[en] OAK-B202: X-ray Spectroscopic Measurements of Areal Density and Modulations of Compressed Shells in Implosion Experiments on OMEGA

[en] Rayleigh-Taylor (RT) instability is one of the major concerns in inertial confinement fusion (ICF) because it amplifies target modulations in both acceleration and deceleration phases of implosion, which leads to shell disruption and performance degradation of imploding targets. This article reviews experimental results of the RT growth experiments performed on OMEGA laser system, where targets were driven directly with laser light. RT instability was studied in the linear and nonlinear regimes. The experiments were performed in acceleration phase, using planar and spherical targets, and in deceleration phase of spherical implosions, using spherical shells. Initial target modulations consisted of two-dimensional (2D) pre-imposed modulations, and 2D and three-dimensional (3D) modulations imprinted on targets by the nonuniformities in laser drive. In planar geometry, the nonlinear regime was studied using 3D modulations with broadband spectra near nonlinear saturation levels. In acceleration-phase, the measured modulation Fourier spectra and nonlinear growth velocities are in good agreement with those predicted by Haan's model (Haan 1989 Phys. Rev. A 39 5812). In a real-space analysis, the bubble merger was quantified by a self-similar evolution of bubble size distributions (Oron et al 2001 Phys. Plasmas 8 2883). The 3D, inner-surface modulations were measured to grow throughout the deceleration phase of spherical implosions. RT growth rates are very sensitive to the drive conditions, therefore they can be used to test and validate drive physics in hydrodynamic codes used to design ICF implosions. Measured growth rates of pre-imposed 2D target modulations below nonlinear saturation levels were used to validate nonlocal thermal electron transport model in laser-driven experiments.

[en] The evolution of shell modulations near peak compression of direct-drive spherical-target implosions has been measured using the 60-beam, 30-kJ UV OMEGA laser system. The spatial size and amplitude of shell-areal-density modulations decrease during the target compression, then increase during its decompression as expected. The shell uniformity at peak compression has been increased by reducing single-beam, laser-drive nonuniformity

[en] Encouraging progress is being made in demonstrating control of ablation front hydrodynamic instability growth in inertial confinement fusion implosion experiments on the National Ignition Facility [E. I. Moses, R. N. Boyd, B. A. Remington, C. J. Keane, and R. Al-Ayat, Phys. Plasmas 16, 041006 (2009)]. Even once ablation front stabilities are controlled, however, instability during the stagnation phase of the implosion can still quench ignition. A scheme is proposed to reduce the growth of stagnation phase instabilities through the reverse of the “adiabat shaping” mechanism proposed to control ablation front growth. Two-dimensional radiation hydrodynamics simulations confirm that improved stagnation phase stability should be possible without compromising fuel compression

[en] We present initial characterization data from a new single-line-of-sight (SLOS) x-ray framing camera. The instrument uses an image-dissecting structure inside an electron optic tube to produce up to four simultaneous dc images from a single image incident on the cathode and a microchannel plate-based device to provide the temporal gating of those images. A series of gated images have been obtained using a short-pulse UV laser source, and the spatial resolution of those images is compared to those obtained using a more traditional-microchannel plate based system

[en] Studies of compressed shell integrity of spherical targets on the 60-beam 30 kJ UV OMEGA laser system involve spatially and temporally resolved measurements of core emission at different x-ray energies. Hot-core emission backlights a titanium-doped shell that is imaged at x-ray energies above and below the titanium K edge. The difference between the two images is related to perturbations in the cold, or absorbing, part of the shell. The core emission has been imaged by a pinhole array on a framing camera and recorded on film. The resolution and noise of all parts of the imaging system have been characterized. Using this information, a Wiener filter that reduces noise, compensates for detector resolution, and facilitates measurement of shell nonuniformities has been formulated

[en] We present new hydrodynamic growth experiments at the National Ignition Facility, which extend previous measurements up to Legendre mode 160 and convergence ratio 4, continuing the growth factor dispersion curve comparison of the low foot and high foot pulses reported by Casey et al. [Phys. Rev. E 90, 011102(R) (2014)]. We show that the high foot pulse has lower growth factor and lower growth rate than the low foot pulse. Using novel on-capsule fiducial markers, we observe that mode 160 inverts sign (changes phase) for the high foot pulse, evidence of amplitude oscillations during the Richtmyer-Meshkov phase of a spherically convergent system. Post-shot simulations are consistent with the experimental measurements for all but the shortest wavelength perturbations, reinforcing the validity of radiation hydrodynamic simulations of ablation front growth in inertial confinement fusion capsules

[en] Current indirect drive implosion experiments on the National Ignition Facility (NIF) [Moses et al., Phys. Plasmas 16, 041006 (2009)] are believed to be strongly impacted by long wavelength perturbations driven by asymmetries in the hohlraum x-ray flux. To address this perturbation source, active efforts are underway to develop modified hohlraum designs with reduced asymmetry imprint. An alternative strategy, however, is to modify the capsule design to be more resilient to a given amount of hohlraum asymmetry. In particular, the capsule may be deliberately misshaped, or “shimmed,” so as to counteract the expected asymmetries from the hohlraum. Here, the efficacy of capsule shimming to correct the asymmetries in two recent NIF implosion experiments is assessed using two-dimensional radiation hydrodynamics simulations. Despite the highly time-dependent character of the asymmetries and the high convergence ratios of these implosions, simulations suggest that shims could be highly effective at counteracting current asymmetries and result in factors of a few enhancements in neutron yields. For higher compression designs, the yield improvement could be even greater.

[en] As the control of the development of Rayleigh-Taylor-type hydrodynamic instabilities is crucial to achieve efficient implosions on the Laser Megajoule, and as the complexity of these instabilities requires an experimental validation of theoretical models and of the associated numerical simulations, the authors briefly present a proposition of experiments aimed at studying the strongly non linear Rayleigh-Taylor instability on the National Ignition Facility (NIF). This should allow a regime of competition between bubbles to be achieved for the first time in direct attack. They evoke the first experiment performed in March 2013

[en] Direct-drive, Rayleigh-Taylor (RT) growth experiments were performed using planar plastic targets on the OMEGA Laser Facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] at laser intensities between ∼2x10¹⁴ and ∼1.5x10¹⁵ W/cm². The primary purpose of the experiments was to test fundamental physics in hydrocodes at the range of drive intensities relevant to ignition designs. The target acceleration was measured with a streak camera using side-on, x-ray radiography, while RT growth was measured with a framing camera using face-on radiography. In a laser-intensity range from 2 to 5x10¹⁴ W/cm², the measured RT growth agrees well with two-dimensional simulations, based on a local model of thermal-electron transport. The RT growth at drive intensities above ∼1.0x10¹⁵ W/cm² was strongly stabilized compared to the local model predictions. The experiments demonstrate that standard simulations, based on a local model of electron thermal transport, break down at peak intensities of ignition designs, although they work well at lower intensities. These results also imply that direct-drive ignition targets are significantly more stable than previously calculated using local electron-transport models at peak intensities of ignition designs. The preheating effects by nonlocal electron transport and hot electrons were identified as some of the stabilizing mechanisms