Session RI1: Direct Drive ICF

Laser-Energy Coupling, Mass Ablation Rate, and Shock-Heating in Direct-Drive ICF

Direct-drive laser-energy coupling, mass ablation rate, and shock-heating are experimentally studied on the OMEGA Laser System to validate hydrodynamics simulations. High-gain, direct-drive inertial confinement fusion (ICF) target implosions require accurate predictions of the shell adiabat \textit{$\alpha $} (entropy), defined as the pressure in the main fuel layer to the Fermi-degenerate pressure. Since the minimum energy for ignition scales as $E_{min}\sim $ \textit{$\alpha $}$^{1.9}$ and the Rayleigh--Taylor ablative stabilization term is proportional to the ablation velocity $V_{a}\sim $ \textit{$\alpha $}$^{3/5}$; a balance must be struck. The temporal pulse shape of the laser irradiation determines the adiabat. A series of experiments in spherical and planar geometries with CH targets have measured the laser absorption, mass ablation rate, and the amount of shock heating in the target. 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. The amount of shock heating is diagnosed in planar-CH-foil targets using two techniques: time-resolved x-ray absorption spectroscopy and noncollective spectrally resolved x-ray scattering. The predicted shell conditions are close to the experimental results. A detailed comparison of the experimental results and the simulations will be presented. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460. Contributors: H. Sawada, D. Li, V. N. Goncharov, R. Epstein, J. A. Delettrez, J. P. Knauer, J. A. Marozas, F. J. Marshall, R. L. McCrory, P. W. McKenty, D. D. Meyerhofer, P. B. Radha, W. Seka, T. C. Sangster, S. Skupsky, V. A. Smalyuk, LLE/UR, R. Mancini, University of Nevada, S.H. Glenzer, O. Landen \textit{LLNL, }G. Gregori, \textit{RAL}   

Time-dependent nuclear measurements of fuel-shell mix in ICF implosions at OMEGA

Fuel-shell mix remains a pivotal concern in inertial confinement fusion (ICF), as it can preclude ignition. Mix is the result of saturation of Rayleigh-Taylor (RT) instability growth at a density interface that leads to small-scale, turbulent eddies and atomic-level mixing of cool, high-density fuel in the shell with hot, low-density fuel in the core. If sufficient mixing occurs, it will disrupt the formation of the ``hot-spot'' required for ignition. To sensitively probe the evolution and extent of mix in spherical implosions, the time dependence of the D$^{3}$He nuclear reaction rate was measured from implosions of capsules filled with pure $^{3}$He. The capsule shell was comprised of a 1-$\mu$m layer of CD inside a 19-$\mu$m layer of CH. Nuclear burn will only occur in such capsules if there is sufficient mixing of D from the shell with hot $^{3}$He in the core. By utilizing novel D$^{3}$He reaction-rate and proton spectrometers, all sensitive to the 14.7 MeV D$^{3}$He protons, a comprehensive, time dependent picture of mix was constructed. Important qualitative features were immediately evident: first, the shock burn of D$^{3}$He, always present for gas fills of D$^{3}$He, was absent, enabling a strong limit to be set on the amount and extent of D penetration into the $^{3}$He. Second, the time necessary for RT instabilities to induce mix and to be heated by the hot core resulted in a 90 ps delay in the D$^{3}$He bang time as compared to bang time for implosions with D$^{3}$He fills. And third, when the gas pressure of $^{3}$He was reduced from 20 to 4 atm, the extent of mix was enhanced by about a factor of 5. \newline \newline This work was supported in part by LLE, LLNL, the U.S. DoE, and the N.Y. State Energy Research and Development Authority.   

1-MJ, Wetted-Foam Target-Design Performance for the NIF

Wetted-foam, direct-drive target designs are a path to high-gain experiments on the National Ignition Facility (NIF). Wetted-foam designs\footnote{ S. Skupsky \textit{et al}., in \textit{Inertial Fusion Sciences and Applications 2001}, edited by K. Tanaka, D. D. Meyerhofer, and J. Meyer-ter-Vehn (Elsevier, Paris, 2002), p. 240.}$^{,}$\footnote{ D. G. Colombant \textit{et al}., Phys. Plasmas \textbf{7}, 2046 (2000).} take advantage of the increased laser absorption provided by the higher-atomic-number elements in the mixture of plastic foam and deuterium--tritium (DT). The fractional absorption is expected to increase by as much as 30{\%} relative to an ``all-DT'' target\footnote{ P. W. McKenty\textit{ et al}., Phys. Plasmas \textbf{8}, 2315 (2001).} for a $\sim $1-MJ design, depending on the density of the foam and the specific target design. With the increased laser coupling, more fuel can be driven with the same incident laser energy, resulting in increased target gain and/or increased hydrodynamic stability. A stability analysis of a 1-MJ design performed using two-dimensional hydrodynamic simulations in the presence of expected levels of laser and target nonuniformities will be shown. For this design, the sources of nonuniformity from the laser include power imbalance between laser beams and the imprint of single-beam nonuniformities on the target. Target nonuniformities include surface finish and inner-surface DT-ice roughness. The relative impact of these sources of nonuniformity on target performance will be examined. Particular emphasis will be placed on identifying the required levels of beam smoothing with regard to smoothing by spectral dispersion. While this emphasizes symmetric illumination, the results are relevant to polar direct drive, where a direct-drive target is driven on the NIF while it is in its indirect-drive configuration.\footnote{ S. Skupsky \textit{et al}., Phys. Plasmas \textbf{11}, 2763 (2004).} This work was supported by U. S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460. Contributors: S. Skupsky, R. Betti, P. W. McKenty, P. B. Radha, V. N. Goncharov, D. R. Harding, J. P. Knauer, J. A. Marozas, R. L. McCrory, UR/LLE.   

Direct-drive laser target designs for sub-MJ energies

New direct-drive laser target designs with KrF laser light take advantage of the shorter wavelength by lowering the laser energy required for substantial gain ($>$20 -30) to sub-MJ levels. These low-energy pellets are useful in systems that form an intermediate step towards fusion energy, such as the proposed Fusion Test Facility [1]. Aside from the lower energy, these designs are similar to previous designs [2]. The short wavelength laser allows higher intensity (and higher pressure) without increasing the risk of laser-plasma instabilities. The higher pressure in turn allows higher velocities (of the order of $4\times10^7$ cm/s) to be achieved while keeping the low pellet aspect ratios required for hydrodynamic stability. The canonical laser energy has been chosen to be 500 kJ. Target designs will be presented and both 1D and 2D simulation results will be shown, and the customary trade-off of gain for stability will be analyzed. Recently developed strategies for improving both gain and stability [3] are combined with recently developed numerical techniques for minimizing noise in the multi-dimensional code [4], allowing full high-resolution simulations of the entire implosion. The sensitivity of these targets to both low- mode (e.g., beam geometry, power imbalance, surface finish) and high mode (pellet uniformity, laser imprint) perturbations will be examined. This paper will show that significant gain can be achieved for these targets even in the presence of hydrodynamic stabilities.\newline [1] S.P. Obenschain et al., Phys. Plasmas \textbf{13}, 056320 (2006).\newline [2] R.Sacks and D.Darling, Nucl. Fusion \textbf{27}, 447 (1987); A.J. Schmitt et al, Phys. Plasmas \textbf{11}, 2716 (2004).\newline [3] K.Anderson and R.Betti, Phys. Plasmas \textbf{10}, 4448 (2003); V.Goncharov et al, Phys. Plasmas \textbf{10}, 1906 (2003)\newline [4] S.Zalesak et al, Phys. Plasmas \textbf{12},056311 (2005)\newline * in collaboration with A.J. Schmitt, S.P. Obenschain, S.T. Zalesak, A. Velikovich, J.W. Bates, D.E. Fyfe, B.B. Afeyan, J.H. Gardner and W. Manheimer