LLE Review 160

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Highlights

This volume of LLE Review 160, covering the period July–September 2019, is sectioned among research areas at LLE and external users of the Omega Laser Facility. Articles appearing in this volume are the principal summarized results of long-form research articles. Readers seeking a more-detailed account of research activities are invited to seek out the primary materials appearing in print, detailed in the publications and presentations section at the end of this volume.

Highlights of research presented in this volume include the following:

• A. R. Christopherson et al. derive an ignition criterion for inertial fusion implosions based on yield amplification from alpha-particle heating (p. 187). They also show that the criterion holds when compared to simulations that include hot-spot perturbations.
• L. Ceurvorst et al. present a hybrid target design that incorporates elements of both direct- and indirect-drive target designs for mitigating the Rayleigh–Taylor instability (p. 190). A planar experiment was completed to show that forming a pre-plasma with a thin, Au-lined CH that is offset from a thicker, CH planar target will reduce instabilities when the latter is shocked by a drive pulse.
• J. J. Ruby et al. describe two modifications to Guderley’s solution to the converging shock problem to extend its utility: (1) the addition of an isentropic release wave and (2) fluid partitioning between an electron fluid and an ion fluid (p. 193). A comparison of the modified solution to LILAC simulations shows agreement over a broad range of solid CH sphere implosions.
• A. V. Maximov, J. G. Shaw, and J. P. Palastro detail a dynamic instability saturation mechanism, allowing for laser light to transmit through coronal plasma despite surpassing the threshold conditions for stimulated Raman scattering (SRS), as observed in planar experiments at the National Ignition Facility (NIF) (p. 195). This resulting enhanced transmission explains why models with no absolute SRS are able to reproduce inferred temperatures in ignition-scale experiments on the NIF.
• A. A. Solodov et al. describe an experimental platform fielded on the NIF for investigating hot-electron production from laser–plasma instabilities at direct-drive ignition-relevant conditions (p. 198). Hot-electron conversion efficiencies between 0.5% and 5% were inferred when quarter-critical intensities were varied from ~4 to 15 × 10¹⁴ W/cm².
• H. Wen et al. use 3-D particle-in-cell (PIC) simulations to investigate the interplay between two-plasmon decay and SRS in conditions relevant to inertial confinement fusion experiments (p. 200). In comparison to 2-D PIC simulations, this results in a fast-electron fraction level much closer to experimental measurements.
• Y. Zhao and W. R. Donaldson detail the fabrication of aluminum–gallium–nitride photodetectors with micrometer-scale metal–semiconductor–metal structures (p. 205). The diagnostic was tested with ultrafast UV laser pulses.
• C. Dorrer, E. M. Hill, and J. D. Zuegel demonstrate efficient parametric amplification of broadband spectrally incoherent pulses (p. 208). These results will be applied to the development of a 1% fractional bandwidth laser that promises smoother profiles and improved performance in direct-drive implosions.
• M. J. Guardalben et al. describe PSOPS, a MATLAB-based semi-analytic model for the OMEGA EP Laser System to predict system performance (p. 211). The model has reaped the benefits of real-time optimization of the laser system configuration and enabled several enhancements to the laser system performance.
• H. Huang et al. present a method for calculating the Raman fluency at the surface of an arbitrary crystal and pump polarization configurations for the purpose of better understanding laser power level limitations in transverse Raman generation and amplification in KDP/DKDP crystals (p. 218). A code using this method is able to provide estimates of maximum laser intensity that will not damage the amplification medium.
• T. Z. Kosc et al. investigate the damage resistance of liquid crystals for laser excitation conditions, including those relevant to OMEGA (p. 220). The data suggest a complex interplay of both multiphoton absorption and excited-state absorption are key components in laser-induced–damage mechanisms.
• S. MacNally et al. study multilayer glancing-angle deposition on a silica substrate for reducing light scattering on film (p. 222). Applying this technique gave measured persistently low-scatter loss compared to single-layer coating runs.
• J. B. Oliver, B. Charles, and J. Spaulding present an optical coating technique using radially nonuniform film deposition that was developed to mitigate deformations induced by high-compressive films and create a nominally flat coated surface (p. 225). Using this technique, surface deformations were reduced by nearly 90% relative to that of a coated surface without a graded corrective layer.
• E. Schiesser et al. describe details for a four-mirror image relay—the theoretical basis for the correction of a field-constant coma and field-constant astigmatism (p. 227). The alignment of the design in a test-bed setup is then presented.
• C. Smith, S. MacNally, and J. B. Oliver describe a process using ellipsometric measurement techniques along with a well-established optical coating design to create precise index models for improved coating design (p. 232). This is made possible by a complex optical coating using only a single material.
• R. B. Spielman and A. B. Sefkow describe the design of a variable-impedance, disk magnetically insulated transmission line (MITL) that allows for the reduction in inductance of disk MITL’s (p. 234). Electron flow and losses are shown using 2-D electromagnetic PIC simulations.
• R. S. Craxton summarizes the 31st LLE Summer High School Research Program. Fourteen students were invited from Rochester-area high schools to participate in the lab’s state-of-the-art research environment (p. 237).
• J. Puth, M. Labuzeta, and D. Canning summarize operations of the Omega Laser Facility during the fourth quarter of FY19 (p. 239).