LLE Review 138


This volume of the LLE Review, covering January–March 2014, features “Method to Measure Young’s Modulus and Damping of Fibers at Cryogenic Temperatures.” This article reports on a novel experimental method that is used to measure key physical properties for fibers used to mount cryogenic implosion targets. Young’s modulus and the critical damping ratio of such fibers, at temperatures from 295 K to 20 K, are needed to design stable targets that are required for high-yield implosions, but these property values do not exist in the literature. The method described enables accurate measurements of the properties of interest for fiber diameters as small as 12 µm at ~20 K, and measurements are reported for fibers made of the following materials: Nicalon™ ceramic grade [silicon carbide (SiC)], Zylon®HM {poly[p-phenlyne-2,6-benzobisoxazole] (PBO)}, M5 {dimidazo-pyridinylene [dihydroxy] phenylene (PIPD)}, and polyimide.

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

  • Radiography with ~10- to 50-MeV protons is used to investigate electric and self-generated magnetic fields in direct-drive–implosion experiments at the Omega Laser Facility. The experiments used plastic-shell targets with imposed surface defects to enhance self-generated fields. The proton radiographs show multiple ring-like structures produced by electric fields ~107 V/cm, as well as fine structures from surface defects, indicating self-generated fields up to ~3 MG. These electric and magnetic fields show good agreement with two-dimensional magnetohydrodynamic simulations.
  • Absolute calibration of the optical response of the streaked optical pyrometer on the OMEGA Laser System has been completed using a NIST–traceable tungsten-filament lamp. Laser-driven high-energy-density-physics experiments typically have time scales of tens of picoseconds, requiring the use of a streak camera, which complicates the already formidable task of absolute calibration. The article reports a simple closed-form equation for the brightness temperature as a function of streak-camera intensity derived from this calibration.
  • The linear stability for multiple coherent laser beams is studied with respect to the two-plasmon–decay instability in an inhomogeneous plasma in three dimensions. Cooperation between beams leads to absolute instability of long-wavelength decays, while shorter-wavelength shared waves are shown to saturate convectively. Nonlinear calculations show that Langmuir turbulence created by the absolute instability modifies the convective saturation of the shorter-wavelength modes, which are seen to dominate at late times.
  • The absorption of near-UV photons by HfO2 monolayer films is reduced by up to 70% following irradiation by continuous-wave (cw) near-UV laser light. Hafnium oxide is the most frequently used high-index material in multilayer thin-film coatings for high-power laser applications; absorption in this high-index material is known to be responsible for nanosecond-pulse laser-damage initiation in multilayers. Nanosecond-pulse laser-damage tests confirm a reduction of absorption by measuring up-to-25%-higher damage.
  • The equation of state (EOS) and optical reflectivity are investigated in shock-compressed polystyrene (CH) up to the unprecedented high pressure of 62 Mbar along the principal Hugoniot. The results from a first-principles quantum molecular dynamics (QMD) method are compared with existing experimental measurements as well as the SESAME EOS model. The predicted pressure/temperature and optical reflectivity of shocked CH from QMD calculations agree well with experiments for pressures below 10 Mbar; above 10 Mbar, the QMD-predicted polystyrene is stiffer than the SESAME model prediction.
  • A new collimated ion-beam–sputtering process is used to demonstrate conformal deposition of a multilayer reflector over the surface relief of a sinusoidal diffraction grating. This process is attractive for high-laser-damage-threshold applications, with each layer acting as a diffractive element that contributes to the overall diffraction efficiency. By depositing hafnia and silica dielectric layers to enhance the reflectance of a silver film on a 1740-lines/mm sinusoidal grating, a diffraction efficiency of 93% (p-polarized light) and 1-on-1 laser-damage threshold of 2.66±0.15 J/cm2 (1053-nm, 65° incidence, 10-ps pulse) have been achieved.