DARHT A Critical Component of Stockpile Stewardship

The weapons programs at Los Alamos have one principal mission: ensure the safety, security, and effectiveness of nuclear weapons in our nation's enduring stockpile. One critical component of this mission is DARHT, the Dual-Axis Radiographic Hydrodynamic Test facility.

DARHT consists of two large x-ray machines that produce freeze-frame radiographs (high-powered x-ray images) of materials that implode at speeds greater than 10,000 miles an hour. Such radiographs help scientists ensure that weapons in the stockpile are safe and effective and that—if ever necessary—they will perform as designed.

Ten years ago, Los Alamos personnel brought DARHT's first axis online. And in December 2009, after surmounting technical challenges, DARHT achieved its first dual-axis, multiframe hydrodynamic test (hydrotest), following a preliminary test in November. The facility has now completed three successful two-axis multiframe hydrotests. Data from the December experiment provides the technical information needed to close a Significant Finding Investigation (SFI) on the W78 warhead.

With DARHT fully operational and producing successful dual-axis hydrotests, scientists can now study full-scale mockups of how nuclear primaries explode.

"You could say that in fiscal year 2010 we finally put the 'D' in DARHT," said Mary Hockaday, LANL's Deputy Associate Director for Weapons.

Overcoming Complex Technical Challenges

Although the first axis of DARHT functioned well for more than 10 years, the second axis experienced monumental challenges. The challenges arose essentially because the second axis was an operational prototype of the world's longest pulsed-electron linear accelerator.

One challenge consisted of fitting the accelerator prototype into the DARHT facility. To reduce the accelerator's size, designers and engineers turned to a novel material, Metglas, which possessed an exceptionally high magnetic susceptibility. By using this novel material, designers fabricated an accelerator five times smaller than the original prototype.

Other challenges included designing a cathode injector system to supply enough electrical current to the accelerator and developing a target robust enough to survive four pulses from the second axis' extremely high energy electron beam. The injector system produces electrons that are drawn from the cathode and are accelerated through a potential of several million electron volts. A magnetic field guides the electrons into the accelerator.

The first-axis injector uses a five-centimeter cathode covered with velvet cloth. Velvet is used because the whiskers perpendicular to the cloth's surface are favorable to electron emission. This simple cathode is only suitable for beams that are shorter than 100 nanoseconds.
DARHT's second axis requires a more complex cathode because its pulse durations last 1,600 nanoseconds—27 times longer than the pulse in the first axis. The injector for this axis uses a 16.5-centimeter-diameter thermionic cathode. This larger-diameter thermionic cathode is maintained typically at 1,150°C, with electrons literally boiled off the cathode's surface.

The technical demands of the second axis are unique. For example, the axis is designed to produce a 17-million-volt electron beam that lasts for 1.5 millionth of a second. The beam is sliced into four pulses, each of which lasts less than 100 billionths of a second, to create four x-ray pulses. Making multiple x-ray pulses from a single-pulse induction accelerator requires creating a single electron pulse of approximately 1,600 nanoseconds. The x-ray dose from just one DARHT pulse is as high as 1,000 roentgens—the equivalent of 60,000 chest x-rays.

The resultant x-rays create images of materials with densities that exceed those at the center of the Earth. Moreover, one of the pulses from the second axis can be synchronized with that of the first axis, so that three-dimensional information can be reconstructed.

Maintenance Worth the Investment

DARHT was shut down this summer to complete a months-long maintenance and upgrade project. As part of this maintenance, workers refurbished the second axis at the 88-stage Marx bank, which consists of a series of large capacitors that can be charged in parallel but can be discharged in series, thus yielding extremely high voltage pulses.

DARHT's accelerator and execution control rooms also underwent extensive upgrades. Workers built a fiber-optic timing and firing communication system, which greatly improved reliability. In addition, the facility incorporated energy-efficient, high-definition video monitors.

"The recent technical upgrades and facility maintenance at DARHT were important investments to NNSA's infrastructure to help solve tough national challenges," said Don Cook, NNSA's Deputy Administrator for Defense Programs. "We congratulate the Los Alamos lab for a job well done."

2010 Yields Three More Successful Hydrotests

In July 2010, DARHT completed a successful two-axis, multiframe hydrotest. This data will allow LANL to close another SFI. Two additional successful tests—one of which was designed by Lawrence Livermore National Laboratory—were performed this year.

"Once again, the people supporting DARHT have demonstrated their exceptional professionalism in facility operations," noted Charles McMillan, Principal Associate Director for Weapons Programs at Los Alamos. "Their focus continues to be the delivery of high-quality data, effectively dealing with complex technical questions as they arise and consistently seeking ways to advance the scientific and technical capability at DARHT."

During a hydrotest, scientists detonate a "mockup" of a pit, the primary stage of a nuclear weapons system. The mockup consists of actual high explosives and most other ingredients, except for plutonium. Instead of plutonium, scientists use a nonfissile substitute material that has similar, weight, density, and other metallurgical properties so that it behaves much like the plutonium. The one thing the mockup does not do is produce a nuclear explosion when detonated.

DARHT's accelerator control room

DARHT's accelerator control room.

Hydrotest Results and Application

With both beams now operational, DARHT can take four sequential radiographs on one axis and one radiograph along a perpendicular axis, providing the first-ever simultaneous views of an implosion from two directions. The exposure time of such radiographs—60 billionths of a second—freezes the action of an imploding mockup to much less than a millimeter.

To do this, DARHT orients the two linear-induction accelerators at right angles to one another. A linear-induction accelerator uses magnetic cores to enable better coupling of electrostatic fields, thus accelerating electrons or other particles to extremely high energies. At DARHT, such electron beams are focused on a metal target. As the high-energy electrons hit the target, the electrons are deflected, converting the beam's kinetic energy to powerful x-rays.

Along with other tools, such as advanced laser interferometers and electronic position indicators, DARHT produces data sets during these hydrotests that are used to verify computer codes for nuclear weapons. The data sets of full-scale implosions are compared to simulations derived from the computer codes.

Current and Future Camera Technology

Capturing such transient events requires special camera systems. DARHT's current camera system converts x-rays to visible light with a scintillator (a material that exhibits the property of luminescence when excited by ionizing radiation). The second-axis scintillator is made from a dense, exotic manmade crystal known as bismuth germinate, which is highly sensitive to x-rays. To prevent lateral blurring of light, the scintillator is made up of 137,000 one-millimeter square rods, each covered with a metal sleeve. For the second axis, this material exhibits a rapid (50 nanoseconds) decay between flashes, thus ensuring that light from one image does not corrupt succeeding frames in the sequence.

Rendering of DARHT's two axes focused on a central containment system.

Rendering of DARHT's two axes focused on a central containment system.

The resultant light from a scintillator is recorded on astronomy-grade charged-couple devices (CCDs). CCDs are the most sensitive optical-recording devices available. The DARHT camera systems (one on each axis) are 100 times more sensitive than film and 40 times more efficient at absorbing x-rays.

To obtain multiple images with the second axis, scientists use a unique CCD architecture that records four images at a rate of two million frames per second. Because there is insufficient time to transfer data off the chip at this high frame rate, the information for each frame must be stored locally on each pixel and then slowly read off after the explosive experiment comes to an end.

In the future, DARHT may take advantage of a new type of camera, known as the Movies of eXtreme Imaging Events, or MOXIE. Known as the world's fastest camera, MOXIE can take pictures closely spaced in time (much like a motion-picture camera), which enable researchers to "see" into the unseen, imaging transient events from start to finish. MOXIE's x-ray images could allow scientists to measure quantities such as velocity, acceleration, and voriticity. This year, MOXIE's inventors received an R&D 100 Award, which R&D Magazine gives to the world's 100 most technologically significant new products each year.

Bolstering the Nuclear Deterrent of the United States

The DARHT program has proposed to increase the rate of hydrotests in the coming year. DARHT personnel also plan to improve its multi-image capability. Such improvement could potentially increase the number of radiographs possible with each test.

The results of these studies will help improve and verify computer models, which in the absence of actual nuclear testing are critical in assessing the effects of aging and remanufactured nuclear weapons.

DARHT and its data-rich radiographs supply real-world validation for the codes and thus effectively enhance the confidence and credibility of stewardship efforts designed to ensure the national security of the United States.

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