LLE Review 146

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Highlights

This volume of the LLE Review, covering January–March 2016, features “Isolating and Quantifying Cross-Beam Energy Transfer in Direct-Drive Implosions on OMEGA and the National Ignition Facility.” This article examines mass-ablation rates and ablation-front trajectories to quantify cross-beam energy transfer (CBET) using a polar-direct-drive configuration. The polar-direct-drive configuration allows CBET to be limited at the target poles while maintaining its influence at the equator. Hydrodynamic simulations performed without CBET agree with the measured ablation rate and ablation-front trajectory at the target pole, confirming that the CBET effects at the pole are small. CBET simulations incorporating a gain multiplier lead to excellent agreement with both polar and equatorial measurements.

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

• • An eight-channel, time-multiplexed pulse-shaping system that generates optical waveforms from a single pulse-shaping unit, demultiplexes, and retimes them is demonstrated. The system is capable of providing pulses to multiple optical systems with low relative jitter and cost. Losses of less than 5 dB and extinction ratios of the order of 50 dB for an eight-channel system are measured for the system, with improved performance for four-channel operation.
  • A design approach for continuous distributed phase plates (DPP) is described using the code Zhizhoo’, which allows rapid DPP design optimization with high-fidelity focal-spot shapes relative to the design objective. Exceptional control of the envelope shape, spectral and gradient control, and robustness from near-field phase aberrations is realized. Phase-dislocation–free DPP designs with low near-field modulation have been achieved with <1% to 2% weighted σ_rms error of the far-field spot shape.
  • A unique approach for permeation filling of non-permeable inertial confinement fusion target capsules with deuterium-tritium (DT) is described. This process uses a permeable capsule coupled into the final target capsule with a tapered 0.1- or 0.08-mm-diameter fill tube, allowing targets of new materials to be filled without the design and construction of a fill-tube–based DT filling station. Permeation filling of glow-discharge polymerization (GDP) targets using this method has been successfully demonstrated, as well as ice layering of the target, yielding an inner ice surface roughness of <1-µm rms.
  • Experimental efforts to correlate the mechanical properties of multilayer diffraction gratings to the laser-induced–damage thresholds (LIDT) are reported. Lower LIDT’s are strongly correlated with greater yield stresses and lower penetration depths as measured by nanoindentation of holographic diffraction gratings etched into silica. This work indicates that mechanical testing may provide guidance on grating cleanliness and damage thresholds for use in high-intensity laser systems.
  • Experimental efforts to correlate the mechanical properties of multilayer diffraction gratings to the laser-induced–damage thresholds (LIDT) are reported. Lower LIDT’s are strongly correlated with greater yield stresses and lower penetration depths as measured by nanoindentation of holographic diffraction gratings etched into silica. This work indicates that mechanical testing may provide guidance on grating cleanliness and damage thresholds for use in high-intensity laser systems.

Magnetorheological (MR) finishing of infrared polycrystalline materials is reported. Acidic, low-viscosity magnetorheological fluids containing alumina or nanodiamond are used to polish infrared materials such as chemical-vapor–deposited (CVD) ZnS in an effort to reduce surface microroughness. Surface roughness and power spectral density results show that the emergence of “pebbles” on the surface of several CVD ZnS substrates finished with the acidic MR fluid containing a nanodiamond abrasive is significantly minimized, and the surface microroughness achieved was as low as ~30 nm peak-to-valley and ~6-nm rms.