LLE Review 94

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Highlights

This volume of the LLE Review, covering January–April 2003, features “Optimization of Deposition Uniformity for Large-Aperture NIF Substrates in a Planetary Rotation System.” For the National Ignition Facility (NIF), coating thickness nonuniformity must not exceed 0.5% peak-to-valley over a 0.85-m aperture. This article describes the design and performance of a thin-film-deposition system used to produce multilayer dielectric thin-film coatings with highly uniform thickness over a full NIF aperture.

Additional highlights of research presented in this issue include the following:

• Multiple OMEGA laser beams are used to study the two-plasmon-decay instability, which is the predominant source of suprathermal electrons in direct-drive inertial confinement fusion experiments. It is shown that the total overlapped intensity governs the scaling of the suprathermal-electron generation regardless of the number of overlapped beams, in contrast to conventional theories that are based on the single-beam approximation.
• The classical Rayleigh–Taylor instability of the interface separating two homogeneous inviscid fluid layers undergoing uniform acceleration is examined. The formulation presented makes a formal distinction between perturbation behavior under acceleration and perturbation behavior as modified by compression and by geometrical convergence of a cylindrical or spherical interface.
• Laser-induced adiabat shaping in ICF spherical targets by a technique referred to as “relaxation” is described. In this approach, the density profile of the capsule’s shell is shaped using a weak prepulse followed by a main pulse with a high-intensity foot. Rayleigh–Taylor growth rates are reduced without significantly degrading 1-D capsule performance.
• The operation of a single-photon source—a key hardware element of quantum information technologies—via photon antibunching in the fluorescence of single terrylene molecules embedded in a cholesteric liquid crystal host is demonstrated. Planar-aligned cholesteric layers provide a one-dimensional photonic band gap, allowing an enhancement of the source efficiency.
• The properties of compressed titanium due to laser-launched shocks by use of extended x-ray absorption fine structure (EXAFS) is studied. Fitting an EXAFS model to the data indicates compression by a factor of 1.3, in agreement with shock-speed measurements and with hydrodynamic simulations. The rate of decay of the modulation with wave number is shown to include a significant contribution from static disorder, in addition to thermal vibration.
• Permeation filling and cooling thin-walled (<3-µm) cryogenic capsules with deuterium– tritium fuel is a critical phase of operation for providing direct-drive targets. A model of the thermal conditions inside the permeation cell is described and used to quantify the forces on the capsule during the cooldown cycle. Results of cooldown cycles of OMEGA cryogenic targets agree well with the simulation, and a cooling program is devised whereby the time for a capsule to reach the frozen state is reduced by 30%.
• A qualitative understanding of the greenhouse effect has long been available through models based on globally and time-averaged quantities. A simple 864-cell climatological model reproduces yearly average temperatures obtained earlier from one of these global models and predicts a locally distributed nonradiative flux when observed temperatures are employed as input data. The model is a useful stepping stone for learning about radiative energy transfer into and out of Earth’s atmosphere.