DARHT: Dual-Axis Radiographic Hydrodynamic Test Facility; Performing Nonnuclear Experiments to Help Ensure the Safety, Reliability, and Performance of the Nuclear Weapons in the US Stockpile [LALP 99-1414]

20 pages plus covers. Illustrations (color). Diagrams. Definitions. Cover has slight wear and soiling. 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. The project became an important priority after the United States stopped testing nuclear weapons in 1992. 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. During a weapon's crucial triggering phase, explosive charges that surround the nuclear fuel are detonated at multiple points. The result is a shock wave that moves inward (implosion) at supersonic speeds, compressing the fuel to higher and higher density. Implosion ends when the fuel reaches a supercritical density, the density at which nuclear reactions in the fuel 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 or lead) 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 extreme implosion forces materials tend to behave like fluids, so this mock implosion is called a hydrodynamic test, or hydrotest. 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. An electron beam moving at near the speed of light is smashed into a tungsten target. The electrons are yanked off course by the strong electrostatic pull of the positively charged nuclei in the tungsten atoms, and their sudden change in direction causes them to give off energy in the form of high-energy x-rays. Scientists already knew how to use a short burst (pulse) of high-energy electrons (rather than a continuous beam) to make a short pulse of high-energy x-rays. The new challenge was for the accelerator to deliver a very large number of electrons in an extremely powerful pulse to generate an x-ray flash that can penetrate the mockup during the ultra-dense implosion. Specifications call for a pulse of 100 billionths of a second, about a million times shorter than exposures achieved with a high-end conventional camera. As with the hole in a pinhole camera, the smaller the beam spot, the more point-like the area producing x-rays, and the sharper the resulting image. Each electron accelerator consists of a long row of doughnut-shaped magnetic induction cells, each connected to a high-voltage generator. There are 74 in total in each accelerator but not all may be used. At the instant of firing, each generator discharges its power, creating a pulse of electric current through its induction cell, which in turn creates a large voltage difference across the gap separating that cell from its neighbor. The electron beam-pulse travels through the central bore of the cells, receiving a 200 keV energy kick each time it passes though a gap. One of the design problems was designing the new induction c.…