David L. Tubbs

Observation of mix in a compressible plasma in a convergent cylindrical geometry

Laser beams that directly drive a cylindrical implosion are used to create a measurable region of mixed material in a compressible plasma state, for the first time in a convergent geometry. The turbulence driven by the Richtmyer–Meshkov instability by shock passage across a density discontinuity mixes marker material that is radiographically opaque. The width of the mix layer is compared between a system with large surface roughness and an initially smooth system. The experiment is described and results are compared to multi-dimensional numerical simulation, including three-dimensional turbulence calculations. The calculations adequately match the observations provided the measured initial conditions are used.

Cylindrical implosion experiments using laser direct drive

Direct-drive cylindrical-implosion experiments are performed to study perturbed hydrodynamic flows in convergent geometry. Two experimental campaigns have been conducted, to demonstrate the advantages of direct over indirect drive and to validate numerical simulations of zeroth-order hydrodynamics and single-mode perturbation growth. Results and analysis of three unperturbed-target shots and two perturbed-target shots are discussed in detail. For unperturbed-target implosions, positions of inner and outer shell edges agree between simulation and experiment during the laser pulse. However, observed shell thickness is greater than simulated in unperturbed targets during deceleration and rebound; the effect appears only at the shell’s exterior edge. For perturbed-target implosions, growth factors ∼10–14 are observed, whereas growth factors near 30 are expected from simulation. Rayleigh–Taylor growth appears to differ between simulation and experiment. Observed zeroth-order flow at the exterior edge of implod...

Experimental configuration of direct drive cylindrical implosions on the OMEGA laser

Details about the cylindrical implosions using direct-drive irradiation on the OMEGA Laser facility are provided. The experimental configuration, including orientation, construction, and mounting of the targets is described. An attempt to characterize the modulation transfer function of the primary x-ray framing camera diagnostic results in insufficient exposure contrast but relative agreement with other determinations. The x-ray intensity of the titanium backlighter driven by the 2.5-nsec linear ramp of the laser beams is described, and the relative intensity on film is compared to similar Nova experiments. The parallax effects of different length marker layers of high-opacity dichloropolystyrene is measured, resulting in the conclusion that the marker layer length should be matched to the laser drive illumination profile.

Spontaneous lepton-number violation and de-leptonization in stellar collapse

Abstract We consider core collapse in weak interaction models where global lepton-number invariance is spontaneously broken. We find that the initial neutrino number prevents finite temperature restoration of lepton-number conservation. Assuming the lepton-number violating reactions occur rapidly, we show that the neutrino number is driven to a sufficiently small value to allow electron capture reactions to de-leptonize the core, and lead to a high-entropy collapse.

Gold wall ablation and hohlraum filling measurements of vacuum and gas-filled hohlraums

An understanding of the timing and dynamics of hohlraum filling by laser-induced gold wall ablation is critical to the performance of indirectly-driven fusion ignition designs for the National Ignition Facility [E. Moses and C. Wuest, Fusion Science and Technology, 43, 420 (2003)]. Hohlraum wall ablation negatively affects ignition hohlraum performance by (1) reducing laser coupling by increasing backscatter by laser plasma instabilities, e.g., stimulated Brillouin scattering, (2) altering where lasers couple by moving the critical surface away from the walls and changing the refractive index, and (3), in the case of vacuum hohlraums, ablating directly into contact with the ablation layer of the fuel capsule. We report on measurements of gold-filling of hohlraums from a series of OMEGA laser [T.R. Boehly, R.L. McCrory, C.P. Verdon et al., Fusion Engineering and Design, 44, 35 (1999)] experiments involving vacuum and gas-filled hohlraums. On-axis x-ray imaging of gold self-emission shows delayed filling for gas-filled hohlraums, as expected. In addition, we present data on the hohlraum temperature penalty incurred with the use of a 1-atmosphere methane-fill. We discuss data and simulation predictions for 1-atmosphere neopentane filled hohlraums driven with a modified laser pulse.

Radiographic image analysis of cylindrical implosion experiments (invited)

Radiography is a heavily used tool for diagnosing laser-based hydrodynamic experiments. A successful experiment relies on the gathering of data over a time window where the relevant physics occurs and on an accurate analysis of those data. Comparison of this experimental data to theory is often best done by generating simulated images from hydrodynamic calculations, including all necessary and important experimental details. Care must be taken to treat both the experimental and theoretical images identically in the analysis. Frequently, image filtering and enhancement routines are used to obtain interface location and perturbation information from the radiographic image. Previous techniques were found to be too sensitive to global image details. New procedures have been developed which utilize local operators that provide better edge or interface identification without bias. These procedures are benchmarked and validated using static radiographic targets of known configuration that mock up experimental si...

Chlorine K-shell spectroscopy of directly driven cylindrical implosions

Abstract K-shell X-ray emission from a chlorinated marker layer is used to diagnose the density and temperature evolution of the inner surface of the ablator in a series of directly driven cylindrical implosions. Because the shape of the emitting region is irregular, the line-of-best-fit (LIBEF) method of Kilkenny et al. [Phys. Rev. A 1980;22:2746.] is applied for the inference of electron density and the results used to find constraints on the temperature of the marker layer.

BEST PRACTICE PROCEDURES FOR MAKING DIRECT DRIVE CYLINDRICAL TARGETS FOR STUDIES OF CONVERGENT HYDRODYNAMICS

Abstract The production of cylindrical targets involves numerous steps. These steps are shared in common with many other types of Inertial Confinement Fusion (ICF) targets but no other single target encompasses such a wide range of fabrication techniques. These targets consist of a large number of individual parts, virtually all fabricated from commercially purchased raw material. As an example, the polystyrene used is synthesized in house from purchased monomer material. This material must be polymerized, purified, characterized and put into solution before it is even first used in the making of a target. Because virtually every manufacturing and assembly process we currently use is involved in the production of these targets, this paper is written as a way documenting the methods used.

Megabar Shocks in Beryllium-Copper Driven by a NIF-Like Foot Pulse in a Vacuum Halfraum

Summary form only given. An understanding of the material dynamics and EoS characteristics of copper-doped beryllium (Be-Cu) ablators is critical for several ignition designs for the National Ignition Facility (NIF). Beryllium offers several advantages as an ablator: (a) high density, (b) low heat capacity, and (c) low opacity. In addition, dopants can be used to optimize these characteristics, for example, copper increases the density and opacity. In such a design, the Be-Cu ablator is subjected to thermal and soft X-ray radiation, delivering a sequence of shocks which compress a deuterium-tritium capsule. The first shock is expected to be between 0.5 and 8.0 Mbar. Be-Cu is predicted to melt on the Hugoniot between 1 and 2 Mbar and may depend upon the time scale. The anisotropic mechanical and thermal expansion properties of Be-Cu may introduce velocity fluctuations as the microstructure is loaded, and may then seed Rayleigh-Taylor instabilities. The uncertainty in Be-Cu melt and instability-growth dependence upon microstructure impacts the predictive capabilities for capsule performance. We have developed and fielded a series of experiments at the Omega laser facility that closely replicate the NIF-foot hohlraum conditions expected for a Be-Cu ablator. The experiments aim to investigate the seeding of instabilities by microstructure. In these experiments, a holhraum is rapidly brought up to a radiation temperature of 70-90 eV and held there for 2.5-3.2 ns. After ~3.0 ns, the temperature is raised to 120-150 eV in order to drive instabilities. Using 25-70 micron thick Be-0.9%Cu samples mounted on one end of the hohlraum, we performed free-surface velocimetry and pyrometry measurements during the foot of the pulse. We will present the velocity and pyrometry data along with inferred pressure, and discuss potential implications for Be-Cu capsule performance at NIF