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

During the Cold War years, one of the Department of Energy's (DOE's) principal priorities was the design, testing, and production of nuclear weapons, with ancillary responsibilities related to assuring 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 need 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), part of the broader Stockpile Stewardship and Management Program. SBSS 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. This task includes 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 stockpiled weapons. An array of existing and planned facilities permit laboratory-level experiments to study the equations of state, opacities, radiative transport, and hydrodynamics that are necessary, together with substantial increases in computational power, to develop improved computer simulation models of nuclear weapons. 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 plasma conditions of a nuclear weapon.

Part of the fundamental understanding required for SBSS is expected to come from the ICF program, whose primary scientific goal as stated by DOE is the release of significant fusion energy in the laboratory. Achieving this release of energy requires that a small mass of deuterium-tritium fuel be compressed and heated to "ignition,"² a condition in which the initial fusion energy produced will induce further fusion in the fuel before the mass disassembles. 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 a capsule of deuterium-tritium fuel 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 total project cost (TPC) of $1.148 billion, is expected to contribute to several DOE mission areas, the SBSS program being foremost among them.

Ignition is relevant to SBSS, but the NIF would make contributions to SBSS independent of ignition: Its experimental program would help attract and train people in weapons-related skills, provide

microphysics data, enable integrated code validation, and create hohlraum conditions unique in a laboratory setting (high radiation temperatures foremost among them).

One of the stated goals of the SBSS program is to maintain core intellectual and technical competencies in nuclear weapons in the absence of nuclear testing. This key aim of SBSS requires a challenging theoretical, computational, and experimental program in the areas of implosion hydrodynamics, instabilities and mix, radiation transport, and theormonuclear burn. In the committee's judgment, ICF provides a unique synthesis of these relevant physics areas, and the NIF would provide unique capabilities for basic experiments in atomic physics, radiation flows, plasma physics, and hydrodynamics. In the collective judgment of the committee, the NIF should stimulate the scientific imagination and attract excellent scientists and engineers more than any other proposed element of SBSS. This assertion is supported by the ICF program's history of attracting and retaining high-quality personnel. Indeed, the attraction of the NIF is so high that it may be a management and leadership challenge to keep the ICF program in proper balance with the rest of SBSS.

Prior to the start of the moratorium on nuclear testing, weapons designers could unambiguously confirm their judgments and refine their skills through the success or failure of their devices in underground tests. In the new era of a complete nuclear test ban, the NIF can provide weapons stewards with a similarly challenging alternative for refining and proving their skills. Predicting ICF results in well-diagnosed experiments and ultimately achieving ignition are in many respects more technically demanding than making a nuclear weapon work.

The NIF will provide unique experimental conditions in a controlled environment from which extensive and relevant data can be extracted for both ICF and SBSS applications. The decision to undertake a policy of SBSS implies moving in the direction of first-principles predictive capability in hydrodynamics and radiation transport. While other facilities are expected to provide similar data, the NIF will access different regimes of such parameters as temperature, density, and scale length. Current and former members of the weapons community are debating the extent to which the data to be gained on facilities such as the NIF are applicable to the stockpile. As SBSS is a venture into uncharted technical and organizational territory, this issue can only be resolved by experience. Although many real engineering issues would not be tested directly on facilities such as the NIF, a skilled practitioner could use NIF experiments to develop an understanding of the relevant physics and apply the judgment he develops to analyze stockpile problems as they arise.

The NOVA Technical Contract (NTC) of 1990³ specifies seven experimental objectives in hohlraum laser physics (HLP 1 to 7) and five in hydrodynamic equivalent physics (HEP 1 to 5) that were to be completed in preparation for proceeding to construction of an ignition facility. Although the completion of the NTC cannot guarantee that ignition will be achieved with the NIF, the completion of each milestone, in the context of the understanding of the underlying physics gained in pursuing it, increases confidence in the extrapolation of the results to the performance of the NIF. DOE's assessment of the technical readiness to proceed to Critical Decision 2 (CD-2)⁴ was based partly on sustained progress on the NTC and on the extrapolation of those results to the NIF. Since CD-2, the NIF baseline target design has been changed to a gas-filled hohlraum. This change introduced unexpected but seemingly reproducible beam bending and increased backscatter so that several of the HLP 1 to 6 milestones are no longer met and others are, at best, barely met. The cause of this unexpected behavior is thought to be understood, and experiments with one NOVA beam smoothed indicate that beam

smoothing will restore the performance to that specified in the NTC; experiments with all 10 beams smoothed are scheduled for completion in FY97. Four of the five NTC milestones in capsule physics (HEP 1 through 4 in Table 1) have been met and the fifth—the convergence ratio of a successful target in the NIF geometry—has reached a value of 10, which is short of the originally projected value of 20.

The HEP 5 implosion experiments and modeling comparisons, intended to demonstrate implosion subject to overall hydrodynamic mix and spatial-mode spectra of capsules similar to ignition target designs, have not been successfully completed. Analysis following the original definition of these milestones, including limited three-dimensional hydrodynamics calculations and experiments, shows that the HEP 5 milestone is unrealistic in that it is beyond the reach of the present NOVA configuration and other currently available facilities. This same analysis shows that the NIF environment is compatible with the high convergence required for ignition, although wall motion issues have not been completely resolved. Fully integrated two-dimensional radiation-hydrodynamics calculations have allayed concerns about time-dependent low-mode number asymmetries. Although the milestone itself has not been reached, understanding of the physics behind it has been advanced significantly. An experimental campaign toward meeting the HEP 5 requirement, involving reconfiguration of the NOVA and OMEGA lasers, is in progress and would increase confidence in reaching ignition with the NIF. However, in the committee's judgment the additional technical confidence that might be gained by completion of milestone HEP 5 is not sufficient to justify delaying the NIF program.

Like current weapons studies, the ICF program supplements and interprets experimental studies with large-scale numerical simulations. The current generation of computer codes is not adequate to perform first-principles three-dimensional integrated calculations of an ignition target. However, the predictions of models that have been developed are in good agreement with the data and are consistent with the objectives of the NIF. The DOE has created the Accelerated Strategic Computing Initiative (ASCI) program to enhance computational resources and to stimulate the development of new and/or improved numerical methods and computational physics, as well as new classes of computer codes, especially three-dimensional codes. Three new high-performance computers have already been contracted for, delivery is in progress, and each will be phased in over the next 2 years. While all of these computers exploit parallel processing, they have very different architectural features that will require attention in developing applications aimed at achieving high performance. Even with these ASCI machines, the demands of three-dimensional calculations will still be severe, and good physical models coupled with good algorithms and code implementations will be required. Advanced developments enabled by the ASCI program in algorithms, models, and codes will be essential to realizing a true three-dimensional simulation capability.

On balance, while the current understanding of the science of the imploding capsule does not guarantee that ignition can be achieved with the NIF, it does give reasonable expectation that ignition will be achieved.

The expected NIF laser peak performance is 2.2-MJ of 350-nm light at 600-TW peak power. (The baseline operation is set at 1.8 MJ and 500 TW.) The NIF is composed of 192 beamlines, arranged into four arrays. Each array is composed of 4 × 12 segments, each with an aperture of 40 × 40 cm². The NIF design was selected from two conceptual designs based on multipass architecture. An original 240-beamline design was deferred and the current design with 192 beamlines was selected as a compromise, based on a reasonable expectation of reaching ignition with the lower energy and power and associated cost trade-offs. The NIF baseline uses a laser architecture selected to meet the required performance levels at acceptable cost while respecting the limits imposed by laser fluence damage.

The NIF laser will operate with larger optics than any previous laser system. It is based on a new laser architecture that reduces the number of optical elements, reduces the volume of the laser, and enhances the laser control and operational capability through a design that allows repair of laser

beamlines between shots. To test these advanced concepts, a single laser beamline (called the Beamlet) was constructed. The Beamlet has been operational since 1994 and has provided a test bed to gain confidence in the NIF laser architecture and design, operating at the fluence levels planned for the NIF laser.

The Beamlet laser has validated almost all aspects of the design of the NIF laser. The exceptions are the final focusing optics and the lower damage threshold of the rapidly grown potassium dihydrogen phosphate (KDP) crystals; there are adequate plans for dealing with these two remaining issues. As designed, the NIF laser will operate at a maximum fluence consistent with an optimization of both cost and reliability and projected performance. The NIF laser architecture allows beamline maintenance and repair without disruption of normal operations. The beam-smoothing experiments under way on NOVA have demonstrated that the technology exists to meet the NIF laser performance specifications.

A baseline target design is in place and was used to specify the necessary operating parameters for the NIF. Possibilities for new and better targets, which would increase the likelihood of ignition, are being explored with the aid of recent advances in computational capabilities and related experiments. New ideas are being proposed regularly, including ''cocktail'' walls, different shapes for hohlraums, and new materials for capsules. The possibility that new and better targets might be developed before actual NIF operation in 5 or more years should not be discounted. Hohlraum designs are expected to evolve and be tested to accommodate the anticipated extension to cryogenic target capsules. Cryogenic layers with adequate smoothness (1 μm) have been demonstrated, albeit in surrogate geometries. Work is in progress on the problems of delivery and in situ characterization of cryogenic targets.

Continuing experiments and simulations are expected to further define capsule fabrication specifications. Given the evolution of the scientific understanding of hydrodynamic instabilities and the coupling between drive uniformity and capsule fabrication specifications, continuing progress in this area will increase the likelihood of achieving ignition with the proposed NIF.

The NIF project organization and its relationship to the Lawrence Livermore National Laboratory (LLNL) appear sound and acceptable, with the critical elements in place to optimize the chances for successful project implementation and completion. The project has support from DOE management, and the project manager has sufficient administrative autonomy to accomplish work in a timely, efficient manner, including $25 million in procurement authority. The project leadership also has a good relationship with both DOE Oakland and DOE headquarters.

Since CD-2, the cost of the NIF has increased by $125 million for additional scope (see the section titled "The National Ignition Facility Project" in Chapter 4) and by $29 million for site-specific costs. However, these changes have increased the TPC by only $74 million (to $1.148 billion), because most of the start-up costs have been moved to the LLNL-projected operating budget for NIF and NOVA operations. In addition, there are NIF-related non-TPC costs of $397 million in LLNL-projected operating funds in FY98-FY02.

Several technical issues should be pursued in parallel with NIF construction. While none is sufficiently serious to constitute a "showstopper," continued attention to all of them will to help ensure the NIF's success and its contributions to SBSS.

• Consistent physical understanding of NIF-relevant phenomena. Efforts should continue to better understand the requirements for convergence and compression with a modified NOVA configuration (HEP 5), laser-plasma interactions and the resulting "spot motion," the

effects of hohlraum wall motion and the physics of plasma instabilities in gas-filled hohlraums, and the prediction, control, and diagnostics of the x-ray drive in hohlraums. Experiments to be conducted include the following:

1. Complete the 10-beam smoothing and wall-motion experiments in gas-filled hohlraums on NOVA. Implementation of the full smoothing by spectral dispersion (SSD) and random phase plates (RPPs) on all 10 beams will demonstrate further understanding of the potential limiting effects of laser-plasma interactions. Demonstrated low levels of stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS), and the resulting acceptable radiation temperatures, are expected.

2. Conduct experiments on retarding wall motion with improved azimuthal symmetry and a wider range of plasma conditions to further investigate the NIF hohlraum design.

3. Vigorously pursue experiments to study the behavior of Rayleigh-Taylor and Richtmyer-Meskov hydrodynamic instabilities in converging geometries.

• Need for and challenges of three-dimensional simulations. Current three-dimensional computer codes are inadequate for integrated calculations of ignition; their improvement will contribute significantly to increasing the probability of ignition. These codes must be adapted to run on ASCI platforms. The development of new, less empirical three-dimensional models and algorithms is necessary, as is benchmarking of three-dimensional codes by ICF experiments and test data. This effort is further challenged by the different characteristics of the machines purchased under the ASCI program.

• Cryogenic capsule technology. Advances should be sought in assembly, fill, and surface smoothness. Continued development of handling and delivery systems is also important to ignition, as are methods of characterizing the fuel in optically opaque cells. Researchers should explore target-design alternatives, advance target design and fabrication technology, and pursue cryogenic target experiments on OMEGA.

• Optics damage thresholds and cost. While the current cost estimate contains funds to purchase slow-growth KDP crystals if necessary, the effect on schedules must be considered if the fast-growth crystals fail to meet damage threshold requirements.

The proposed NIF is a flexible, high-power, high-energy laser facility that will address fundamental high-energy-density physics issues while creating in the laboratory conditions approaching those relevant to a nuclear weapon. The challenge of achieving ignition should help attract and sustain a cadre of talented scientists and engineers with weapons-relevant experience and expertise. The challenge of predicting results of NIF experiments will provide a "certification" of future weapons stewards analogous to that provided by the underground test experience of the present designers. The NIF's experimental capabilities will complement other SBSS activities by allowing unique experiments probing weapons-related physics.

In assessing the scientific and technical readiness of the proposed NIF project, the committee attempted to balance the NIF's potential value against the risk inherent in extrapolation from the present

base of experimental and computational experience. The committee believes that the NIF can be delivered to specifications within the stated TPC, as augmented by LLNL-projected operating funds, allowing the high-energy-density and ignition experimental programs to proceed; there are no identifiable "show stoppers." The achievement of ignition appears likely, but not guaranteed. The steady scientific and technological progress in ICF during the 6 years since the last National Research Council review,⁵ the plausibility of ignition estimates based on the experimental and modeling results and capabilities in hand, and the flexibility of the proposed facility all support the committee's finding that the NIF project is technologically and scientifically ready to proceed as planned with reasonable confidence in the attainment of its objectives.