LLE Review 137

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Highlights

This volume of the LLE Review, covering October–December 2013, features “Theory of Hydro-Equivalent Ignition for Inertial Fusion and Its Applications to OMEGA and the National Ignition Facility.” This article reports the development of the theory of hydrodynamic similarity, which is used to scale the performance of direct-drive cryogenic implosions conducted on the Omega Laser Facility to National Ignition Facility (NIF) energy scales. The theory of hydrodynamic similarity is tested with hydrodynamic simulations, and then used to determine the requirements for demonstrating hydro-equivalent ignition (implosions with the same implosion velocity, adiabat, and laser intensity) on OMEGA. Hydro-equivalent ignition on OMEGA is represented by a cryogenic implosion that would scale to ignition on the NIF at 1.8 MJ of laser energy symmetrically illuminating the target. It is found that a reasonable combination of neutron yield and areal density for OMEGA hydro-equivalent ignition is 3 to 6 × 10¹³ and ~0.3 g/cm², respectively, depending on the level of laser imprinting, though this performance has not yet been achieved on OMEGA.

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

• The physics of direct-drive implosions that are hydrodynamically equivalent to ignition designs on the National Ignition Facility (NIF) were studied on the Omega Laser System. It is shown that the highest hot-spot pressures (up to 40 Gbar) are achieved in moderate-fuel adiabat (α ~ 4) target designs, which are well understood using 2-D hydrocode simulations. The performance of lower-adiabat implosions is significantly degraded relative to the code predictions, and simplified theoretical models are developed to gain physical understanding of the implosion dynamics that dictate the target performance.
• Angular filter refractometry (AFR), a novel diagnostic technique, has been developed and used on OMEGA EP to characterize high-density, long-scale–length plasmas relevant to high-energy-density physics experiments. AFR is used to study the plasma expansion from CH foil and spherical targets that are irradiated with ~9 kJ of ultraviolet (351-nm) laser energy.
• The experimental evidence for multibeam laser–plasma instabilities of relevance to laser-driven inertial confinement fusion at the ignition scale, is reviewed for both the indirect- and direct-drive approaches. The instabilities described are cross-beam energy transfer, multibeam stimulated Raman scattering, and multibeam two-plasmon–decay instability. Advances in theoretical understanding, and in the numerical modeling of these multibeam instabilities are discussed.
• New quantum molecular-dynamics (QMD) calculations are presented for the thermal conductivity (κ) of deuterium, over the broad density (ρ = 1.0 to ~700 g/cm³) and temperature (T = 5 × 10³ K to T = 8 × 10⁶ K) conditions undergone by ICF imploding fuel shells. Over the wide ranges of conditions in such coupled and degenerate plasmas, the extensively used Spitzer model and a variety of other thermal conductivity models break down. The differences resulting from the use of κ_QMD are shown to be particularly relevant for lower adiabat implosions and shell conditions during the early stages of an implosion.
• Cascaded nonlinearities in a regenerative laser amplifier are demonstrated to compensate for intracavity self-phase modulation. Without compensation, self-phase modulation limits the generation of high-quality short optical pulses because of spatial self-focusing and spectral broadening. Experimental results obtained on two Nd:YLF regenerative amplifiers achieve a significant reduction in spectral broadening, and are in good agreement with predictions from simulations performed as part of this study.