The capsule targets for ignition experiments at the National Ignition Facility must meet very exacting requirements. Primary among them is an extremely high degree of symmetry at all length scales for the 2-mm-diameter 150-μm-walled capsule. At LLNL work is in progress to produce both polyimide and sputtered beryllium targets that meet these specifications. Both of these targets require a thin-walled spherical-shell plastic mandrel upon which the beryllium or polyimide ablator is deposited. In this paper we report on recent progress in developing NIF capsules that meet the demanding design requirements.

We are studying the feasibility of using boron doping to refine the grain structure of sputter-deposited Be for NIF ignition capsule ablators. The goal is to improve the surface finish and homogeneity of these coatings. Films deposited on flat silicon substrates display a pronounced change in structure at a concentration of similar to 11 at.% B. At lower levels of B, grain sizes of about 200 mm are observed. AFM images show the roughness of these films to be about 20 nm rms. At higher levels of B, the grains size drops to below 50 nm and the roughness decreases to less than 2.5 nm rms. Films deposited on capsules do not show the same behavior. In particular, at 15 at.% B, the capsule coatings have nodular structure with an rms roughness of greater than 50 nm. When viewed in cross section, however, no structure is seen with either the flat films or the capsule coatings. We believe that differences in substrate temperature may be largely responsible for the observed behavior.

Several choices exist in the design and production of capsules intended to ignite and propagate fusion burn of the deuterium–tritium (D–T) fuel when imploded by indirect drive at the National Ignition Facility (NIF). These choices include ablator material, ablator dopant concentration and distribution, capsule dimensions, and X-ray drive profile (shock timings and strengths). The choice of ablator material must also include fabrication and material characteristics, such as attainable surface finishes, permeability, strength, transparency to radio frequency and infrared radiation, thermal conductivity, and material homogeneity. Understanding the advantages and/or limitations of these choices is an ongoing effort for LLNL and LANL designers. At this time, simulations in one-, two-, and three-dimensions show that capsules with either a copper-doped beryllium or a polyimide (C22H10N2O4) ablator material have both the least sensitivity to initial surface roughnesses and favorable fabrication qualities. Simulations also indicate the existence of capsule designs based on these ablator materials which ignite and burn when imploded by less than nominal laser performance (900-kJ energy, 250-TW power, producing 250-eV peak radiation temperature). We will describe and compare these reduced-scale capsules, in addition to several designs which use the expected 300-eV peak X-ray drive obtained from operating the NIF laser at 1.3 MJ and 500 TW.

Targets for Inertial Confinement Fusion (ICF) typically consist of a hollow, spherical capsule filled with a mixture of hydrogen isotopes. Typically, these capsules are irradiated by short, intense pulses of either laser light ("direct drive") or laser-generated X-rays ("indirect drive"), causing them to implode. This compresses and heats the fuel, leading to thermonuclear fusion. This process is highly sensitive to hydrodynamic (e.g., Rayleigh-Taylor) instabilities, which can be initiated by imperfections in the target. Thus, target capsules must be extremely spherical and smooth. One of the lead capsule designs for the National Ignition Facility, a 1.8 MJ laser being built at Livermore, calls for a 2-mm-diam capsule with a 150-/spl mu/m-thick copper-doped beryllium wall. These capsules can be fabricated by sputter depositing the metal onto a spherical plastic mandrel. This results in surfaces with measured R/sub q/'s of 50 to 150 nm, as measured with an atomic force microscope. For optimal performance the roughness should be below 10 nm rms. We have begun studying the use of ion cluster beam polishing as a means of improving the surface finish of as-deposited capsules. In this approach, a batch of capsules would be agitated in a bounce pan inside a vacuum chamber during exposure to the cluster beam. This would ensure a uniform beam dose around the capsule. We have performed preliminary experiments on both Be flats and on a stationary Be capsule. On the capsule, the measured R/sub q/ went from 64 nm before polishing to 15 nn after. This result was obtained without any effort at process optimization. Similar smoothing was observed on the planar samples.

Several inertial confinement fusion (ICF) capsule designs have been proposed as possible candidates for achieving ignition by indirect drive on the National Ignition Facility (NIF) laser [Paisner et al., Laser Focus World 30, 75 (1994)]. This article reviews these designs, their predicted performance using one-, two-, and three-dimensional numerical simulations, and their fabricability. Recent design work at a peak x-ray drive temperature of 250 eV with either 900 or 1300 kJ total laser energy confirms earlier capsule performance estimates [Lindl, Phys. Plasmas 2, 3933 (1995)] that were based on hydrodynamic stability arguments. These simulations at 250 eV and others at the nominal 300 eV drive show that capsules having either copper doped beryllium (Be+Cu) or polyimide (C22H10N2O4) ablators have favorable implosion stability and material fabrication properties. Prototypes of capsules using these ablator materials are being constructed using several techniques: brazing together machined hemishells (Be+Cu), sputter deposition (Be+Cu), and monomer deposition followed by thermal processing (polyimide).

In this article we describe the design and simulated performance characteristics of an indirectly-driven inertial confinement fusion capsule which utilizes only 900 kJ of laser energy and 250 TW of laser power from the National Ignition Facility (NIF) [Paisner et al., Laser Focus World 30, 75 (1994)]. This intentional reduction in laser performance from the nominal NIF specifications of 1.8 MJ and 500 TW results in lowering the hohlraum x-ray drive temperature from 300 eV to 250 eV. These energy and radiation temperature reductions are believed to define a “lower bound” on the successful implosion of an ignition capsule. This reduced scale capsule has a beryllium ablator containing a radially varying copper dopant, and a cryogenic solid deuterium–tritium fuel layer surrounding a cavity filled with equilibrium vapor pressure gaseous deuterium and tritium. Two-dimensional simulations predict ignition and propagated burn from this capsule when either Rayleigh–Taylor instability or time-dependent drive asymmetry effects are included.

We have performed a series of preliminary experiments to determine whether sputter deposition of doped Be is a practical route to producing NIF target capsules with Be ablators. Films ranging in thickness from 7 to similar to 120 mu m have been deposited on spherical polymer mandrels using a bounce pan to ensure uniform coating. With no voltage bias applied to the pan, relatively porous coatings were formed that were highly permeable to hydrogen. The surface finish of these films ranged from similar to 250 nm rms for 13-mu m-thick films to a minimum of similar to 75 nm rms for an 80-mu m-thick film. Application of a voltage bias was found to significantly modify the film morphology. At a bias of 120 V, 7-mu m-thick films with a dense, fine-grained microstructure were produced. These capsules had a reflective surface with a 50 nm rms roughness. Finally, to demonstrate the ability to produce a graded dopant profile, a coating was produced in which the concentration of added Cu was varied from 2.5 atom % at the beginning to zero after 40 mu m of deposition.

The performance of indirectly driven fusion capsules has been improved by mid Z doping of the plastic capsule ablator. The doping increases x-ray preheat shielding leading to a more isentropic compression, higher convergence, and higher neutron yield. A 4× increase in neutron yield is both calculated and observed as the Ge doping level is increased from 0% to 3% by atomic fraction. A predicted 40% decrease in x-ray image core size with increasing Ge content is confirmed.

Capsule implosion experiments carried out on the Nova laser [E. M. Campbell et al., Rev. Sci. Instrum. 57, 2101 (1986)] are simulated with the three-dimensional HYDRA radiation hydrodynamics code [NTIS Document No. DE-96004569 (M. M. Marinak et al. in UCRL-LR-105821-95-3)]. Simulations of ordered, near single mode perturbations indicate that structures which evolve into round spikes can penetrate farthest into the hot spot. Bubble-shaped perturbations can burn through the capsule shell fastest, in which case they cause even more damage. A simulation of a capsule with a multimode perturbation of moderate amplitude shows spike amplitudes evolving in good agreement with a saturation model during the deceleration phase. The presence of sizable low mode asymmetry, caused either by drive asymmetry or perturbations in the capsule shell, can dramatically affect the manner in which spikes approach the center of the hot spot. Three-dimensional coupling between the low mode shell perturbations intrinsic to Nova capsules and the drive asymmetry is found to be important, bringing the simulated neutron yields into closer agreement with the experimental values.

In this paper we present measured energy and angular distributions for Na{sup +} scattering from Cu(001) with incident energies ranging from 10 to 100 eV. Excellent agreement with the measured spectra over the full range of incident energies is achieved with simulations using a scattering potential that consists of two parts, both of which we discuss in detail in this paper. The first is a sum of Hartree-Fock (Na-Cu){sup +} pair potentials where the sum runs over the surface atoms nearest the scattering ion. To this we add an attractive potential that approaches the classical image potential far from the surface, but saturates close to the surface. From these spectra we extract detailed information about the scattering dynamics, such as the scattering trajectories, energy transfer to the surface, and particle trapping. For energies below 100 eV we find that the scattering is particularly sensitive to the attractive term in the potential. In particular, as the incident energy is reduced the scattered angular distributions broaden, the fractional energy transfer to the surface increases, and trapping of the ions by the surface is observed. This sensitivity enables us to put bounds on the depth of the attractive well in the potential. Accordingmore » to the simulations there is a minimum in the trapping probability at incident energies between 15 and 30 eV. Furthermore, they indicate that the trajectories that lead to trapping at energies below and above the minimum differ markedly, particularly in the energy transfer in the initial collision with the surface. {copyright} {ital 1996 The American Physical Society.}« less

Indirectly-driven implosions for which the predicted Rayleigh-Taylor (RT) instability growth rates of pre-imposed capsule surface perturbations are substantially increased by mid-Z-doping of the ablators have been fielded on the Nova laser. The multiple effects on implosion performance of the additional x-ray opacity provided by the ablator dopant is discussed. For best surface finish capsules, the addition of increasing ablator dopant levels is shown to improve the neutron yield. However, as capsule surface roughness is increased, so that RT instability growth increases, this trend is reversed, leading to decreasing yields with increased dopant content. The RT-induced mixing between shell and fuel is further investigated by diagnosing the x-ray emission levels and time histories from Ti and Ar dopants in capsules with predetermined surface roughness. The x-ray line ratios show the expected decrease in fuel temperature with increasing surface roughness. The spectral content, intensity and duration of the Ti spectra, however, suggest 2- or 3-D rather than just 1-D effects are important so that higher than 1-D models of the mix region may be needed.

The degradation in implosion performance of surface-pitted, indirectly driven Br-doped capsules which have been intentionally designed to be susceptible to large hydrodynamic instability growth has been systematically and reproducibly measured by a wide array of x-ray and neutron diagnostics. Primary and secondary neutron yields, and x-ray line ratios and durations from Ar and Ti fuel and pusher dopants are presented, showing clear sensitivity to increasing surface roughness and by inference, increasing pusher-fuel mix. The sensitivity of core x-ray emissivity and neutron bang-times to micron variations in capsule ablator thickness is also demonstrated. The experimental trends are compared to expectations based on two different models for the effects of instability growth at the pusher-fuel interface (atomic mixing versus Rayleigh–Taylor bubble-and-spike growth).

Rayleigh–Taylor instabilities during implosions of inertially confined fusion capsules affect capsule performance. During acceleration, surface imperfections grow and can, if large enough, lead to an asymmetric implosion or even shell breakup. For this reason, characterizing the topography of target capsules is extremely important. We have developed a profilometer based on an atomic force microscope combined with a precision rotary air bearing. Averaged one-dimensional (1D) surface height power spectra obtained with this instrument are converted to two-dimensional mode spectra that are used as input to hydrodynamic simulations. We describe the design of the system and its performance in terms of runout and repeatability. We also discuss the simulation of these measurements and the statistics involved in averaging 1D power spectra. Finally, we show the application of this measurement technique to capsules whose surfaces have been modified by laser ablation, resulting in a well-defined surface topography. This special case provides an excellent test for the system since the expected results are exactly calculable.