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When it comes to building lasers to trigger nuclear reactions, “the bigger, the better,” says Stefano Atzeni, a physicist at the University of Rome, Italy. Larger facilities can produce higher energies, which means that materials can be subjected to higher temperatures or pressures, or that larger quantities of materials can be tested. Expanding the boundaries of experiments is likely to give nuclear researchers more useful data.
In experiments, these lasers blast target materials into a high-energy state of matter known as plasma. In gases, solids, and liquids, electrons are usually tightly locked in the nuclei of their atoms, but they roam freely in the plasma. Plasma emits electromagnetic radiation, such as flashes of light and x-rays, and particles such as electrons and neutrons. So lasers also need detection equipment that can record when and where these events occur. These measurements then allow the scientists to extrapolate how an entire warhead would behave.
So far Russia’s lack of such a laser has not been a major flaw in ensuring its weapons work. That’s because Russia is committed to constantly reshaping plutonium “pits,” the explosive cores found in many nuclear weapons, named after the hard centers of fruits like peaches. If you can easily replace old blast craters with new ones, there will be less need to use a laser to check how much they have deteriorated over the years. “In the United States, we’ll also be re-manufacturing our nuclear weapons, except we don’t have the capacity to produce large numbers of craters,” Lewis says. America’s largest production facility, in Rocky Flats, Colorado, closed in 1992.
Researchers have used lasers in nuclear weapons testing since at least the 1970s. First they combined them with underground tests of actual weapons, using data from both to build theoretical models of how plasmas would behave. But after the United States halted live tests of nuclear weapons in 1992 as it sought agreement on the Comprehensive Nuclear-Test-Ban Treaty, it turned to “science-based stockpile supervision”—that is, using supercomputer simulations of detonating warheads to assess their safety and reliability. .
But the United States and other countries taking this approach still needed to physically test some nuclear materials, using lasers, to ensure that their models and simulations matched reality and that their nuclear weapons were off. And they still need to do it today.
These systems are not perfect. “The models they use to predict gun behavior are not entirely predictive,” Atzeni says. There are different reasons why. The first is that it is very difficult to simulate plasma. Another is that plutonium is an exotic metal, unlike any other element. Unusually, when heated, plutonium changes through six solid forms before melting. In each shape, its atoms occupy a completely different volume than the previous one.
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