Inertial Fusion Implosions Research Articles

Considerations and Requirements for Providing Cryogenic Targets for Direct-Drive Inertial Fusion Implosions at the National Ignition Facility

Modifications to the National Ignition Facility (NIF) Cryogenic Target Positioner (Cryo-TarPos) are needed to provide polar-drive-ignition targets; ideally, these modifications will be completed and tested by 2017, the earliest date anticipated for polar-drive-ignition experiments. The extent of these modifications is defined by the mechanical and thermal requirements needed for the target to conform to the ignition design and the capabilities of the existing equipment. This paper describes the design of the polar-drive target assembly and the surrounding cryogenic environment that meets many of the specifications and requirements for the ignition target. Further work is necessary to optimize the design and provide more-detailed guidance for modifying the NIF Cryo-TarPos; however, there is sufficient information to begin the redesign effort at the conceptual level.A specialized facility has been constructed to test different target assembly and cryogenic hardware designs. The equipment provides the mechanical and cryogenic functionality available at the NIF, making it possible to test different target designs with deuterium in a configuration suitable for integration with the NIF Cryo-TarPos. The polar-drive target assembly has demonstrated a stable ice layer (170 to 350 μm thick) and the ability to control the thickness to ±3 μm of the desired value. The target is rotatable to fully characterize the D2 ice surface using shadowgraphy and X-ray phase contrast. Thermal models of the target and its environment indicate that (a) it should be possible to achieve the desired 1-μm root-mean-square smoothness using D-T, (b) the fill tube has little effect on the ice smoothness, and (c) it is possible to shape the isotherms surrounding the target sufficiently to form an oblate ice layer that may be more desirable for polar-drive implosions.

Overview of Recent Tritium Target Filling, Layering, and Material Testing at Los Alamos National Laboratory in Support of Inertial Fusion Experiments

The Tritium Science and Engineering (AET-3) Group at Los Alamos National Laboratory (LANL) performs a variety of activities to support Inertial Fusion (IF) research - both to further fundamental fusion science and to develop technologies in support of Inertial Fusion Energy (IFE) power generation.Inertial fusion ignition target designs have a smooth spherical shell of cryogenic Deuterium-Tritium (DT) solid contained within a metal or plastic shell that is a few mm in diameter. Fusion is attained by imploding these shells under the symmetric application of energy beams. For IFE targets the DT solid must also survive the process of injecting it into the power plant reactor. Non-ignition IF targets often require a non-cryogenic DT gas fill of a glass or polymeric shell. In this paper an overview will be given of recent LANL activities to study cryogenic DT layering, observe tritium exposure effects on IF relevant materials, and fill targets in support of IF implosion experiments.

Proton Radiography of Inertial Fusion Implosions

A distinctive way of quantitatively imaging inertial fusion implosions has resulted in the characterization of two different types of electromagnetic configurations and in the measurement of the temporal evolution of capsule size and areal density. Radiography with a pulsed, monoenergetic, isotropic proton source reveals field structures through deflection of proton trajectories, and areal densities are quantified through the energy lost by protons while traversing the plasma. The two field structures consist of (i) many radial filaments with complex striations and bifurcations, permeating the entire field of view, of magnetic field magnitude 60 tesla and (ii) a coherent, centrally directed electric field of order 10(9) volts per meter, seen in proximity to the capsule surface. Although the mechanism for generating these fields is unclear, their effect on implosion dynamics is potentially consequential.

Fast ignition studies at Sandia National Laboratories

Sandia National Laboratories is developing a combination of experimental and theoretical capabilities useful for the study of fast ignition physics. Pulsed power machines such as the present Z machine have demonstrated the ability to drive inertial fusion implosions. The Z-beamlet laser is currently being used to create multi-kiloelectronvolts photons for backlighting, which can be used to diagnose the compression of deuterium/tritium to high densities. A high-energy petawatt capability is presently being added to the Z-beamlet to extend the backlighting x-ray energy up to 10–50 keV and to enable integrated fast ignition experiments. We are also developing a capability to implode capsules filled with liquid deuterium/tritium, which avoids the stringent temperature control required for β-layered capsules.In preparation for such experiments, the theory group at Sandia is modelling various aspects of fast ignition physics. Numerical simulations of laser/plasma interaction, electron transport and ion generation are being performed. Simulations of the compression of deuterium/tritium fuel in various geometries are being performed. Analytic and numerical modelling has been performed to determine the conditions required for fast ignition breakeven scaling. These results indicate that breakeven will require about 5% of the laser energy needed for ignition and might be an achievable goal with an upgraded Z-beamlet laser in short pulse mode.

Theory of laser-induced adiabat shaping in inertial fusion implosions: The relaxation method

The theory of the adiabat shaping induced by a strong shock propagating through a relaxed density profile is carried out for inertial confinement fusion (ICF) capsules. The relaxed profile is produced through a laser prepulse, while the adiabat-shaping shock is driven by the foot of the main laser pulse. The theoretical adiabat profiles accurately reproduce the simulation results. ICF capsules with a shaped adiabat are expected to benefit from improved hydrodynamic stability while maintaining the same one-dimensional performances as flat-adiabat shells.

Laser-induced adiabat shaping by relaxation in inertial fusion implosions

The theory of laser-induced adiabat shaping is carried out for inertial confinement fusion (ICF) capsules. It is shown that a significant improvement of the stability characteristics of ICF implosions can be achieved by shaping the adiabat inside the imploding shell. The optimized adiabat profile has a maximum on the outer ablation surface to lower Rayleigh–Taylor growth rates, and a minimum on the shell inner surface for high compressibility and high neutron yields. Laser-induced adiabat shaping is produced via relaxation using a weak prepulse followed by laser shut-off and the main laser pulse.

Theory of laser-induced adiabat shaping in inertial fusion implosions: The decaying shock

The evolution of a decaying shock driven by a laser prepulse in inertial confinement fusion (ICF) capsules is investigated including the effects of mass ablation and residual ablation pressure. The adiabat (entropy) profile left behind by a decaying shock is calculated and compared with the results of one-dimensional simulations. It is found that the adiabat shape follows a simple power law of the areal density α=αinn(mshell/m)δa, where αinn is the shell inner surface adiabat, mshell is the initial total shell areal density, and m is the local shell areal density. The power law index δa varies from 1.12 for prepulses in the 100 ps range to 1.20 for prepulses in the 300 ps range. This adiabat profile accurately reproduces the simulation results to within a few percent. A simple procedure aimed at designing a laser pulse inducing the maximum adiabat ratio between the outer- and inner-shell surface is also developed. ICF capsules with a shaped adiabat are expected to benefit from improved hydrodynamic stability while maintaining the same one-dimensional performance as flat adiabat shells.