This volume of the LLE Review, covering October to December 2004, highlights the significance of shaped adiabats to inertial confinement fusion. Theory suggests that inertial confinement fusion (ICF) capsules compressed by shaped adiabats will exhibit improved hydrodynamic stability. Although the theoretical formulas for the adiabat profiles generated by the relaxation method of adiabat shaping have been previously derived, the formulas presented in this Review are simplified to power-law expressions. Experiments demonstrate that target stability improves when picket pulses are used to increase and shape the ablator adiabat. Rayleigh–Taylor (RT) growth of nonuniformities is suppressed in both planar and spherical targets with picket-pulse laser illumination. Two types of picket pulses—a “decaying-shock-wave picket” and a “relaxation” picket—are used to shape the adiabat in spherical targets. Planar growth measurements using a wide, intense picket to raise the adiabat of a CH foil show that the growth of short-wavelength perturbations is reduced, and even stabilized, by adjusting the intensity of the picket. Data from planar imprint experiments show that the imprint level is reduced when a picket is added and, for short wavelengths, is as effective as 1-D, 1.5-Å SSD. Results from relaxation-picket implosions show larger yields from fusion reactions when the picket drive is used.
Additional research developments presented in this issue include the following:
- Crater formation in SiO2 thin films containing artificial defects by UV-pulsed-laser irradiation depends on the lodging depth of the defects. At laser fluences close to the crater-formation threshold and for lodging depths of a few particle diameters, the dominating material-removal mechanism is melting and evaporation.For absorbing defects lodged deeper than ~10 particle diameters, a two-stage material-removal mechanism occurs. The process starts with the material melting within the narrow channel volume, and, upon temperature and pressure buildup, film fracture takes place.
- Altering the fluid composition of magnetorheological (MR) fluids prepared with a variety of magnetic and nonmagnetic ingredients minimizes artifact formation on the surface of CVD ZnS flats and greatly improves the smoothing performance of magnetorheological finishing. A nanoalumina abrasive used with soft carbonyl iron and altered MR fluid chemistry yields surfaces with roughness that do not exceed 20 nm p–v and 2-nm rms after removing 2 µm of material. The formation of “orange peel” and the exposure of “pebble-like” structure inherent in ZnS from the CVD process are suppressed.
- A 63-channel, high-resolution, ultraviolet (UV) spectrometer to check the tuning state of KDP triplers has been designed and tested. The spectrometer accepts an input energy of 1 µJ per channel, has a dispersion at the detector plane of 8.6 X 10–2 picometers (pm)/µm, and has a spectral window of 2.4 nanometers (nm) at λ = 351 nm. The wavelength resolution varies from 2.5pm at the center of the field of view to 6 pm at the edge.
- The quantum efficiency (QE) and the noise equivalent power (NEP) of the latest-generation, nanostructuredNbN, superconducting, single-photon detectors (SSPD’s) operated at temperatures in the 2.0- to 4.2-K range in the wavelength range from 0.5 to 5.6 µm has been measured. The detectors are designed as 4-nm-thick, 100-nm-wide NbN meander-shaped stripes, patterned by electron-beam lithography. Their active area is 10 X 10 µm2. The best-achieved QE at 2.0 K for 1.55-µm photons is 17%, and the QE for 1.3-µm infrared photons reaches its saturation value of ~30%. The SSPD NEP at 2.0 K is as low as 5 X 10–21 W/Hz–1/2. These SSPD’s, operated at 2.0 K, significantly outperform their semiconducting counterparts. Together with their GHz counting rate and picosecond timing jitter, they are the devices-of-choice for practical quantum key distribution systems and quantum optical communications.
- The first measurements of electron preheat in direct-drive laser implosions of cryogenic deuterium targets are reported. Preheat due to fast electrons generated by nonlinear laser–plasma interactions can reduce the gain in laser-imploded fusion targets. The preheat level is derived directly from the measured hard-x-ray spectrum. The fraction of the incident laser energy that preheats the deuterium fuel is found to be less than 0.1%, suggesting that the preheat will have a negligible impact on target performance.