A new era of nuclear weapons policy, prohibiting testing and the development of new types of weapons, raises this challenge for the nation: ensuring that our current weapons stockpile remains safe, secure, and reliable into the indefinite future, regardless of aging. Confidence in the U.S. nuclear arsenal now must depend on our fundamental understanding of weapons science and technology, pursued without recourse to the detonation of full-scale nuclear devices.
Under the Department of Energy's Stockpile Stewardship Program, scientists at Lawrence Livermore are addressing the issues inherent in this challenge. They are developing advanced computer modeling and simulation to predict weapons performance and are establishing benchmarks against existing underground test data. However, the best gauge of this computer modeling capability is how well it predicts experimental results. Several experimental efforts are under way to enhance the accuracy of our computer modeling. They include advanced hydrotesting, subcritical experiments, and superlasers.
High-power lasers are unique in that they can produce energy density (energy per unit volume) approaching that of nuclear devices. The lasers can produce momentary, microscopic versions of some important aspects of nuclear detonations. The National Ignition Facility (NIF), now under construction at Livermore, will incorporate such a laser. Based on nearly 30 years of fusion laser development, NIF is being designed to deliver 1.8 million joules of energy with micrometer precision onto millimeter-size targets. (See the article entitled On Target: Designing for Ignition for a description of work on designing and fabricating fusion targets.) The intent is to produce thermonuclear burn that, for a few trillionths of a second, produces conditions found only in the center of the sun and in the core of a burning nuclear weapon. Achieving ignition outside a nuclear device will be a landmark achievement for stockpile stewardship.
More than four decades of dedicated work by weapons scientists has provided us with sophisticated scientific knowledge critical to understanding the design and performance of nuclear weapons. Fusion ignition is a complex physical process that will be a challenging integrated test of this knowledge and of our modeling and experimental capabilities. Before the actual ignition experiments, scientists will spend several years on simulations and preliminary experiments to calibrate the details of target physics that are impossible to model, even with modern supercomputers. Experiments preliminary to ignition can start early because the facility is being built so that some of the laser's 192 beams can be used to conduct target experimentation even as other parts are being constructed and installed. These preparatory results, along with decades of experimental data from smaller lasers and the powerful modeling and simulation capability developed by the Accelerated Strategic Computing Initiative-another arm of the Stockpile Stewardship Program-will be used to determine the final target and laser configuration used to drive ignition.
Once ignition is achieved, it can be used in several important ways. The Stockpile Stewardship Program can explore the physics of thermonuclear burn and its effects. The burning capsules can be used as sources of neutrons and x rays for studies on nuclear weapon effects. NIF also can be used for basic science applications and to develop nuclear fusion as an energy source.
NIF is the next step in laboratory thermonuclear fusion. It will provide a detailed, integrated test of our understanding of nuclear detonation processes. It will also further our understanding of the fundamental physics of nuclear weapons, thereby enhancing our ability to predict weapons performance and help to provide a sound basis for assuring the safety and reliability of the nuclear stockpile.


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