M. L. Hoppe

The experimental plan for cryogenic layered target implosions on the National Ignition Facility—The inertial confinement approach to fusion

Ignition requires precisely controlled, high convergence implosions to assemble a dense shell of deuterium-tritium (DT) fuel with ρR>∼1 g/cm2 surrounding a 10 keV hot spot with ρR ∼ 0.3 g/cm2. A working definition of ignition has been a yield of ∼1 MJ. At this yield the α-particle energy deposited in the fuel would have been ∼200 kJ, which is already ∼10 × more than the kinetic energy of a typical implosion. The National Ignition Campaign includes low yield implosions with dudded fuel layers to study and optimize the hydrodynamic assembly of the fuel in a diagnostics rich environment. The fuel is a mixture of tritium-hydrogen-deuterium (THD) with a density equivalent to DT. The fraction of D can be adjusted to control the neutron yield. Yields of ∼1014−15 14 MeV (primary) neutrons are adequate to diagnose the hot spot as well as the dense fuel properties via down scattering of the primary neutrons. X-ray imaging diagnostics can function in this low yield environment providing additional information about ...

The PAMS/GDP Process for Production of ICF Target Mandrels

AbstractAn improved process for production of ICF Target Mandrels has been developed. Shells made from PAMS (poly-α-methylstyrene) are coated with GDP (glow discharge polymer). The PAMS is then removed by depolymerization and volatilization at 300°C, leaving a GDP mandrel. Compared to past polymer mandrels, this process yields GDP mandrels with significant improvements in wall thickness control, sphericity and concentricity, and the complete absence of vacuoles. The process is capable of making GDP shells with a wide size range (from 300 < o.d. < 2700 µm), and an independently controlled wall thickness (from 1 to 30 µm). The GDP can be doped with a variety of elements.

National Ignition Facility target design and fabrication

The current capsule target design for the first ignition experiments at the NIF Facility beginning in 2009 will be a copper-doped beryllium capsule, roughly 2 mm in diameter with 160-{micro}m walls. The capsule will have a 75-{micro}m layer of solid DT on the inside surface, and the capsule will driven with x-rays generated from a gold/uranium cocktail hohlraum. The design specifications are extremely rigorous, particularly with respect to interfaces, which must be very smooth to inhibit Rayleigh-Taylor instability growth. This paper outlines the current design, and focuses on the challenges and advances in capsule fabrication and characterization; hohlraum fabrication, and D-T layering and characterization.

Progress toward fabrication of graded doped beryllium and CH capsules for the National Ignition Facilitya)

Current ignition designs require graded doped beryllium or CH capsules. This paper reports on the progress toward fabricating both beryllium and CH capsules that meet the current design criteria for achieving ignition on the National Ignition Facility (NIF) [S. Hann et al., Phys. Plasmas 12, 056316 (2005)]. NIF scale graded copper doped beryllium capsules have been made by sputter coating, while graded germanium doped CH capsules have been made by plasma polymer deposition. The sputtering process used for fabricating graded beryllium shells was produced with a void fraction of ∼5%. Varying the deposition parameters can lead to several different beryllium microstructures, which have been tuned to reduce the void size and fraction to within specifications. In addition, polishing of beryllium-coated shells reduces the outer surface roughness of shells to ignition specifications. Transmission electron microscopy has been used to characterize void fraction and grain structure of beryllium coatings. The plasma ...

Large Glass Shells from GDP Shells

Abstract An entirely new process was discovered starting from M-doped glow discharge polymer (GDP) deposited by plasma polymerization1 (where M = Si or Ti) to make M-oxide shells. This process utilizing Si-GDP was developed to make large, uniform, thick-walled glass shells which are suitable for use in cryogenic layering experiments at LLNL and are unobtainable by the routinely utilized drop-tower method. We have found that in addition to controlling the geometry, the permeability and opacity may be controllable over very wide ranges by varying the processing conditions. Preliminary tests to determine the strength of SiO2 glass shells made by this process are consistent with that expected of pure silica glass.

Systematic Fuel Cavity Asymmetries in Directly Driven Inertial Confinement Fusion Implosions

We present narrow-band self-emission x-ray images from a titanium tracer layer placed at the fuel-shell interface in 60-laser-beam implosion experiments at the OMEGA facility. The images are acquired during deceleration with inferred convergences of ∼9-14. Novel here is that a systematically observed asymmetry of the emission is linked, using full sphere 3D implosion modeling, to performance-limiting low mode asymmetry of the drive.

Single-shot divergence measurements of a laser-generated relativistic electron beam

The relativistic electron transport induced by an ultraintense picosecond laser is experimentally investigated using an x-ray two-dimensional imaging system. Previous studies of the electron beam divergence [R. B. Stephens et al. Phys. Rev. E 69, 066414 (2004), for instance] were based on an x-ray imaging of a fluorescence layer buried at different depths in the target along the propagation axis. This technique required several shots to be able to deduce the divergence of the beam. Other experiments produced single-shot images in a one-dimensional geometry. The present paper describes a new target design producing a single-shot, two-dimensional image of the electrons propagating in the target. Several characteristics of the electron beam are extracted and discussed and Monte Carlo simulations provide a good understanding of the observed beam shape. The proposed design has proven to be efficient, reliable, and promising for further similar studies.

Fabrication of Pressurized 2 mm Beryllium Targets for ICF Experiments

Abstract We have successfully fabricated 2 mm beryllium targets pressurized with a gas mixture of ~20 atm deuterium and ~0.1 atm argon. These targets have been used for indirect drive Inertial Confinement Fusion (ICF) experiments on the Z-pinch machine at Sandia National Laboratories leading to record neutron yields of ~3.5 × 1011 [J.E. Bailey, et al., “Be Capsule Implosions Driven by Dynamic Hohlraum X-rays,” Bull. Am. Phys. Soc. 51, 107 (2006)]. This paper will discuss the process of fabricating such targets from intact shells (Be sputter coated CH mandrels). These processes include laser drilling a ~6 μm diameter fill hole in a shell, removing the CH mandrel by pyrolysis, pressurizing the target with a deuterium/argon gas mixture and sealing the fill hole using UV glue while under pressure. The targets were characterized for gas pressure and deuterium gas permeation half-life by utilizing techniques including mass spectrometry, x-ray fluorescence spectroscopy and controlled shell bursting.

White Light Interferometry for the Optical Characterization of Transparent ICF Shells

Abstract White light interferometry has been adapted to the characterization of transparent ICF shells and their precursor mandrels. The combination of an interferometric microscope, a precision z-stage, and simulation-derived analysis algorithms allow determination of the diameters of the inner and outer surfaces, their non-concentricity, the location of interfacial layers, the average index of refraction of the walls, and the thickness of discrete layers within the shell wall. The hard- and soft-ware required for these measurements are described.

Development of an inertial confinement fusion platform to study charged-particle-producing nuclear reactions relevant to nuclear astrophysics

This paper describes the development of a platform to study astrophysically relevant nuclear reactions using inertial-confinement fusion implosions on the OMEGA and National Ignition Facility laser facilities, with a particular focus on optimizing the implosions to study charged-particle-producing reactions. Primary requirements on the platform are high yield, for high statistics in the fusion product measurements, combined with low areal density, to allow the charged fusion products to escape. This is optimally achieved with direct-drive exploding pusher implosions using thin-glass-shell capsules. Mitigation strategies to eliminate a possible target sheath potential which would accelerate the emitted ions are discussed. The potential impact of kinetic effects on the implosions is also considered. The platform is initially employed to study the complementary T(t,2n)α, T(3He,np)α and 3He(3He,2p)α reactions. Proof-of-principle results from the first experiments demonstrating the ability to accurately measur...