J. D. Lindl

Development of the indirect‐drive approach to inertial confinement fusion and the target physics basis for ignition and gain

Inertial confinement fusion (ICF) is an approach to fusion that relies on the inertia of the fuel mass to provide confinement. To achieve conditions under which inertial confinement is sufficient for efficient thermonuclear burn, a capsule (generally a spherical shell) containing thermonuclear fuel is compressed in an implosion process to conditions of high density and temperature. ICF capsules rely on either electron conduction (direct drive) or x rays (indirect drive) for energy transport to drive an implosion. In direct drive, the laser beams (or charged particle beams) are aimed directly at a target. The laser energy is transferred to electrons by means of inverse bremsstrahlung or a variety of plasma collective processes. In indirect drive, the driver energy (from laser beams or ion beams) is first absorbed in a high‐Z enclosure (a hohlraum), which surrounds the capsule. The material heated by the driver emits x rays, which drive the capsule implosion. For optimally designed targets, 70%–80% of the d...

The physics basis for ignition using indirect-drive targets on the National Ignition Facility

The 1990 National Academy of Science final report of its review of the Inertial Confinement Fusion Program recommended completion of a series of target physics objectives on the 10-beam Nova laser at the Lawrence Livermore National Laboratory as the highest-priority prerequisite for proceeding with construction of an ignition-scale laser facility, now called the National Ignition Facility (NIF). These objectives were chosen to demonstrate that there was sufficient understanding of the physics of ignition targets that the laser requirements for laboratory ignition could be accurately specified. This research on Nova, as well as additional research on the Omega laser at the University of Rochester, is the subject of this review. The objectives of the U.S. indirect-drive target physics program have been to experimentally demonstrate and predictively model hohlraum characteristics, as well as capsule performance in targets that have been scaled in key physics variables from NIF targets. To address the hohlrau...

Point design targets, specifications, and requirements for the 2010 ignition campaign on the National Ignition Facility

Point design targets have been specified for the initial ignition campaign on the National Ignition Facility [G. H. Miller, E. I. Moses, and C. R. Wuest, Opt. Eng. 443, 2841 (2004)]. The targets contain D-T fusion fuel in an ablator of either CH with Ge doping, or Be with Cu. These shells are imploded in a U or Au hohlraum with a peak radiation temperature set between 270 and 300 eV. Considerations determining the point design include laser-plasma interactions, hydrodynamic instabilities, laser operations, and target fabrication. Simulations were used to evaluate choices, and to define requirements and specifications. Simulation techniques and their experimental validation are summarized. Simulations were used to estimate the sensitivity of target performance to uncertainties and variations in experimental conditions. A formalism is described that evaluates margin for ignition, summarized in a parameter the Ignition Threshold Factor (ITF). Uncertainty and shot-to-shot variability in ITF are evaluated, and...

Progress toward Ignition and Burn Propagation in Inertial Confinement Fusion

For the past four decades, scientists throughout the world have pursued the dream of controlled thermonuclear fusion. The attraction of this goal is the enormous energy that is potentially available in fusion fuels and the view of fusion as a safe, clean energy source. The fusion reaction with the highest cross section uses the deuterium and tritium isotopes of hydrogen, and D‐T would be the fuel of choice for the first generation of fusion reactors. (See the article by J. Geoffrey Cordey, Robert J. Goldston and Ronald R. Parker, January, page 22.)

Design and modeling of ignition targets for the National Ignition Facility

Several targets are described that in simulations give yields of 1–30 MJ when indirectly driven by 0.9–2 MJ of 0.35 μm laser light. The article describes the targets, the modeling that was used to design them, and the modeling done to set specifications for the laser system in the proposed National Ignition Facility. Capsules with beryllium or polystyrene ablators are enclosed in gold hohlraums. All the designs utilize a cryogenic fuel layer; it is very difficult to achieve ignition at this scale with a noncryogenic capsule. It is necessary to use multiple bands of illumination in the hohlraum to achieve sufficiently uniform x‐ray irradiation, and to use a low‐Z gas fill in the hohlraum to reduce filling of the hohlraum with gold plasma. Critical issues are hohlraum design and optimization, Rayleigh–Taylor instability modeling, and laser–plasma interactions.

Review of the National Ignition Campaign 2009-2012

The National Ignition Campaign (NIC) was a multi-institution effort established under the National Nuclear Security Administration of DOE in 2005, prior to the completion of the National Ignition Facility (NIF) in 2009. The scope of the NIC was the planning and preparation for and the execution of the first 3 yr of ignition experiments (through the end of September 2012) as well as the development, fielding, qualification, and integration of the wide range of capabilities required for ignition. Besides the operation and optimization of the use of NIF, these capabilities included over 50 optical, x-ray, and nuclear diagnostic systems, target fabrication facilities, experimental platforms, and a wide range of NIF facility infrastructure. The goal of ignition experiments on the NIF is to achieve, for the first time, ignition and thermonuclear burn in the laboratory via inertial confinement fusion and to develop a platform for ignition and high energy density applications on the NIF. The goal of the NIC was to develop and integrate all of the capabilities required for a precision ignition campaign and, if possible, to demonstrate ignition and gain by the end of FY12. The goal of achieving ignition can be divided into three main challenges. The first challenge is defining specifications for the target, laser, and diagnostics with the understanding that not all ignition physics is fully understood and not all material properties are known. The second challenge is designing experiments to systematically remove these uncertainties. The third challenge is translating these experimental results into metrics designed to determine how well the experimental implosions have performed relative to expectations and requirements and to advance those metrics toward the conditions required for ignition. This paper summarizes the approach taken to address these challenges, along with the progress achieved to date and the challenges that remain. At project completion in 2009, NIF lacked almost all the diagnostics and infrastructure required for ignition experiments. About half of the 3 yr period covered in this review was taken up by the effort required to install and performance qualify the equipment and experimental platforms needed for ignition experiments. Ignition on the NIF is a grand challenge undertaking and the results presented here represent a snapshot in time on the path toward that goal. The path forward presented at the end of this review summarizes plans for the Ignition Campaign on the NIF, which were adopted at the end of 2012, as well as some of the key results obtained since the end of the NIC.

Symmetric Inertial Confinement Fusion Implosions at Ultra-High Laser Energies

Ignition Set to Go One aim of the National Ignition Facility is to implode a capsule containing a deuterium-tritium fuel mix and initiate a fusion reaction. With 192 intense laser beams focused into a centimeter-scale cavity, a major challenge has been to create a symmetric implosion and the necessary temperatures within the cavity for ignition to be realized (see the Perspective by Norreys). Glenzer et al. (p. 1228, published online 28 January) now show that these conditions can be met, paving the way for the next step of igniting a fuel-filled capsule. Furthermore, Li et al. (p. 1231, published online 28 January) show how charged particles can be used to characterize and measure the conditions within the imploding capsule. The high energies and temperature realized can also be used to model astrophysical and other extreme energy processes in a laboratory settings. Laser-driven temperatures and implosion symmetry are close to the requirements for inertial-fusion ignition. Indirect-drive hohlraum experiments at the National Ignition Facility have demonstrated symmetric capsule implosions at unprecedented laser drive energies of 0.7 megajoule. One hundred and ninety-two simultaneously fired laser beams heat ignition-emulate hohlraums to radiation temperatures of 3.3 million kelvin, compressing 1.8-millimeter-diameter capsules by the soft x-rays produced by the hohlraum. Self-generated plasma optics gratings on either end of the hohlraum tune the laser power distribution in the hohlraum, which produces a symmetric x-ray drive as inferred from the shape of the capsule self-emission. These experiments indicate that the conditions are suitable for compressing deuterium-tritium–filled capsules, with the goal of achieving burning fusion plasmas and energy gain in the laboratory.

A review of the ablative stabilization of the Rayleigh–Taylor instability in regimes relevant to inertial confinement fusion

It has been recognized for many years that the most significant limitation of inertial confinement fusion (ICF) is the Rayleigh–Taylor (RT) instability. It limits the distance an ablatively driven shell can be moved to several times its initial thickness. Fortunately material flow through the unstable region at velocity vA reduces the growth rate to √kg/1+kL−βkvA with β from 2–3. In recent years experiments using both x‐ray drive and smoothed laser drive to accelerate foils have confirmed the community’s understanding of the ablative RT instability in planar geometry. The growth of small initial modulations on the foils is measured for growth factors up to 60 for direct drive and 80 for indirect drive. For x‐ray drive large stabilization is evident. After some growth, the instability enters the nonlinear phase when mode coupling and saturation are also seen and compare well with modeling. Normalized growth rates for direct drive are measured to be higher, but strategies for reduction by raising the isentr...

High yield inertial confinement fusion target design for a z-pinch-driven hohlraum

Calculations are presented for a high yield inertial fusion design, employing indirect drive with a double-ended z-pinch-driven hohlraum radiation source. A high current (∼60 MA) accelerator implodes z pinches within an enclosing hohlraum. Radial spoke arrays and shine shields isolate the capsule from the pinch plasma, magnetic field, and direct x-ray shine. Our approach places minimal requirements on z-pinch uniformity and stability, usually problematic due to magneto-Rayleigh–Taylor instability. Large inhomogeneities of the pinch and spoke array may be present, but the hohlraum adequately smooths the radiation field at the capsule. Simultaneity and reproducibility of the pinch x-ray output to better than 7% are required, however, for good symmetry. Recent experiments suggest a pulse shaping technique, through implosion of a multishell z pinch. X-ray bursts are calculated and observed to occur at each shell collision. A capsule absorbing 1 MJ of x rays at a peak drive temperature of 210 eV is found to ha...

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 ...