The Dual-Axis Radiographic Hydrodynamic Test facility

Powerful accelerators help ensure national security.

By Jill Gibson | April 28, 2023

Dahrt Feature Opt
The individual induction cells that form the DARHT Axis 2 accelerator weigh 17,000 pounds each. Los Alamos National Laboratory

The soaring ponderosa pines, expansive views, towering mountain ranges, and dramatic canyons of northern New Mexico offer many opportunities for breathtaking photographs. Tucked into that scenery is the sprawling campus of Los Alamos National Laboratory, where scientists are taking breathtaking pictures of an entirely different sort.

The Lab’s Dual Axis Radiographic Hydrodynamic Test (DARHT) facility is used to take high-speed images of mock nuclear devices and other materials as they implode. Howard Bender, a group leader in the Lab’s Integrated Weapons Experiments Division, says the facility’s two massive electron accelerators are used to produce critical data—in the form of radiographs—that help ensure the safety, security, and effectiveness of the United States’ nuclear deterrent. “When it comes to nuclear weapons, scientists have constant questions regarding the safety, materials, aging, new designs, and other aspects of the various systems,” Bender says. “DARHT helps us answer these questions.”

Currently, scientists are using DARHT to carry out hydrodynamic experiments on plutonium surrogates—materials that behave similarly to plutonium but do not create a nuclear reaction. These tests provide essential experimental data to constrain computational codes for nuclear weapons, which is a critical component of maintaining the nuclear weapons stockpile. These tests also provide useful data for experiments at the Nevada National Security Site, which is the only place where subcritical experiments using actual plutonium can take place.

Dahrt Body 2
A confinement vessel is lowered into place at DARHT. To see a replica vessel, visit the Bradbury Science Museum in downtown Los Alamos.

DARHT’s two accelerators, located in above-ground concrete bunkers, intersect at a right angle. Each bunker stretches about the length of a football field. During an experiment, each accelerator generates a stream of electrons that travel down the length of the machine, gaining energy along the way. In the Axis 1 accelerator, the electrons reach a kinetic energy of 20 million electron volts and are moving close to the speed of light when they hit a heavy metal target that converts them into a pulse of high-powered x-rays. Simultaneously, a mock nuclear weapon is detonated inside a steel confinement vessel just beyond the accelerator. A high-tech camera concurrently captures an image—a radiograph—that shows what’s happening inside the confinement vessel. The radiograph looks like something you’d see at the dentist’s office, but instead of molars, the image shows a cloud of weapons parts and materials moving more than 2.5 miles per second.

The Axis 2 accelerator works similarly, but it creates a longer stream of electrons that are sliced into four shorter pulses. Each pulse is converted into x-rays, and a camera captures four images in rapid succession. Bender says there is a slight delay in between the ring of the test and the time the images appear on the computer screens in the facility’s control room. “Until the images pop up, we are on pins and needles,” he says.

DARHT is the only linear accelerator facility in the nation that uses a dual-axis approach to generate images. The single image from Axis 1 can be combined with any of the four images from Axis 2 to create dual-angle digital images that can be used to generate three-dimensional computer simulations. Both the images and the simulations provide a unique view of what happens to mock nuclear weapons as they detonate. Bender says a long-term goal for DARHT is to produce radiographs from additional angles, which will improve 3D modeling and continue to advance scientists’ understanding of weapons implosion. “We are never done with science at Los Alamos,” he says.

Since 1999, when Axis 1 was completed, scientists have run 77 full experiments and more than 70,000 reliability and preparation tests. “It can take up to three years and millions of dollars to design and fabricate a device for a test,” says Bender, noting that after all the preparation work, the accelerators must perform perfectly to capture radiographs. “We get one shot at a specific experiment. One shot and that’s it.” The reliability and repeatability of each of the thousands of engineering subsystems that make up the accelerators are crucial to achieving the goals of each experiment.

To ensure consistent and predictable performance, the facility requires constant upkeep and upgrading. In 2020, a weather enclosure was built around the firing point (the area where detonations occur), creating a controlled environment for experiments. The enclosure also shields the facility’s cameras and other diagnostic equipment from poor weather conditions.

Bender says that although the enclosure contributes to the facility’s success, the key to DARHT’s performance is the people. “Everybody here is really good at what they do—tremendous experts,” he says. “The engineers, scientists, technicians, operators, diagnostics people, data specialists—everyone is here to make this work, and it works because they do a phenomenal job.” ★