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Lab gets second beam going

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By Roger Snodgrass

Under construction for 20 years, a major scientific facility at Los Alamos National Laboratory is living up to its name.The Dual Axis Radiographic Hydrodynamic Test Facility (DARHT) has operated its first axis of one electron accelerator beam since 1999. This week, DARHT won approval to begin operating with its second linear accelerator, achieving dual axis status at last.“We’ll be doing our first dual-axis hydro test this summer,” said Ray Scarpetti, the DARHT Axis Two project director. “We’ve been generating an intense beam, very reliably and very repeatably, but we were not authorized to do X-rays.”DARHT was intended to be a key ingredient in assuring the reliability of stockpiled nuclear weapons in the absence of testing, but with only one wing it has played only a minor role until now.The laboratory announced Tuesday that test firings of the second axis electron beam could begin as early as this week, scaling up to a full-blown test of the two beams performing in concert within months.DARHT uses accelerated electrons to produce high energy X-rays that can peer inside a mock nuclear implosion.The second axis produces a 17-million-volt electron beam for 1.5 millionth of a second, according to the laboratory press release. An electromagnetic kicker slices out four separate pulses of the beam, less than 100 billionths of a second each.“These X-rays then create images of material moving at 10,000 miles per hour with densities that exceed those at the center of the earth,” the announcement stated.The tests create such high temperatures that the components melt into liquids, which is why they are called “hydro tests.”The second beam enables the complex machinery to take high-speed mini-movies (a few frames of movement), as well as microscopic and stereoscopic images of a flash in the flow of a powerful implosion.The announcement marks another stage in the comeback of a project that was nearly written off as a “botched debacle,” as years went by and multiples of its original budget continued to mount.“We finally put to bed all those problems,” Scarpetti said. “LANL realized that they had to step up and get the job done.”With help from sister laboratories Berkeley and Lawrence Livermore and outside industry, he said, a lot of hurdles and challenges were overcome.“Some of it wasn’t clear that we could overcome,” he said, calling it a construction project with a research and development component.A story in the laboratory’s science magazine, 1663, described the comeback as a “heroic effort.”DARHT was originally conceived to provide two individual pulses of X-rays. Twin buildings were built on that assumption. But by the time work began on the second axis, the Department of Energy had changed the scope to the longer, four-pulse concept, which required an accelerator “four to five times longer than the space planned for,” the 1663 article recounted.To solve that problem and a subsequent discovery that the acceleration modules blew out when the required voltage was applied, major revisions had to be made.In October 2006, the Jasons, highly specialized consultants on defense and nuclear issues, delivered a review that gave DARHT’s come back a strong endorsement, with only a few remaining reservations.“We find that LANL has done a good job of analyzing and understanding the problems, improving the cell design and rebuilding the modules, and has instituted a good test program,” the study team noted in their unclassified report.As the program resumes, each test to be documented will be contained inside a steel vessel, unlike earlier tests that were staged behind protective barriers in the open air.“We’re looking at doing 12 shots per year,” said Scarpetti, noting that the shots would be cleaner. The vessel protects the expensive camera equipment and makes the tests cleaner.“You can’t hear them or feel them or see them,” he said.