E. R. Mapoles

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 ...

A high-resolution integrated model of the National Ignition Campaign cryogenic layered experiments

A detailed simulation-based model of the June 2011 National Ignition Campaign cryogenic DT experiments is presented. The model is based on integrated hohlraum-capsule simulations that utilize the best available models for the hohlraum wall, ablator, and DT equations of state and opacities. The calculated radiation drive was adjusted by changing the input laser power to match the experimentally measured shock speeds, shock merger times, peak implosion velocity, and bangtime. The crossbeam energy transfer model was tuned to match the measured time-dependent symmetry. Mid-mode mix was included by directly modeling the ablator and ice surface perturbations up to mode 60. Simulated experimental values were extracted from the simulation and compared against the experiment. Although by design the model is able to reproduce the 1D in-flight implosion parameters and low-mode asymmetries, it is not able to accurately predict the measured and inferred stagnation properties and levels of mix. In particular, the measu...

Shock timing experiments on the National Ignition Facility: Initial results and comparison with simulation

Capsule implosions on the National Ignition Facility (NIF) [Lindl et al., Phys. Plasmas 11, 339 (2004)] are underway with the goal of compressing deuterium-tritium (DT) fuel to a sufficiently high areal density (ρR) to sustain a self-propagating burn wave required for fusion power gain greater than unity. These implosions are driven with a carefully tailored sequence of four shock waves that must be timed to very high precision in order to keep the DT fuel on a low adiabat. Initial experiments to measure the strength and relative timing of these shocks have been conducted on NIF in a specially designed surrogate target platform known as the keyhole target. This target geometry and the associated diagnostics are described in detail. The initial data are presented and compared with numerical simulations. As the primary goal of these experiments is to assess and minimize the adiabat in related DT implosions, a methodology is described for quantifying the adiabat from the shock velocity measurements. Results ...

Infrared redistribution of D2 and HD layers for inertial confinement fusion

We describe a technique to form uniform solid D2 or HD layers for inertial confinement fusion targets. Pumping the infrared (IR) collision induced vibration–rotation band in solid D2 or HD redistributes the solid into a relatively uniform layer depending on the IR intensity profile. Measured redistribution time constants are near the calculated values. We have observed redistribution time constants in HD up to ten times smaller than the DT value.

Deuterium-Tritium Fuel Layer Formation for the National Ignition Facility

Abstract Inertial confinement fusion requires very smooth and uniform solid deuterium-tritium (D-T) fuel layers. The National Ignition Facility (NIF) point design calls for a 65- to 75-μm-thick D-T fuel layer inside of a 2-mm-diam spherical ablator shell to be 1.5 K below the D-T melting temperature (Tm) of 19.79 K. We find that the layer quality depends on the initial crystal seeding, with the best layers grown from a single seed. The low modes of the layer are controlled by thermal shimming of the hohlraum and meet the NIF requirement with beryllium shells and nearly meet the requirement with plastic shells. The remaining roughness is localized in grain-boundary grooves and is minimal for a single crystal layer. Once formed, the layers need to be cooled to Tm - 1.5 K. We have studied dependence of the roughness on the cooling rate and found that cooling at rates of 0.03 to 0.5 K/s is able to preserve the layer structure for a few seconds after reaching the desired temperature. The entire fuel layer remains in contact with the shell during this rapid cooling. Thus, rapid cooling of the layers is able to satisfy the NIF ignition requirements.

Progress in the indirect-drive National Ignition Campaign

We have carried out precision optimization of inertial confinement fusion ignition scale implosions. We have achieved hohlraum temperatures in excess of the 300 eV ignition goal with hot-spot symmetry and shock timing near ignition specs. Using slower rise pulses to peak power and extended pulses resulted in lower hot-spot adiabat and higher main fuel areal density at about 80% of the ignition goal. Yields are within a factor of 5–6 of that required to initiate alpha dominated burn. It is likely we will require thicker shells (+15–20%) to reach ignition velocity without mixing of ablator material into the hot spot.

Progress toward ignition at the National Ignition Facility

Progress toward ignition at the National Ignition Facility (NIF) has been focused on furthering the understanding of implosion performance. Implosion performance depends on the capsule fuel shape, on higher mode asymmetries that may cause hydrodynamic instabilities to quench ignition, on time-dependent asymmetries introduced by the hohlraum target, and on ablator performance. Significant findings in each of these four areas is reported. These investigations have led to improved in-flight capsule shape, a demonstration that a capsule robust to mix can generate high levels of neutrons (7.7 × 10 14 ), hohlraum modifications that should ultimately provide improved beam propagation and better laser coupling, and fielding of capsules with high-density carbon (HDC) ablators. A capsule just fielded with a HDC ablator and filled with DT gas generated a preliminary record level of neutrons at 1.6 × 10 15 , or 5kJ of energy. Future plans include further improvements to fuel shape and hohlraum performance, fielding robust capsules at higher laser power and energy, and tuning the HDC capsule. A capsule with a nanocrystalline diamond (HDC) ablator on a DT ice layer will be fielded at NIF later this year.

REDUCTION OF ISOLATED DEFECTS ON Ge DOPED CH CAPSULES TO BELOW IGNITION SPECIFICATIONS

Abstract The NIF Ge-doped CH capsule should be free of isolated defects on the outer surface. The allowed number and dimensions of large isolated defects over the entire capsule surface is given by the isolated feature specification. To date NIF-thickness (146 μm) capsules are plagued by a few isolated large domes on the outer surfaces that otherwise meet the atomic force microscope (AFM) spheremap modal power spectra specification. The large domes on the capsule surfaces were mostly caused by particulate contamination from the wear of an agitation tapping solenoid inside the coater. By eliminating the solenoid and using an alternate rotation agitation, most thick-walled capsules become free of large isolated defects and meet the AFM spheremap modal power spectra standard. The number and size of the isolated defects on the outer surface were characterized with a high resolution phase-shifting diffractive spherical interferometer and checked against the NIF isolated defect specification. The results show the isolated defects on the rolled capsule are below the isolated defect specification. The growth modeling of the remaining nanometer-height domes on the capsules indicates most of these small domes come from the mandrel surface. The rolled capsules meet the layer thickness, doping levels and wall thickness specifications and have good wall uniformity of ±0.1.0.2 μm.

Target Development for the National Ignition Campaign

Abstract Complex and precise research targets are required for the inertial confinement fusion (ICF) experiments conducted at the National Ignition Facility. During the National Ignition Campaign (NIC) the target development team embarked on and completed a science and technology campaign to provide the capability to produce the required targets at the rate needed by the NIC. An engineering design for precision, manufacturing, and fielding was developed. This required new processes, new tooling, and equipment to metrologize and assemble components. In addition, development of new processing technology was also required. Since the NIC had to respond to new results from ICF experiments, the target development team had to respond as well. This required target designs that allowed for flexibility in accommodating changes in the targets for capsule dimensions and doping levels, hohlraum dimensions and materials, and various new platforms to investigate new physics. A continuous improvement of processes was also required to meet stringent specifications and fielding requirements.

Optical and X-Ray Characterization of Groove Profiles in D-T Ice Layers

Abstract Deuterium-tritium (D-T) single-crystal ice layers in spherical shells often form with localized defects that we believe are vapor-etched grain boundary grooves built from dislocations and accommodating slight misorientations between contacting lattice regions. Ignition implosion target requirements limit the cross-sectional areas and total lengths of these grooves, and since they are often the dominant factor in determining layer surface quality, it is important that we be able to characterize their depths, widths, and lengths. We present a variety of ray-tracing and diffraction image modeling results that support our understanding of the profiles of the grooves, which is grounded in X-ray and optical imaging data. We also describe why these data are nevertheless insufficient to adequately determine whether or not a particular layer meets the groove requirements for ignition. We present accumulated data showing the distribution of groove depths, widths, and lengths from a number of layers, and we discuss how these data motivate the adoption of layer rejection criteria in order to ensure that layers that pass these criteria will almost certainly meet the groove requirements. We also describe future improvements that will provide more quantitative information about grooves in D-T ice layers.