Laser absorption, mass ablation rate, and shock heating in direct-drive inertial confinement fusion

Abstract

Direct-drive laser absorption, mass ablation rate, and shock heating are experimentally studied on the OMEGA Laser System [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] to validate hydrodynamics simulations. High-gain, direct-drive inertial confinement fusion target implosions require accurate predictions of the shell adiabat α (entropy), defined as the pressure in the main fuel layer to the Fermi-degenerate pressure, and the implosion velocity of the shell. The laser pulse shape determines the shell adiabat and the hydrodynamic efficiency determines the implosion velocity. A comprehensive set of measurements tracking the flow of energy from the laser to the target was conducted. Time-resolved measurements of laser absorption in the corona are performed on spherical implosion experiments. The mass ablation rate is inferred from time-resolved Ti K-shell spectroscopic measurements of nonaccelerating, solid CH spherical targets with a buried tracer layer of Ti. Shock heating is diagnosed in planar-CH-foil targets using time-resolved x-ray absorption spectroscopy and noncollective spectrally resolved x-ray scattering. The highly reproducible experimental results achieved with a high level of laser drive uniformity [S. P. Regan et al., J. Opt. Soc. Am. B 22, 998 (2005)] constrain the modeling of direct-drive energy coupling. A detailed comparison of the experimental results and the simulations reveals that a single-value flux limiter in the thermal transport model cannot explain all of the experimental observables. Simulations of laser absorption measurements need a time-dependent flux limiter to match the data. Modeling of both resonance absorption and nonlocal effects in the electron thermal conduction from the critical density to the ablation front are underway to resolve the observed discrepancies.

Al 1 s - 2 p absorption spectroscopy of shock-wave heating and compression in laser-driven planar foil

“…Although simulations with a constant flux limiter and experiments agree well, simulations with the same value of f do not consistently match to the all experimental data. 7 For instance, shock-velocity measurements in CH foils on OMEGA 8 agree with the simulation with f = 0.06, while the Richtmyer-Meshkov-growth measurements are in agreement with f = 0.1 (Ref. 9).…”

Time history prediction of direct-drive implosions on the Omega facility

“…25 But it has been known for several years that a flux-limited thermal conduction model is not consistent with all experimental observables. 21 A nonlocal model is an improvement of this theory and allows accounting for the nonlocal aspect of the electron thermal conduction, 26 without using flux-limiters. In our 2D hydrodynamic code, FCI2, 27 we use the model proposed by Schurtz-Nicola€ ı-Busquet (SNB).…”

Design of the high-resolution soft X-ray imaging system on the Joint Texas Experimental Tokamak

“…4,5 Within the fundamental science program at the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory (LLNL) it was proposed to use XRTS as a temperature diagnostic in a series of equation-of-state (EOS) experiments that explore matter at extreme pressures that approach and exceed 1 Gbar. 6 At sufficiently large x-ray photon energies and scattering angles, XRTS is probing the response of individual electrons, and the shape of the inelastically down-scattered Compton profile reflects their velocity distribution in the plasma. 4 At the conditions expected in these EOS experiments the plasma is non-degenerate for temperatures greater than ≈100 eV.…”