W. W. Hsing

Rayleigh–Taylor instability evolution in ablatively driven cylindrical implosions

The Rayleigh–Taylor instability is an important limitation in inertial confinement fusion capsule designs. Significant work both theoretically and experimentally has been done to demonstrate the stabilizing effects of material flow through the unstable region. The experimental verification has been done predominantly in planar geometry. Convergent geometry introduces effects not present in planar geometry such as shell thickening and accelerationless growth of modal amplitudes (e.g., Bell–Plesset growth). Amplitude thresholds for the nonlinear regime are reduced, since the wavelength λ of a mode m decreases with convergence λ∼R/m, where R is the radius. Convergent effects have been investigated using an imploding cylinder driven by x-ray ablation on the NOVA laser [J. L. Emmet, W. F. Krupke, and J. B. Trenholme, Sov. J. Quantum Electron. 13, 1 (1983)]. By doping sections of the cylinder with opaque materials, in conjunction with x-ray backlighting, the growth and feedthrough of the perturbations from the ...

Shock structuring due to fabrication joints in targets

The use of copper-doped beryllium ablators on National Ignition Facility [J. A. Paisner et al., Laser Focus World 30, 75 (1994)] targets, in place of plastic, can require the bonding together of hemispheres with a joint of differing composition. Indirect drive experiments have been conducted on the Nova laser [J. L. Emmet, W. F. Krupke, and J. B. Trenholme, Sov. J. Quantum Electron. 13, 1 (1983)], and the resulting shock structuring compared with code simulations. It is concluded that one of the available codes, the RAGE code [R. M. Baltrusaitis et al., Phys. Fluids 8, 2471 (1996)] provides useful insight into the effect of joints. This code is then employed to obtain a physical picture of the shock front nonuniformity in terms of a secondary rarefaction and an oblique shock interacting with the main shock that propagates in the absence of the joint. A simple analysis reinforces this picture.

Gas‐filled targets for large scale‐length plasma interaction experiments on Nova

Stimulated Brillouin backscatter from large scale‐length gas‐filled targets has been measured on the Nova laser. These targets were designed to approximate conditions in indirect drive ignition target designs in underdense plasma electron density (ne∼1021/cm3), temperature (Te≳3 keV), and gradient scale lengths (Ln∼2 mm, Lv≳6 mm) as well as calculated gain for stimulated Brillouin scattering (SBS). The targets used in these experiments were gas‐filled balloons with polyimide walls (gasbags) and gas‐filled hohlraums. Detailed characterization using x‐ray imaging and x‐ray and optical spectroscopy verifies that the calculated plasma conditions are achieved. Time‐resolved SBS backscatter from these targets is <3% for conditions similar to ignition target designs.

Review of drive symmetry measurement and control experiments on the Nova laser system (invited)

Good radiation drive symmetry is crucial for achieving ignition in laboratory inertial fusion experiments. X‐ray drive symmetry in hohlraums has been the subject of investigation for more than four years and a great deal of progress has been made. Over the last two to three years, a concerted series of (indirect) drive symmetry experiments has been performed on the Nova laser system and is the subject of the present paper. The goals of this work have been to develop measurement techniques and to apply them to symmetry variation and control experiments. The principal diagnostic has utilized the symmetry signature impressed on the dense core of a target imploded by the hohlraum x‐ray environment. The core is distorted by drive asymmetries and x‐ray imaging of this core provides a mapping that can be compared with theoretical modeling and thus related to specific amounts of drive asymmetry. We will describe the instruments and measurement techniques used in these experiments and present representative data a...

Production and characterization of large plasmas from gas bag targets on Nova

Large plasmas are created by illuminating gas‐filled thin‐walled balloon‐like targets using the Nova laser [E. Campbell et al., Rev. Sci. Instrum. 57, 2101 (1986)]. The targets consist of a 5000–6000 A skin surrounding 1 atm of neopentane, which, when ionized, becomes a plasma with an electron density of 1021 electrons/cm3. X‐ray images of the gas bag target are used to evaluate the size and uniformity of the plasma by comparison with LASNEX [R. M. More, J. Quant. Spectrosc. Radiat. Transfer 27, 345 (1982)] simulations. The gas bags are heated with converging and diverging beam spots. The most uniform plasmas are created by illuminating the target with large converging beam spots that overlap to cover most of the surface of the gas bag. The gas bag plasma is heated to a peak temperature of approximately 3.5 keV, with 25 kJ of 3ω laser light in a 1 ns square pulse.

Production of enhanced pressure regions due to inhomogeneities in inertial confinement fusion targets

It is shown that regions of enhanced pressure have been produced in targets with indirect radiation drive in planar and cylindrically convergent geometry through the interaction between the flows caused by target inhomogeneities and the main target drive. Design calculations for National Ignition Facility (NIF) [J. A. Paisner et al., Laser Focus World 30, 75 (1994)] targets with beryllium ablators formed by bonded hemi-shells [D. C. Wilson et al., Bull. Am. Phys. Soc. 43, 1667 (1998)] indicate that related behavior produces a seed perturbation in the ablator which can in some cases lead to the suppression of ignition. From simulation and analysis of the NIF problem in the planar geometry analog, a scaling for the perturbation, which should be useful for validation of the behavior with lower energy drive and smaller-scale geometries, is derived.

X‐ray imaging of uniform large scale‐length plasmas created from gas‐filled targets on Nova

We report on the production and characterization of large scale‐length plasmas created by illuminating gas‐filled thin‐walled balloonlike targets using the Nova laser. The targets consisted of a 5–6000 A skin surrounding 1 atm of neopentane which when ionized becomes a plasma with 1021 electrons/cm3. Results are presented from x‐ray imaging used to evaluate the uniformity of the plasma. The most uniform plasmas were produced by illuminating the target with large converging beams that overlapped to cover most of the surface of the gas bag. An alternate focus geometry using small beam spots resulted in a less uniform plasma with low density holes in it.

The design and characterization of toroidal‐shaped NOVA hohlraums that simulate National Ignition Facility plasma conditions for plasma instability experiments

Special Nova hohlraums have been designed to simulate the plasma conditions calculated for various NIF hohlraum point designs. These hohlraums attempt to maximize the laser pathlength for parametric instability measurements. A toroidal‐shaped hohlraum with a diameter of 3200 microns and a length of 1600 microns allows a laser pathlength of about 2 mm. Filling the hohlraum with 1 atmosphere of neopentane gas gives an electron temperature of 3 keV and electron density near 0.1 of critical. Detailed LASNEX calculations for these hohlraums and comparisons to the NIF point design will be presented. Comparisons between data and calculations that characterize the plasma conditions (electron, radiation, and ion temperatures, electron density, etc.) in these Nova hohlraums will also be shown.

Bragg‐diffraction x‐ray spectrographs for the determination of Te in 2–3‐mm‐sized laser‐produced plasmas on NOVA

We have developed spectrographs to measure the electron temperature in gas‐filled targets and low‐density foams, and find it to nominally fall in the 2–4‐keV range. The instrument we designed, built, and fielded can simultaneously record the Ti Heα and Cr Heα line emission. After compensating for the instrumental response, we can estimate the electron temperature from this line ratio to within ±15%.

Improved optical diagnostics for the NOVA laser

This paper describes three diagnostics detecting optical scatter from NOVA laser targets. Detecting such scatter can help to understand instabilities in laser plasmas and to characterize such plasmas, particularly hohlraum plasmas. These diagnostics are the full aperture backscatter station (FABS), presently being built; the oblique scatter array (OSA), just starting operation; and the axial imager, also just starting operation. FABS will allow imaging at high resolution of Brillouin and Raman backscatter. The OSA provides a quantitative measurement of Brillouin and Raman scatter in many directions. The axial imager allows high‐resolution imaging of Brillouin, two‐plasmon decay and Raman scatter emitted toward the direction of the symmetry axis of the NOVA laser beams.