1997)]. The targets are energy scaled from the base line ignition design developed for the National Ignition Facility [W. J. Hogan et al., Nucl. Fusion 41, 567 (2001)]. Thin-walled (∼4μm), ∼860μm diam deuterated polymer shells are permeation filled with D2 gas and cooled to the triple point (∼18.7K). Cryogenic ice layers with a uniformity of ∼2μm rms are formed and maintained. The targets are imploded with high-contrast pulse shapes with full single-beam smoothing (1THz bandwidth, two-dimensional smoothing by spectral dispersion with polarization smoothing) to study the effects of the acceleration- and deceleration-phase Rayleigh–Taylor growth o...