The Dual-Axis Radiographic Hydrodynamic Test Facility (DARHT) is a facility at

Los Alamos National Laboratory Los Alamos National Laboratory (often shortened as Los Alamos and LANL) is one of the sixteen research and development laboratories of the United States Department of Energy (DOE), located a short distance northwest of Santa Fe, New Mexico, ...

which is part of the

Department of Energy A Ministry of Energy or Department of Energy is a government department in some countries that typically oversees the production of fuel and electricity; in the United States, however, it manages nuclear weapons development and conducts energy-re ...

stockpile stewardship Stockpile stewardship refers to the United States program of reliability testing and maintenance of its nuclear weapons without the use of nuclear testing. Because no new nuclear weapons have been developed by the United States since 1992, even ...

program. It uses two large

X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

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".
This article incorporates material from LANL: © Copyright 2010 Los Alamos National Security, LLC All rights reserved. ''Copyright Notice''. Unless otherwise indicated, this information has been authored by an employee or employees of the Los Alamos National Security, LLC (LANS), operator of the Los Alamos National Laboratory under Contract No. DE-AC52-06NA25396 with the U.S. Department of Energy. The U.S. Government has rights to use, reproduce, and distribute this information. The public may copy and use this information without charge, provided that this Notice and any statement of authorship are reproduced on all copies 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.

History

Planning for DARHT began in the early 1980s. Based upon the success of Livermore's FXR, induction-linac facility, in 1987 Los Alamos chose the same type of accelerator to replace PHERMEX, an RF accelerator commissioned in 1963. 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 (initially to be a twin of the first) in 1997. This plan was changed when 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. Construction was halted between 1995 and 1996 due to lawsuits by Los Alamos Study Group and Concerned Citizens for Nuclear Safety, two anti-nuclear weapons organizations demanding that the laboratory produce an Environmental Impact Statement for its construction and operation. Activists argued that DARHT is in violation of the

Comprehensive Test Ban Treaty The Comprehensive Nuclear-Test-Ban Treaty (CTBT) is a multilateral treaty to ban nuclear weapons test explosions and any other nuclear explosions, for both civilian and military purposes, in all environments. It was adopted by the United Nat ...

and potentially the

Nuclear Non-Proliferation Treaty The Treaty on the Non-Proliferation of Nuclear Weapons, commonly known as the Non-Proliferation Treaty or NPT, is an international treaty whose objective is to prevent the spread of nuclear weapons and weapons technology, to promote cooperation ...

, though the lab and the DOE reject this view. When completed in 1999, the first-axis accelerator produced a 60ns electron pulse with a current of 2 kA and an energy of 20 MeV focused to 1mm diameter spot on the target - 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 FXR facility. The second machine (second axis) is more complicated and, when first completed in 2003, was found to be unusable due to electrical breakdown. The origin of the electrical breakdown turned out to be unexpectedly high electric fields between the high-voltage plate and the oil-insulated magnetic cores and at sites where metal, high-voltage insulator, and vacuum meet inside the cells. After much analysis, the design error was tracked to faulty equipment used when doing voltage calibrations. An extensive design overhaul and rebuild was required, which was completed in 2008. The project was initially expected to cost $30 million in 1988, but costs ultimately rose to $350 million by 2008 when the facility became completely operational.

Description

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 Depleted uranium (DU; also referred to in the past as Q-metal, depletalloy or D-38) is uranium with a lower content of the fissile isotope than natural uranium.: "Depleted uranium possesses only 60% of the radioactivity of natural uranium, hav ...

lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cut, ...

) 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 An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nbs ...

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 in a process called

bremsstrahlung ''Bremsstrahlung'' (), from "to brake" and "radiation"; i.e., "braking radiation" or "deceleration radiation", is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typical ...

. 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 recorded on conventional x-Ray films. 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 width of 60 billionths of a second. 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 Kev can refer to: Given name * Kev Adams, French comedian, actor, screenwriter and film producer born Kevin Smadja in 1991 * Kevin Kev Carmody (born 1946), Indigenous Australian singer-songwriter * Kev Coghlan (born 1988), Scottish Grand Prix moto ...

energy kick each time it passes through a gap. One of the design problems was designing the new induction cores to fit in the confines of the previous facility. The design team had to replace the ferrite used in the first axis cores with "

metglas Metglas is a thin amorphous metal alloy ribbon produced by using rapid solidification process of approximately . This rapid solidification creates unique ferromagnetic properties that allows the ribbon to be magnetized and de-magnetized quickly an ...

"—paper-thin ribbons of amorphous iron tape. The maximum magnetic field strength (saturation point) in metglas is five times higher than in ferrite. The magnetic tape was insulated by thin layers of

mylar BoPET (biaxially-oriented polyethylene terephthalate) is a polyester film made from stretched polyethylene terephthalate (PET) and is used for its high tensile strength, chemical and dimensional stability, transparency, reflectivity, gas and a ...

and wound up into a roll of 20,000 turns to make mammoth six-foot-diameter cores, each four inches wide and weighing more than one and a half tons. Four cores fit into each induction cell. Perhaps the most significant technical advance achieved at the DARHT facility is the high-speed camerahttps://permalink.lanl.gov/object/tr?what=info:lanl-repo/lareport/LA-UR-03-4985 used to image x-rays on the second axis. This camera utilizes the world's largest LSO crystal array to convert the x-rays into visible light which are then imaged with the world's highest speed CCD camera (developed jointly by MIT and Los Alamos) at more than two million frames per second. This scintillation camera is further augmented by a large anti-scatter (“Bucky”) grid to improve image contrast. The unique combination of diagnostics solves technical surveillance problems that have persisted since the Manhattan Project enabling the United States to assert higher confidence in its nuclear stockpile performance and safety margins without the need for nuclear testing.

Images

File:DARHTaxis2.jpg, DARHT electron accelerator File:Darht accelerator.jpg, A technician examining one of the refurbished accelerator cells for DARHT's second-axis accelerator.

References

{{coord, 35, 50, 02, N, 106, 18, 09, W, display=title Nuclear weapons infrastructure of the United States Nuclear stockpile stewardship Los Alamos National Laboratory