6
Findings and Conclusions

THE NIF WOULD MAKE IMPORTANT CONTRIBUTIONS TOWARD THE STATED LONG-TERM GOALS OF THE SBSS PROGRAM

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.

THE SCIENCE AND TECHNOLOGY HAVE PROGRESSED SUFFICIENTLY TO ALLOW THE NIF PROJECT TO PROCEED AS PLANNED

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

The science supporting the NIF fusion objectives includes the physics of the imploding capsule and the physics of laser-hohlraum interactions. The issues in these two areas and the requirements for scientific readiness were delineated as milestones in the NOVA Technical Contract (NTC) of 1990, which was based on the point target design at that time. The fulfillment of almost all of the NTC's terms with surrogate targets that precede the current NIF design has led to substantial and encouraging progress in the understanding of both areas, although some issues have not yet been resolved completely.

The NOVA experimental campaign failed to meet its objective of high-convergence implosions (HEP 5). The computational advances of the last 5 years have pinpointed the cause as lack of symmetry in the 10-beam NOVA system. The three-dimensional simulations that successfully describe the NOVA experiments predict that the 192-beam NIF system will have adequate symmetry for high-convergence implosions. This has been confirmed in small capsule experiments. The development of two-dimensional integrated codes during the last 5 years has addressed issues of time-dependent long-wavelength asymmetry.

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. The NIF baseline design has been changed to include beam smoothing, and experiments with all 10 NOVA beams smoothed are planned within the next 6 months to confirm the favorable effects of smoothing. The committee believes that the prognosis for these experiments is sufficiently favorable to justify proceeding with the NIF without delay.

Hohlraum physics issues are absent in the direct-drive approach, which nevertheless imposes more stringent requirements on beam balance and smoothness. Direct-drive experiments are currently being pursued on the 60-beam OMEGA facility and will also be explored with the 192-beam NIF.



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Review of the Department of Energy's Inertial Confinement Fusion Program: The National Ignition Facility 6 Findings and Conclusions THE NIF WOULD MAKE IMPORTANT CONTRIBUTIONS TOWARD THE STATED LONG-TERM GOALS OF THE SBSS PROGRAM 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. THE SCIENCE AND TECHNOLOGY HAVE PROGRESSED SUFFICIENTLY TO ALLOW THE NIF PROJECT TO PROCEED AS PLANNED In assessing the scientific and technical readiness of the NIF project, the committee attempted to balance the NIF's potential value against the risk inherent in the extrapolation from the present base of experimental and computational experience. The science supporting the NIF fusion objectives includes the physics of the imploding capsule and the physics of laser-hohlraum interactions. The issues in these two areas and the requirements for scientific readiness were delineated as milestones in the NOVA Technical Contract (NTC) of 1990, which was based on the point target design at that time. The fulfillment of almost all of the NTC's terms with surrogate targets that precede the current NIF design has led to substantial and encouraging progress in the understanding of both areas, although some issues have not yet been resolved completely. The NOVA experimental campaign failed to meet its objective of high-convergence implosions (HEP 5). The computational advances of the last 5 years have pinpointed the cause as lack of symmetry in the 10-beam NOVA system. The three-dimensional simulations that successfully describe the NOVA experiments predict that the 192-beam NIF system will have adequate symmetry for high-convergence implosions. This has been confirmed in small capsule experiments. The development of two-dimensional integrated codes during the last 5 years has addressed issues of time-dependent long-wavelength asymmetry. 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. The NIF baseline design has been changed to include beam smoothing, and experiments with all 10 NOVA beams smoothed are planned within the next 6 months to confirm the favorable effects of smoothing. The committee believes that the prognosis for these experiments is sufficiently favorable to justify proceeding with the NIF without delay. Hohlraum physics issues are absent in the direct-drive approach, which nevertheless imposes more stringent requirements on beam balance and smoothness. Direct-drive experiments are currently being pursued on the 60-beam OMEGA facility and will also be explored with the 192-beam NIF.

OCR for page 36
Review of the Department of Energy's Inertial Confinement Fusion Program: The National Ignition Facility 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 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. With regard to NIF project management, the committee finds that there is a competent, well-supported project team and a well-aligned project organization. The project has undergone a stringent Title I Review, and an Independent Cost Estimate review has verified the TEC cost estimates. Although the contingency currently provided is unusually tight for such a project, the availability of support from the ICF program during the construction period provides the margin to systems and components necessary for the NIF to the point that a low contingency might be acceptable. The operating budget is defined precisely, but in relatively narrow terms, and is consistent with LLNL's bottom-up analysis. Additional target physics program funds need to be available in order to provide an operating budget comparable to those at other large facilities. In sum, 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, the plausibility of ignition estimates based on the experimental and modeling results and capabilities in hand, and the flexibility of the 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.