LLE Review 116

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Highlights

This volume of the LLE Review, covering July–September 2008, features a report on expressions for the yield of electron–positron pairs, their energy spectra, and production rates have been obtained in the interaction of multi-kJ pulses of high-intensity laser light interacting with solid targets. The Bethe–Heitler conversion of hard x-ray bremsstrahlung is shown to dominate over direct production (trident process). The yields and production rates have been optimized as a function of incident laser intensity, by the choice of target material and dimensions, indicating that up to 5 × 10¹¹ pairs can be produced on the OMEGA EP Laser System. The corresponding production rates are sufficiently high that the possibility of pair-plasma creation is shown to exist.

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

• Simulations describing neutron yields of direct-drive, low-adiabat (α ≅ 2 to 3) cryogenic D₂ target implosions on OMEGA have been systematically investigated using the two-dimensional (2-D) radiation hydrodynamics code DRACO. Low-mode (l × 12) perturbations, including initial target offset, ice-layer roughness, and laser-beam imbalance, were found to be the primary source of yield reduction in implosions for thin-shell (5-µm), low-α, cryogenic targets. These 2-D numerical results provide a reasonably good guide to understanding the yield degradation in direct-drive, low-adiabat, cryogenic, thin-shell-target implosions and track experimental results. Simulations including the effect of laser-beam nonuniformities indicate that high-l-mode perturbations caused by laser imprinting play a role in further degrading the neutron yield of thick-shell implosions.
• A time-resolved Al 1s–2p absorption spectroscopy is used to diagnose direct-drive, shock-wave heating and compression of planar targets having nearly Fermi-degenerate plasma conditions (Te ~ 10 to 40 eV, ~3 to 11 g/cm³) on the Omega Laser System. The laser-ablation processes launch 10- to 70-Mbar shock waves into the CH/Al/CH target. The Al 1s–2p absorption spectra were analyzed using the atomic physics code PrismSPECT to infer Te and in the Al layer, assuming uniform plasma conditions during shock-wave heating, to determine when the heat front penetrated the Al layer. The predictions of simulated shock-wave heating and the timing of heat-front penetration are compared with the observations. The experimental results for a wide variety of laser-drive conditions and buried depths have shown that the LILAC predictions using f = 0.06 and the nonlocal model accurately model the shock-wave heating and timing of the heat-front penetration while the shock is transiting the target.
• The ignition condition (Lawson criterion) for inertial confinement fusion can be cast in a form dependent on the only two measurable parameters of the compressed fuel assembly: the hot-spot ion temperature and the total areal density (ρR_tot) that includes the cold shell contribution. A marginal ignition curve is derived in the ρR_tot, plane and current implosion experiments are compared with the ignition curve. Such a criterion can be used to determine how surrogate D₂ and sub-ignited DT target implosions perform with respect to the one-dimensional ignition threshold.
• Ultrafast THz-pulse time-domain spectroscopy (TDS) and femtosecond optical-pump THz-probe (OPTP) studies of Hg-Ba-Ca-Cu-O (HBCCO) high-temperature, superconducting thin films. The time-resolved OPTP spectroscopy experiments showed that the quasiparticle relaxation process exhibited an intrinsic single-picosecond dynamics with no phonon bottleneck, which is a unique feature among superconductors and makes the HBCCO material very promising for ultrafast radiation detector applications.
• This volume concludes with a summary of LLE’s Summer High School Research Program, the FY08 Laser Facility Report, and the National Lasers Users’ Facility and External Users’ Programs.