LLE Review 95

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Highlights

This volume of the LLE Review, covering April–June 2003, features “Reduction of the Ablative Rayleigh–Taylor Growth Rate with Gaussian Picket Pulses.” This article reports on the seminal work to experimentally validate the reduction in the Rayleigh–Taylor (RT) growth rate using a prepulse, or picket, preceding the main laser-drive pulse in planar-target experiments. The experimental results showed that a high-intensity picket (~50% of the drive-pulse intensity) significantly reduced the RT growth rate for a 20-µm-wavelength surface perturbation but had no effect on the growth rates of longer-wavelength perturbations (30 and 60 µm). However, both the 20- and 30-µm-wavelength perturbations showed no appreciable growth rate with a prepulse intensity equal to the drive-pulse intensity. These results suggest that the acceleration-phase RT growth rates for short-wavelength, laser-induced imprint perturbations may be virtually eliminated in spherical implosions by modifying the drive pulse to include a high-intensity picket on the leading edge. This work will be applied to spherical implosions in the near future.

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

• An analytic analysis of laser-induced adiabat shaping in inertial fusion implosions shows that the adiabat profile between the ablation surface and the fuel–shell interface induced by a decaying shock follows a simple power law of the shell areal density. Significantly, the calculated profiles are nearly identical to those observed in 1-D hydrodynamic simulations. This similarity suggests that the calculated profiles can be used to quickly and easily design an optimal laser prepulse to maximize the adiabat ratio between the inner- and outer-shell surfaces, leading to improved hydrodynamic stability.
• The development and application of a numerical model to systematically investigate the performance of an optical parametric chirped-pulse amplification (OPCPA) system are described. The model uses both Gaussian and super-Gaussian spatial and temporal pump laser pulse shapes and includes the effects of pump– signal spatial walk-off and spatiotemporal noise. The results of this numerical investigation show that good energy stability, good beam quality, and high overall conversion efficiency can be obtained by carefully designing the OPCPA configuration and optimizing the spatiotemporal profile of the pump laser.
• The nonlinear propagation of light through a plasma near the critical density is described using a model that includes filamentation, forward stimulated Brillouin scattering (SBS), backward SBS, the reflection of light from the critical-density surface, and the absorption of light. Because the model incorporates nonparaxial propagation of light, it can describe the reflection of light from the critical-density surface and the propagation of crossing laser beams. The study shows that the model successfully describes experimentally observed features of scattered light and is well suited to describe the oblique incidence of laser beams on a critical-density surface.
• Ultrafast voltage transients in optically thick YBCO superconducting microbridges driven into the resistive flux state by nanosecond-wide supercritical current pulses and synchronously excited by femtosecond optical pulses have been investigated. Using a flexible experimental setup, the dynamics of the YBCO photoresponse have been measured. These measurements demonstrate that a YBCO superconductor in the flux-flow state can operate as a GHz-rate, high-power, optically triggered switch for microwave-based telecommunication applications.
• The fabrication and testing of ultrafast photodetectors fabricated on high-energy-nitrogen–implanted gallium-arsenide (GaAs) are described. By direct comparison, it is shown that this novel photodetector is significantly more sensitive than commercially available low-temperature GaAs photodevices used extensively in high-speed applications.
• By incorporating tritium—the radioactive isotope of hydrogen—into standard hydrogenated amorphous silicon (a-Si:H) devices, it may be possible to establish a new family of devices in which the energy output of the tritium decay is integrated with the optoelectronic properties of a-Si:H (e.g., photovoltaics and active matrix displays). This article describes the fabrication process and shows unequivocally that tritium bonds stably in amorphous silicon.