M. Schoff

Optimization of a high-yield, low-areal-density fusion product source at the National Ignition Facility with applications in nucleosynthesis experiments

Polar-direct-drive exploding pushers are used as a high-yield, low-areal-density fusion product source at the National Ignition Facility with applications including diagnostic calibration, nuclear security, backlighting, electron-ion equilibration, and nucleosynthesis-relevant experiments. In this paper, two different paths to improving the performance of this platform are explored: (i) optimizing the laser drive, and (ii) optimizing the target. While the present study is specifically geared towards nucleosynthesis experiments, the results are generally applicable. Example data from T2/3He-gas-filled implosions with trace deuterium are used to show that yield and ion temperature (Tion) from 1.6 mm-outer-diameter thin-glass-shell capsule implosions are improved at a set laser energy by switching from a ramped to a square laser pulse shape, and that increased laser energy further improves yield and Tion, although by factors lower than predicted by 1 D simulations. Using data from D2/3He-gas-filled implosion...

High-resolution measurements of the DT neutron spectrum using new CD foils in the Magnetic Recoil neutron Spectrometer (MRS) on the National Ignition Facility

The Magnetic Recoil neutron Spectrometer (MRS) on the National Ignition Facility measures the DT neutron spectrum from cryogenically layered inertial confinement fusion implosions. Yield, areal density, apparent ion temperature, and directional fluid flow are inferred from the MRS data. This paper describes recent advances in MRS measurements of the primary peak using new, thinner, reduced-area deuterated plastic (CD) conversion foils. The new foils allow operation of MRS at yields 2 orders of magnitude higher than previously possible, at a resolution down to ∼200 keV FWHM.

Improvements in Fabrication of Elastic Scattering Foils Used to Measure Neutron Yield by the Magnetic Recoil Spectrometer

Abstract The magnetic recoil spectrometer uses a deuterated polyethylene polymer (CD2) foil to measure neutron yield in inertial confinement fusion experiments. Higher neutron yields in recent experiments have resulted in primary signal saturation in the detector CR-39 foils, necessitating the fabrication of thinner CD2 foils than established methods could provide. A novel method of fabricating deuterated polymer foils is described. The resulting foils are thinner, smoother, and more uniform in thickness than the foils produced by previous methods. These new foils have successfully been deployed at the National Ignition Facility, enabling higher neutron yield measurements than previous foils, with no primary signal saturation.

Atomic Layer Deposition Coating for Permeation Half-Life Control of GDP Ablator Capsules

Abstract The atomic layer deposition technique generates very thin Al2O3 films to control the hydrogen diffusion half-life of glow discharge polymer (GDP) inertial confinement fusion shells. The films generated by this process have an easily controlled thickness and are pinhole free. As a result, they can be used to set the hydrogen diffusion half-life of a GDP shell to the required value of hours, from an uncoated value of minutes. Such diffusivity control is much harder to achieve with the currently used sputtered Al coating, which also renders the shell opaque, causing difficulties with ice-layer characterization. The [approximately]10-nm oxide is also less intrusive to target performance than an [approximately]100-nm (and highly nonuniform) metal coating such that it can be safely ignored by the target designer.

Diffusion-dominated mixing in moderate convergence implosions

High-Z material mixed into the fuel degrades inertial fusion implosions and can prevent ignition. Mix is often assumed to be dominated by hydrodynamic instabilities, but we report Omega data, using shells with ∼150nm deuterated layers to gain unprecedented resolution, which give strong evidence that the dominant mix mechanism is diffusion for these moderate temperature (≲6 keV) and convergence (∼12) implosions. Small-scale instability-driven or turbulent mix is negligible.

The National Direct-Drive Program: OMEGA to the National Ignition Facility

Abstract The goal of the National Direct-Drive Program is to demonstrate and understand the physics of laser direct drive (LDD). Efforts are underway on OMEGA for the 100-Gbar Campaign to demonstrate and understand the physics for hot-spot conditions and formation relevant for ignition at the 1-MJ scale, and on the National Ignition Facility to develop an understanding of the direct-drive physics at long scale lengths for the MJ Direct-Drive Campaign. The strategy of the National Direct-Drive Program is described; the requirements for the deuterium-tritium cryogenic fill-tube target being developed for OMEGA are presented; and preliminary LDD implosion measurements of hydrodynamic mixing seeded by laser imprint, the target-mounting stalk, and microscopic surface debris are reported.

Fabrication of Large-Area Glow Discharge Polymer–Deposited Foils

Abstract Planar components made from glow discharge polymer (GDP) are commonly used in laser inertial fusion experiments. The thickness profile of GDP coatings on flat substrates follows a nearly Gaussian distribution with a full-width at half-maximum only 20 mm across, restricting the number of usable components with the same thickness that can be made from each substrate. A new coating method to improve the thickness uniformity and increase the usable area of GDP coatings has been simulated and involves rotating the planar substrate while it is offset from the center of the coating chamber. While the coating rate drops to about 40% at the center, the thickness variation is less than 10% out to a radius of 10 mm, increasing the usable area by a factor of over 7, thereby increasing the number of components that can be made from a single coating.

Measurement of apparent ion temperature using the magnetic recoil spectrometer at the OMEGA laser facility

The Magnetic Recoil neutron Spectrometer (MRS) at the OMEGA laser facility has been routinely used to measure deuterium-tritium (DT) yield and areal density in cryogenically layered implosions since 2008. Recently, operation of the OMEGA MRS in higher-resolution mode with a new smaller, thinner (4 cm2, 57 μm thick) CD2 conversion foil has also enabled inference of the apparent DT ion temperature (T ion) from MRS data. MRS-inferred T ion compares well with T ion as measured using neutron time-of-flight spectrometers, which is important as it demonstrates good understanding of the very different systematics associated with the two independent measurements. The MRS resolution in this configuration, ΔE MRS = 0.91 MeV FWHM, is still higher than that required for a high-precision T ion measurement. We show how fielding a smaller foil closer to the target chamber center and redesigning the MRS detector array could bring the resolution to ΔE MRS = 0.45 MeV, reducing the systematic T ion uncertainty by more than a factor of 4.

Progress Toward Fabrication of Machined Metal Shells for the First Double-Shell Implosions at the National Ignition Facility

Abstract The double-shell platform fielded at the National Ignition Facility requires developments in new machining techniques and robotic assembly stations to meet the experimental specifications. Current double-shell target designs use a dense high-Z inner shell, a foam cushion, and a low-Z outer shell. The design requires that the inner shell be gas filled using a fill tube. This tube impacts the entire machining and assembly design. Other intermediate physics designs have to be fielded to answer physics questions and advance the technology to be able to fabricate the full point design in the near future. One of these intermediate designs is a mid-Z imaging design. The methods of designing, fabricating, and characterizing each of the major components of an imaging double shell are discussed with an emphasis on the fabrication of the machined outer metal shell.

Fabrication of the Metal Particle in Plastic Bead Target for the LLE Ultra-Strong-Spherical Shock Campaign

Abstract The challenge of fabricating a shock convergence target is embedding the metal particle at the center of a plastic bead with ≤10-µm concentricity between the metal particle and plastic bead. Two types of the metal particle in plastic bead target were fabricated for the Ultra-Strong-Spherical Shock campaign: (1) a metal particle 50 µm in diameter embedded in the center of a 430-µm-diameter plastic bead and (2) the same metal particle and a 430-µm-diameter plastic bead with an embedded conical shield with the metal particle located at the tip of the conical shield. This paper describes the fabrication of these two target types; it includes the selection of the plastic bead material, how the metal particle was embedded in the plastic material, how the metal particle was attached to the end of the cone, how the plastic material was machined into a bead 430 µm in diameter, and how X-ray images were used to establish the particle position in the plastic material and how it was used for final metrology to determine the concentricity of the metal particle with respect to the plastic bead and the metal particle position with respect to the tip of the conical shield.