Session QI3: Direct-Drive ICF; X-Ray Diagnostics

Low-Adiabat, High-Compression Cryogenic Deuterium--Tritium Implosions on OMEGA

The performance of cryogenic deuterium--tritium (DT) spherical implosions using triple-picket designs is reported. These designs facilitate control of shock heating in low-adiabat inertial confinement fusion targets. The perturbation growth in triple-picket designs is controlled by the adiabat shaping imposed by a series of decaying shocks launched by individual pickets. Taking advantage of enhanced ablative stabilization of Rayleigh--Taylor instability makes it possible to simplify target design by replacing wetted foam in the pusher material with solid plastic (CH). As reported previously,\footnote{V.N. Goncharov \textit{et al., }Phys. Rev. Lett. \textbf{104}, 165001 (2010).} areal density up to 300 mg/cm$^{2}$, the highest ever measured in cryogenic-DT implosions, have been measured using these designs with the implosion velocity $\sim $3 $\times $ 10$^{7}$ cm/s. This talk will summarize the results on improving target performance and the progress in theoretical understanding of cryogenic implosions on OMEGA. To identify the main limiting sources in target performance and to improve target yield and ion temperature at the peak compression, extensive 2-D simulations using the hydrocode \textit{DRACO} have been performed. Using these simulations, target sensitivities to different levels of laser-nonuniformity smoothing, ice roughness, laser-power imbalance, and target offset were identified. To minimize both the effective beam mispointing and pulse-shape distortion induced by a 1-Thz, 1-color-cycle SSD system, 0.3-Thz, 3-color-cycle SSD smoothing was implemented on the OMEGA Laser System. In addition, to improve target quality, isolated defects in the ice at the point of contact of the stalk mount with target surface were mitigated by applying IR heating (ice shimming). This work was supported by the U.S. Department of Energy (DOE) Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

Inertial Fusion Target Physics Advantages with the Krypton Fluoride Laser

The krypton fluoride (KrF) laser's short wavelength, broad bandwidth and capability to provide extremely uniform target illumination are advantages towards obtaining high gain direct drive implosions. The short wavelength helps suppress deleterious laser-plasma instabilities, and allows one to employ higher ablation pressures. In addition, the KrF architecture allows one to zoom down the focal diameter to follow the size of the imploding pellet, thereby improving the coupling efficiency. The NRL researchers have been conducting theoretical and experimental studies to quantify the beneficial effects of utilizing KrF light. Experiments using the Nike facility have confirmed that KrF light significantly increases the threshold for laser-plasma instability. This presentation will discuss the observed target physics with KrF light and its effects towards facilitating the high gains needed for power production with inertial fusion. Simulations indicate that shock ignited designs can achieve gains above 200 with KrF energies as low a 1 megajoule. For fusion energy a laser driver must be capable of high repetition rates (5-10 Hz) along with adequate efficiency and durability. The Electra KrF 30-cm aperture electron-beam-pumped amplifier has demonstrated long duration continuous operation at high-repetition rates. This and other advances show that the KrF laser should be able to meet the requirements.   

Absolute measurements of short-pulse, long-pulse, and capsule-implosion backlighter sources at x-ray energies greater than 10 keV

Laser-generated x-ray backlighters with x-ray energies $>$ 10 keV are becoming essential diagnostic tools for many high energy density experiments. Examples include studies of high areal density cores for ignition designs, mid- to high-Z capsule implosion experiments, absolute equation of state experiments, dynamic diffraction under extreme pressures, and the study of material strength. Significant progress has been made recently using short pulse lasers, coupled to metal foil targets [1], and imploding capsules for producing high energy backlighters. Measuring the absolute x-ray flux and spectra from these sources is required for quantitative analysis of experimental data and for the design and planning of future experiments. We have performed an extensive series of experiments to measure the absolute x-ray flux and spectra on the Titan, Omega, Omega-EP, and NIF laser systems, employing single-photon-counting detectors, crystal spectrometers, and multichannel differential filtering (Ross-pair) and filter stack bremsstrahlung spectrometers. Calibrations were performed on these instruments [2] enabling absolute measurements of backlighter spectra to be made from 10 keV to 1 MeV. Various backlighter techniques that generate either quasi-monochromatic sources or broadband continuum sources will be presented and compared. For Molybdenum K$\alpha $ backlighters at x-ray energy of $\sim $17 keV we measure conversion efficiencies of 1.3x10$^{-4}$ using 1 $\mu $m wavelength short-pulse lasers at an intensity of $\sim $1x10$^{17}$ W/cm$^{2}$. This is a factor of $\sim $2 high than using 0.3 $\mu $m wavelength long-pulse lasers at an intensity of $\sim $1x10$^{16}$ W/cm$^{2}$. Other types of backlighter targets include capsule implosion backlighters that can generate a very bright ``white-light'' continuum x-ray source and high-Z gas filled capsules that generate a quasi-line-source of x rays. We will present and compare the absolute laser energy to x-ray conversion efficiencies for these different backlighter techniques and give examples of the science experiments that they enable. Lawrence Livermore National Laboratory is operated by Lawrence Livermore National Security, LLC, for the U.S. Department of Energy, National Nuclear Security Administration under Contract DE-AC52-07NA27344. \\[4pt] [1] H-S. Park, PoP, 15, 072705 (2008). \\[0pt] [2] B. R. Maddox, RSI, submitted (2010).   

Compton Radiography of Imploded Capsule Shells

In Inertial Confinement Fusion (ICF), a number of concurrent processes can contribute to degrading the degree and uniformity of compression of the imploding shell and fuel and hence the yield, namely hydro-instabilities, increase in entropy and residual asymmetries in the drive or target. Obtaining images at stagnation time of the compressed and relatively cold deuterium-tritium fuel surrounding the hot-spot is therefore fundamental to distinguishing between the degradation mechanisms so they may be mitigated on later shots. Here we report on the development and first demonstration of hard x-ray radiography of implosions obtained at photon energies around and above 100keV, where the Compton effect is the dominant contributor to the opacity. The radiographs of plastic shell implosions were obtained at the OMEGA/EP laser facility using gold micro wires in a point projection geometry and have a spatial resolution of $\sim$10$\mu$m and a temporal resolution of $\sim$10ps. This novel ``Compton'' radiography technique is an invaluable diagnostic tool for ICF targets and will be deployed at the National Ignition Facility (NIF).