David E. Fyfe

Surface tension and viscosity with Lagrangian hydrodynamics on a triangular mesh

Abstract Numerical algorithms for surface tension and viscosity are presented in the context of a Lagrangian treatment of incompressible hydrodynamics with a dynamically restructuring grid. New algorithms are given which update previous Lagrangian approaches in the code SPLISH. Test problems involving internal gravity and capillary waves, an oscillating droplet and a viscous shear layer are described. An example is given of a flow calculated in and around a viscous droplet with surface tension in a shear flow.

Shock ignition target design for inertial fusion energy

Continuing work in the design of shock ignition targets is described. Because of reduced implosion velocity requirements, low target adiabats, and efficient drive by short wavelength lasers, these targets produce high gain (>100) at laser energies well below 1 MJ. Effects of hydrodynamic instabilities such as Rayleigh–Taylor or Richtmyer–Meshkov are greatly reduced in these low-aspect ratio targets. Of particular interest is the optimum ratio of ignitor to compression pulse energy. A simple pellet model and simulation-derived coupling coefficients are used to analyze optimal fuel assembly, and determine that shock ignition allows enough control to create theoretically optimum assemblies. The effects on target design due to constraints on the compression and ignitor pulse intensities are also considered and addressed. Significant sensitivity is observed from low-mode perturbations because of large convergence ratios, but a more powerful ignitor can mitigate this.

Three-dimensional multimode simulations of the ablative Rayleigh--Taylor instability

Multimode simulations of the evolution of the laser‐driven, ablative Rayleigh–Taylor instability on planar, plastic targets are performed in three dimensions, with FAST3D–CM. The initial mass density target perturbations are random, with a power law dependence of k−2, a RMS surface finish of 0.1 μm, and perturbation wave numbers ranging from 2π/dmax to √2×(12π/dmax), for dmax=128 μm. At early nonlinear times, the perturbations grow to tile the target with approximately hexagonal bubbles that are of the shortest, initially seeded wavelengths not stabilized by density gradients. This tiling occurs on a time scale that is comparable to the eddy turnover time of the dominant bubble wavelength. When the target thickness is large compared to the dominant, short wavelengths, the bubbles continue to burn into the target and to evolve to progressively longer spatial scales. Predictions from second‐order mode coupling and saturation models are found to be consistent with the simulation results.

Large-scale high-resolution simulations of high gain direct-drive inertial confinement fusion targets

Targets have been designed that produce moderate to high gain when directly driven by lasers. The intrinsic sensitivity of these targets to hydro instabilities is found using the FAST(2D) multidimensional radiation hydrocode [J. H. Gardner, A. J. Schmitt, J. P. Dahlburg et al., Phys. Plasmas 5, 1935 (1998)], which simulates the simultaneous behavior of a large bandwidth (e.g., l=2–256) of perturbations from compression to acceleration, and then to stagnation and burn. The development of the structure in these multimode simulations is benchmarked to theoretical analysis and single-mode calculations, which reveals the need to “renormalize” the simulation after compression. The simulations predict that a direct drive point design is expected to degrade significantly from its one-dimensional clean yield, yet still ignite and give appreciable gain. Simulations of high-gain pellets using a spike prepulse to inhibit Richtmyer–Meshkov growth show a considerable robustness, with high (>100) gains possible even wit...

DIRECT DRIVE FUSION ENERGY SHOCK IGNITION DESIGNS FOR SUB-MJ LASERS

New approaches in target design have increased the possibility that useful fusion power can be generated with sub-MJ lasers. We have performed many 1D and 2D simulations that examine the characteristics of target designs for sub-MJ lasers. These designs use the recently-proposed shock-ignition target scheme, which utilizes a separate high-intensity pulse to induce ignition. A promising feature of these designs is their significantly higher gains at lower energies (one dimensional (1D) gain ˜ 100 at Elaser ˜ 250kJ) than can be expected for the conventional central ignition scheme. The results of these simulations are shown and we discuss the implications for target fabrication and laser design. Of particular interest are the constraints on the target and laser from asymmetries due to target imperfections and laser imprint.

Direct-drive laser target designs for sub-megajoule energies

New direct-drive laser target designs with KrF laser light take advantage of the shorter wavelength to lower the laser energy required for substantial gain (>30×) to sub-MJ level. These low laser-energy pellets are useful in systems that could form an intermediate step towards fusion energy, such as the proposed Fusion Test Facility [S. P. Obenschain et al., Phys. Plasmas 13, 056320 (2006)]. The short wavelength laser should allow higher intensity (and higher pressure) without increasing the risk of laser-plasma instabilities. The higher pressure in turn allows higher velocities to be achieved while keeping the low aspect ratios required for hydrodynamic stability. The canonical laser energy has been chosen to be 500kJ. A target design is presented with various laser pulse shapes and both 1D and 2D simulation results are shown. The sensitivity of these targets to both low-mode and high-mode perturbations is examined. The analysis and simulations in this paper indicate that significant gain (G=57) can be a...

Numerical simulations of fuel droplet flows using a Lagrangian triangular mesh

Abstract : This report describes work done to convert the incompressible, Lagrangian, triangular grid code, SPLISH, to the study of flows in and about fuel droplets. This has involved developing, testing and incorporating new algorithms for surface tension and viscosity. The major features of the Lagrangian method and the new algorithms are described. Benchmarks of the new algorithms are given. Several calculations are presented for kerosene droplets in air. Finally, extensions which make the code compressible and three dimensional are discussed.

Efficient Utilization of a CPU-GPU Cluster

This paper will investigate the performance of a mixture of central processing unit (CPU) and graphical processing unit (GPU) codes on a multi-CPU, multi-GPU cluster. This cluster attempts to balance IO, GPU, and CPU performance to accommodate a wide variety of codes. When designing this cluster, the design goal of a balanced system was one of many options that could have been taken. The GPU, is essentially a video graphics card, found in every desktop or laptop computer. High-end graphics cards such as those used by a computer gamer are capable of extremely high floating point performance. The GPU utilizes the CPU to initialize the GPU, to transfer data from memory/storage to and from the GPU, and to launch the computation kernels that run on the GPU. The Jet Engine Noise Reduction (JENRE) code implements a compressible flow solver which is under development for the simulation of supersonic jet flow and its acoustic properties. A major emphasis of this codes development is on ensuring that the code is capable of fully exploiting emerging massively parallel, high-performance computing architectures for either GPUs or multi-core CPUs. The JENRE codes performance using GPUs is currently 2.1 times that with CPUs, and thus is run typically on the GPUs in the cluster. The cluster is also used for a variety of MPI-based jobs as well as single node OpenMP shared-memory jobs. These jobs utilize the CPU only, and the GPUs are left idle. Typically, a user requests that an entire node (or set of nodes) is allocated to a single job (CPU or GPU) so that there is no contention for resources with other jobs. Since jobs are either CPU or GPU-based, this leads to significant under-utilization of the computational resources. This paper will examine the overall utilization of the cluster and performance of a mix of CPU codes with the GPU-based JENRE code running simultaneously on the same nodes of the cluster. Results indicate that careful and cooperative scheduling of jobs can result in a tripling of the computational capability of the cluster.