|Fig. 1: This diagram illustrates the indirect-drive strategy employed at NIF, in which lasers irradiate the interior of a hohlraum to produce a bath of x-rays that compress the target.  (Adapted from Hurricane et al. by permission from Macmillan Publishers Ltd: Nature, © 2014.)|
The National Ignition Facility (NIF) is an inertial confinement fusion (ICF) research facility constructed at Lawrence Livermore National Laboratory (LLNL) starting in 1997. After years of delays and cost overruns, "first light" of its 192 laser beam lines was demonstrated in 2009. Since then, NIF has periodically attempted to compress a cryogenic hydrogen target contained in a gold vessel, called a hohlraum, to induce the ignition of nuclear fusion (defined as the release of more fusion energy than is input to the hohlraum).
In order to ignite fusion, it is necessary to produce very high pressures and temperatures in a target, while containing the matter under such conditions for as long as possible. This is achieved at NIF via an indirect drive system, which involves sending lasers into a hohlraum rather than directly onto a target, as illustrated in Fig. 1. This increases the containment time of the target plasma by creating a more uniform bath of x-rays than is possible with a direct drive system, at the cost of an energy inefficiency in transferring laser energy to the hohlraum.
Over the past 5 years, NIF has demonstrated increasing fusion yields, by manipulating these parameters in its experiments, but has thus far come up well short of achieving its titular goal. Despite the funding issues that plagued the project, these ended up having little effect on NIF's science capability. Instead, the codes used to predict the ignition performance of NIF failed to predict the appearance of a large Rayleigh-Taylor instability at the surface of the fusion capsule, limiting NIF's ability to achieve ignition. We describe the nature of this instability, current and future mitigation strategies, and overall future prospects for NIF.
The Comprehensive Nuclear-Test-Ban Treaty, signed by the United States in 1996, prohibited explosive testing and use of nuclear devices and weapons. In light of this treaty, it became necessary for the US to find ways of maintaining and assuring the performance of its nuclear stockpile without direct testing via detonation. It was in this context that the proposal for NIF was presented to the Department of Energy (DOE).
Tellingly, the branch of the DOE that decided to fund NIF was not the Office of Science, which primarily funds civilian high-energy physics projects such as the Large Hadron Collider, but the National Nuclear Security Administration (NNSA), the agency in charge of developing and maintaining the American nuclear stockpile.  This made it clear that although much civilian hype has accompanied the development and operation of NIF, its fundamental purpose has always been to maintain nuclear expertise and advance understanding of weapons physics, rather than to achieve ignition for civilian energy purposes. 
The primacy of NIF's military purpose was made even clearer in 2000, by which time mismanagement of the project had led to significant delays and more than $2 billion in cost overruns.  Energy secretary Bill Richardson, with few other options, decided to redirect funds from "other Stockpile Stewardship Program activities" to cover the new costs.  Having now drawn over $4 billion from military sources, NIF would have to deliver returns to its primary stakeholders (interested in validating their weapons simulation codes) before much concern was paid to achieving ignition for civilian energy generation. Indeed, despite having failed to achieve ignition, NIF has performed many experiments aimed at improving military understanding of nuclear stockpile maintenance. 
Despite the mismanagement at NIF, its performance specifications were never compromised. The conceptual design report (CDR), submitted in 1994, called for a 1.8MJ laser system, with a minimum of 192 beamlines.  The actual performance of NIF modestly surpasses this specification, with actual performance capability measured at 1.9 MJ. 
|Fig. 2: This plot shows the expected performance of NIF at the time of its design. The performance specifications (red star) were achieved, but the simulations used to compute the black area were proven incorrect by eventual NIF experiments. After Paisner, et al. |
The failure of NIF to achieve ignition thus far has not been due to a de-scope of the experiment; rather, NIF has encountered limitations due to the fundamental physics of plasma dynamics. In the CDR, the researchers claimed: "The baseline design at 500 TW/1.8 MJ has approximately a factor of two safety margin for ignition. This energy and power safety margin above threshold provides room to trade off asymmetry, laser-plasma instabilities, and other uncertainties." Fig. 2 shows a plot from the CDR illustrating this claimed 'safety margin,' showing the NIF specification design well inside the ignition region of parameter space. Unfortunately, this estimate was based on an understanding of plasma dynamics that would later be proven wrong by NIF experiments. In some sense, this was the primary purpose of the experiment - to test simulation codes for plasma dynamics relevant to nuclear stockpile management - but was nonetheless disappointing to many in the scientific community. 
At the beginning, NIF was expected to achieve ignition with room to spare. But as of February 2015, the best results at NIF are a factor or three short of achieving the Lawson criterion.  How did NIF end up missing its ignition target by an order of magnitude? Part of answer lies in the fundamental fluid dynamics of Rayleigh-Taylor instabilities.
A NIF target capsule is a deuterium-tritium (DT) sphere encased in an ablative shell, illustrated in Fig. 3. When the capsule is compressed by the bath of X-rays from the hohlraum, Rayleigh-Taylor instabilities can cause the ablative shell to mix with the DT fuel, interfering with the further compression of the fuel and radiating away heat, reducing overall performance.  The growth of these instabilities is governed by the following equation given in: 
|Growth Rate||=||α (||k g
1 + k L
|)1/2 - β k v|
where k is the wavenumber of the Rayleigh-Taylor perturbation, L is the length scale of the ablative front's density gradient, and v and g are the velocity and the acceleration of the ablator, respectively.
During the National Ignition Campaign (NIC), a technique known as a "high-foot" implosion method was developed to dampen the effect of the observed Rayleigh-Taylor instabilities.  In a "high-foot" shot, L and v are increased due to stronger initial shocks that precede the main burst. By the equation above, both of these changes lead to slower instability growth, meaning that the desired uniform compression of the fuel lasts longer during a "high-foot" laser shot. Sample temperature profile for "high-foot" and "low-foot" implosion strategies, illustrating the pre- shocks associated with slower growth of Rayleigh-Taylor instabilities, are shown in Fig. 3.
|Fig. 3: The top image shows a cross-sectional view of a NIF fuel capsule with plastic ablative shell. The bottom plot illustrates the difference between a "high-foot" and "low-foot" implosion strategy.  (Adapted from Hurricane et al. by permission from Macmillan Publishers Ltd: Nature, © 2014.)|
This "high-foot" implosion strategy led to NIF's biggest step toward ignition to date: achieving "fuel gain" greater than unity.  This does not meet our definition of ignition, where the energy released must exceed the laser energy input to the hohlraum. Instead, this achievement means that the energy released was observed to be greater than the energy input to the fuel capsule, based on a model of hohlraum inefficiency. While this result is just a small step toward true ignition, it provides an example of post-construction tweaks to NIF experiments leading to improved performance, suggesting that further performance improvements may be possible.
In the absence of funding for major improvements to NIF, further work to tweak parameters to improve performance is ongoing. NIF is investigating adjustments to hohlraum geometry, composition, and the gases present in the chamber during laser shots, in addition to considering fuel capsules coated in diamond or beryllium rather than plastic ablator.  However, the primary focus of NIF has shifted back to its original purpose: sub-critical nuclear testing. Having ended the NIC unsuccessfully, NIF is also transforming itself into a part-time user facility for nuclear physics, laboratory astrophysics, and materials science at exotic temperatures and pressures. 
In a larger scope, the dream of near limitless energy from nuclear fusion power remains behind many more technical hurdles. Even if NIF is able to formally achieve ignition by tuning the parameters described in the previous section, it will remain a long way from providing a usable, much less affordable energy source. For one, useful power output from fusion will require more than mere ignition - a fusion gain of ~100 will be required for a reactor to be economically viable.  What's more, the affordability and rate of firings will need to increase by orders of magnitude in order for ICF to provide economical energy. At present, each test at NIF requires a custom precision-manufactured hohlraum and cryogenic target; the cost of both of these is much higher than will be necessary in a full-scale power generation operation. These factors, in addition to the necessary laser cooling time, mean that the maximum rate of firings at NIF is ~1 per day. This would need to increase by many orders of magnitude, to a repetition rate of 10s of shots per second in a power plant setting. 
Despite the large gap between even the design specifications of NIF (which optimistically estimated a total gain only slightly higher than one) and the requirements for a useful fusion power facility, LLNL broadcast the promise of fusion-driven energy production via ignition at NIF very widely. [6,11] The lab funded a program called LIFE to study practical issues surrounding producing energy at NIF once ignition had been achieved.  In 2014, after the NIC had failed to achieve ignition, LLNL quietly killed the program. Robert McCrory, director of Rochester University's Laboratory for Laser Energetics, said "In my opinion, the over-promising and overselling of LIFE did a disservice to Lawrence Livermore Laboratory." Though LLNL may have overstated the direct application of NIF to fusion energy production, the advances in engineering knowledge and fundamental science from NIF development and operations do constitute a step toward this far-off goal. 
Despite confident predictions in its early stages, NIF has yet to achieve its titular goal of fusion ignition due to a Rayleigh-Taylor instability at the ablative surface of the target capsule. However, it has succeeded in its primary funding goal of providing non-critical testing and validation of simulation codes for the design and upkeep of nuclear weapons. Going forward, NIF will focus on improving performance for weapons code validation as well as fundamental science as a part-time user facility. The promise of internal confinement fusion as a limitless source of civilian energy therefore remains a distant prospect for the foreseeable future.
© Michael Baumer. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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