DARHT: Validating Weapons Performance without Nuclear Testing

12 pages, plus covers. Includes: illustrations (most in color), diagrams. Cover has slight wear and soiling. Report number LA-UR-08-04937. DARHT stands for Dual-Axis Radiographic Hydrodynamic Test. It is a major above ground experimental facility developed as part of the science foundation needed to enable the United States to continue certifying its nuclear weapons without underground nuclear weapons testing. During a weapon's triggering phase, explosive charges are detonated. The result is a shock wave, compressing the fuel to higher density. Implosion ends when the fuel reaches a supercritical density, the density at which nuclear reactions build up an uncontainable amount of energy, which is then released in a massive explosion. To make the mockup non-nuclear, a heavy metal surrogate (such as depleted uranium) stands in for the nuclear fuel, but all other components can be exact replicas. Sub-critical masses of plutonium may also be used. Under such implosion forces materials behave like fluids, so this mock implosion is called a hydrodynamic test. Standard practice is to take a single stop-action snapshot of the weapon mockup's interior as the molten components rush inward at thousands of meters per second. X-rays that can penetrate the heavy metal in a weapon mockup are made with an electron accelerator. The Dual-Axis Radiographic Hydrodynamic Test Facility (DARHT) is a facility at Los Alamos National Laboratory which is part of the Department of Energy's stockpile stewardship program. It uses two large X-ray machines to record three-dimensional interior images of materials. In most experiments, materials undergo hydrodynamic shock to simulate the implosion process in nuclear bombs and/or the effects of severe hydrodynamic stress. The tests are described as "full-scale mockups of the events that trigger the nuclear detonation". The powerful pulsed X-ray beams allow for an ultra-fast motion picture to be constructed showing the details of the process being studied in three dimensions. The tests are often compared with computer simulations to help improve the accuracy of the computer code. Such testing falls under the category of sub-critical testing. Planning for DARHT began in the early 1980s. The project became an important priority after the United States stopped testing nuclear weapons in 1992. Approval for an overhaul and new axis came in stages, with the first axis approved for construction in 1992 and the second axis in 1997. The Department of Energy decided it wanted the second axis to deliver not one view of the implosion, but a series of views in rapid succession. When completed in 1999, the first-axis accelerator could produce one short electron pulse lasting 60 ns with a current of 2 kA and an energy of 20 MeV. The beam could be focused to 2-millimeter diameter spot on the target. It was the smallest spot size and shortest pulse length ever achieved at that intensity. As a result, image quality was about three times higher than at Livermore's x-ray facility. The second machine (second axis) is more complicated and, when first completed in 2003, was found to be unusable due to electrical breakdown. An extensive design overhaul and rebuild was required, which was completed in 2008.…