Review of the Department of Energy's Inertial Confinement Fusion Program: The National Ignition Facility (1997)

The end of the Cold War in the early 1990s has brought fundamental and far-reaching changes to the Department of Energy's (DOE's) national security mission and to the National Laboratories that are the department's scientific arm. Among the important factors leading to these changes are a decline in defense spending, increasingly far-reaching arms-control treaties, and a growing fear of the proliferation of weapons of mass destruction.

During the Cold War years, one of DOE's principal priorities was the design, testing, and production of nuclear weapons, with ancillary responsibilities related to ensuring the safety, reliability, and performance of the stockpile. Nuclear testing was the major tool used to verify new designs, to assess design or construction flaws, and to certify design and manufacturing changes made to correct those flaws.

The end of U.S. nuclear testing brought a new challenge to the DOE: the stated national needs to ensure the safety, reliability, and performance of an aging inventory of nuclear weapons, and to maintain core intellectual and technical competencies in nuclear weapons without conducting nuclear tests. It is stated national policy that this challenge will be met through Science Based Stockpile Stewardship (SBSS), a program that seeks to provide the fundamental technical understanding and capabilities required to manage a safe and reliable stockpile of nuclear weapons under the Comprehensive Test Ban Treaty.

Briefly outlined below are the circumstances that have led to the U.S. nuclear test moratorium, the requirements imposed on the SBSS program, and a brief description of that program, particularly its Inertial Confinement Fusion (ICF) and National Ignition Facility (NIF) components.

In July 1993, President Clinton announced that a safe, secure, and reliable U.S. nuclear deterrent will remain a cornerstone of U.S. national security policy. He further extended a moratorium on U.S. nuclear testing initiated under President Bush and stated his wish to establish a comprehensive test ban. To support these actions he directed both the Department of Defense (DOD) and the DOE to explore means to maintain confidence in the safety, reliability, and performance of the stockpile in the absence of nuclear tests. The FY94 National Defense Authorization Act (P.L. 103-160) called on the Secretary of Energy to "establish a stewardship program to ensure the preservation of the core intellectual and technical competencies of the United States in nuclear weapons."

In August 1995, President Clinton made an additional statement concerning the Comprehensive Test Ban Treaty:¹

In September 1996, the United States and more than 90 nations signed the Comprehensive Test Ban Treaty. According to international law, the United States, by virtue of its signing of the treaty, is already enjoined from nuclear testing, although the treaty has not entered into full force.

There has long been a formal program² to assess the nuclear stockpile and to deal with problems of safety and reliability as they have arisen. This program has included destructive and nondestructive studies of the physical, dynamic, geometrical, mechanical, metallurgical, and chemical properties of weapons and their components; testing, both nuclear and nonnuclear, has played a central role, but only nonnuclear testing will continue under the Comprehensive Test Ban Treaty. This surveillance program has, in fact, uncovered problems in the U.S. stockpile, some of which have affected large numbers of devices and required significant resources to resolve. To date, the DOE has been able to address all such identified problems and to certify the safety and reliability of the existing stockpile. However, it is almost certain that other problems will arise and be identified as the current stockpile ages and as advancing analytical skills are brought to the surveillance program. Only very limited nuclear test data are available on weapons with long stockpile sojourns.

The SBSS program is one element of the Stockpile Stewardship Management Program designed to ensure that the no-testing regime remains robust into the future and that the United States will not have to invoke its "supreme national interests" option and resume nuclear testing. ³ Its central challenge is to maintain a continuing capability to anticipate, detect, and evaluate actual and potential problems related to aging in the enduring nuclear stockpile and to plan for refurbishment and remanufacture as required.⁴ Meeting this challenge requires the development of increased technical understanding of weapons and weapons-related technologies, including the underlying science, to permit confident prediction, without nuclear testing, of the effects of aging on the safety and performance of weapons. This responsibility includes preserving the core intellectual and technical competencies of the DOE weapons laboratories. It involves enhanced surveillance of the stockpile and remediation of defects as they arise. SBSS entails improving the National Laboratories' experimental capabilities and enhancing their computational capabilities. The decision to undertake a policy of SBSS implies moving in the direction of first-principles predictive capability in hydrodynamics and radiation transport. The advanced stockpile surveillance and manufacturing and materials capabilities of the broader Stockpile Stewardship Management Program are also necessary, as are the maintenance of system engineering and infrastructure and the preservation of nuclear design and experimentation skills. ⁵

The principal facilities of the SBSS program, as provided to the committee by DOE, are listed in Appendix B. This array of planned facilities and programs is designed to carry out above-ground experimentation (AGEX) relevant to weapons. These laboratory-level experiments permit studies of equations of state, opacities, radiative transport, and hydrodynamics that are necessary, together with substantial increases in computational power, to eventually construct predictive computer models of nuclear weapons. Previously, nuclear tests were a source of these data and the primary validation of such models. Several AGEX facilities create and explore confined plasmas at conditions close to, but not coincident with, those of a nuclear weapon. The proposed National Ignition Facility (NIF), part of the Inertial Confinement Fusion (ICF) program, is intended to develop plasma conditions that in many respects would be closer than those of any other facility to the conditions of a nuclear weapon, thereby reducing, but not eliminating, uncertainties in the extrapolation to the conditions in a nuclear weapon.

The National Laboratories must carry out the tasks of SBSS consistent with the broad nonproliferation goals of the United States. In this regard, others have assessed the NIF with the conclusion that the concerns about nonproliferation were manageable, that the risks could be made acceptable, and that the NIF could contribute positively to U.S. arms control and nonproliferation goals.⁶

The physics of matter and radiation at high energy density and the physics of thermonuclear fusion are central issues for SBSS. One goal of the ICF program is to explore all of these elements in a laboratory setting. The following sections, included to make this report relatively self-contained, briefly describe the ICF program's goals and present facilities, as well as the NIF.

The DOE has stated that the primary scientific goal of the ICF program is the release of significant fusion energy in the laboratory. ICF requires that a small mass of deuterium-tritium fuel be rapidly compressed and heated to ignition, so that the initial fusion energy produced will induce further fusion in the fuel before the mass disassembles. In ICF, a spherical fuel capsule of a few millimeters in size is compressed by ablation of material from the capsule surface. Compression is achieved by either direct or indirect drive. In direct drive, laser or ion beams impinge directly on the capsule surface, causing ablation and compression. In "indirect drive, the capsule is placed in an enclosure (hohlraum) made of a high-Z (high atomic number) material such as gold. Laser energy is directed to the hohlraum's inner surface, where it is converted to x-rays; these x-rays impinge on the capsule and cause ablation. Relative to direct drive, indirect drive suffers the inefficiency of x-ray conversion, but it imposes less stringent requirements on beam balance and uniformity.

The DOE's ICF program began formally in 1963 and was established as a separate budget line item in 1976. Several of the National Laboratories, universities, and commercial firms have participated in this program. Several major facilities are currently active in performing experiments relevant to both direct and indirect drive.

The NOVA laser facility at Lawrence Livermore National Laboratory is a 10-beam Nd:glass laser system that is being used to explore indirect drive. Each beam can deliver 8 to 10 kJ of 1050-nm light in variable pulse shapes to 5 ns; up to 40 kJ of frequency-tripled, 350-nm light can be delivered routinely. The OMEGA laser facility at the University of Rochester's Laboratory for Laser Energetics (LLE) is a 60-beam, 30-kJ, pulse-shaped, 350-nm glass laser system intended primarily to explore the direct drive option. The Naval Research Laboratory is also pursuing studies of direct drive with its NUCE KrF gas laser, which produces 4 to 5 kJ of 248-nm light in a 4-ns pulse. The Particle Beam Fusion

Accelerator Z at Sandia National Laboratories in Albuquerque, New Mexico, is a facility that uses pulsed power to create an imploding plasma that generates x-rays. This novel and promising technology has demonstrated more than 150-TW power and 1.8-MJ energy, albeit at significantly lower energy density than that envisioned for the NIF. In addition to these driver facilities, capsule fabrication development is ongoing at General Atomics and at Los Alamos National Laboratory.

Experiments conducted thus far have proved the concept of ICF, but none of the current facilities is capable of creating the conditions necessary to drive present capsule configurations to ignition. With the goal of achieving ignition, the DOE has proposed building a new, 192-beam Nd:glass laser system capable of routinely delivering 1.8 MJ of 350-nm light at a power of 500 TW. This National Ignition Facility, at a stated TPC of $1.148 billion,⁷ is expected to contribute to several DOE mission areas. Beyond the primary SBSS role discussed in this report, the NIF has relevance to fusion energy: even though a flash-lamp-pumped glass laser is too inefficient to be the driver in a prospective inertial fusion energy plant, achievement of ignition with the NIF would help establish design requirements for commercially relevant drivers and other components of an eventual inertial confinement fusion power plant. The NIF is also expected to contribute to basic science and to attract scientists worldwide through its unique ability to provide experimental conditions relevant to atomic, nuclear, and stellar physics, supernova explosions, and cosmology.

The definition of ignition, while seeming straightforward at first glance, is not necessarily a point of consensus within the community of ICF researchers. Therefore, the committee has adopted an operative definition for the purposes of this report.

Figure 1 Predicted nominal energy output as a function of energy incident on the National Ignition Facility point design target. Source: Lawrence Livermore National Laboratory.

Figure 1 shows the predicted nominal fusion energy output as a function of the laser energy incident on the NIF point design target presented to the committee. It indicates that the predicted gain (ratio of fusion yield to laser energy) at the nominal laser energy is about 6. The shape of the yield curve is cliff-like, in that fusion yield increases very rapidly, from near zero to its full value, over a relatively small range of incident laser energy. A plot of fusion yield as a function of other relevant drive parameters (such as laseruniformity or capsule surface finish) would exhibit a similar structure. This curve leads to the operative definition of ignition adopted by the committee: gain greater than unity.

However, there are two diagnosable milestones on the yield curve. At a gain of about 0.1, energy deposited by fusion alpha particles is sufficient to double the central temperature. At a gain of about 0.3, fusion reactions occur over a sufficient region to induce propagation of the thermonuclear burn into the denser, colder, outer fuel.