A day at DARHT

On the afternoon of Monday, May 21, the skies opened up and nearly an inch of rain—and in some places, hail—pounded the dry and dusty Pajarito Plateau. Amid thunderclaps and lightning strikes, the parched earth soaked up the first precipitation in 55 days, the first of the summer monsoons that might curb the chronic drought conditions plaguing northern New Mexico.

In the middle of the plateau, on a narrow mesa pinched between Water Canyon and Cañon de Valle, operations at Los Alamos National Laboratory’s Dual-Axis Radiographic Hydrodynamic Test (DARHT) facility ground to a halt. After a productive day and a half of setting up an experiment—during which time a crane lowered a high-explosive test device into a spherical confinement vessel on an exposed concrete pad—the facility operations manager deemed the weather too dangerous to continue preparations.

And so the roughly $7.5 million experiment was postponed until Thursday, May 24.

At 8 a.m. on that Thursday, 58 people—50 men and 8 women—gathered in the DARHT accelerator control room for roll call. Nearly all of them wore jeans with sneakers or hiking boots. A few wore tropical-print Hawaiian shirts, and a few more wore suspenders—the unofficial dress code for the day. Some stood and sipped coffee while others sat in rolling office chairs and ate breakfast burritos. Along the room’s perimeter, 73 screens lit up with numbers and graphs and live feeds of the facility’s firing point.

Terry Priestley, the operations manager, stood in the south corner and called each person’s name before explaining that from this point forward, no one would be allowed to leave the building. Then he turned to Omar Wooten, a short bespectacled physicist who looks like he graduated college much more recently than 2000.

“Omar,” Priestley said, “Can you please tell us why we are here today?”

THE B61 BACKSTORY

Wooten’s answer—to take radiographs of a hot B61—has a complicated backstory that most people in the room already knew.

In 1963, Los Alamos designed and engineered the B61, a thermonuclear gravity bomb that can be dropped by a plane at high speeds from as low as 50 feet. Most B61s were produced in the 1970s with a life expectancy of 10 years. Decades later, however, the B61 is still part of America’s nuclear weapons stockpile.

The B61 is currently undergoing a life-extension program (LEP) at Los Alamos in partnership with other nuclear weapons laboratories to convert four versions of the bomb (models B61-3, -4, -7, and -10) into a single, updated version: the B61-12. By refurbishing key components through a combination of reuse, redesign, and remanufacturing, the LEP will help ensure that the B61 remains a safe, secure, and effective part of the stockpile until at least 2040.

To explain why the LEP is necessary, Wooten, the lead physicist on this particular DARHT experiment, compares the B61 to a 1964 Ford Mustang. “If you were to go to Ford to get new parts for your antique car, you’d have a problem,” Wooten says. “Those parts don’t exist anymore. Materials have changed. Rules about emissions and safety have also changed—what was OK in the ’60s isn’t up to current standards.”

The same is true for the B61. Like parts of an antique car, weapons components degrade with age. Metals corrode and fatigue, plastics become brittle and crack, rubber dries out and crumbles, and adhesives no longer bond. But these weapons are still expected to work, and it’s the job of Los Alamos scientists to make sure each part of the weapon functions appropriately. “If we are going to change something about the B61, we need to make sure it performs correctly,” Wooten explains. “We are trying to characterize how the system performs as we introduce these new components.”

But how can scientists be certain that a refurbished weapon works as well as the original? The obvious answer is by testing the weapon, but the Comprehensive Nuclear Test-Ban Treaty has prohibited nuclear testing since 1992. (Although the United States did not ratify the treaty, President Bill Clinton signed the treaty, and America has not tested a nuclear weapon for more than 26 years.) In lieu of testing, Los Alamos and other nuclear weapons laboratories developed the science-based Stockpile Stewardship Program, which combines scientific and experimental capabilities with high-performance supercomputing simulations. These simulations, however, are only as good as the data that go into them. These data come from a variety of experiments, including the experiments performed at DARHT.

SOME LIKE IT HOT

DARHT is only a seven-mile drive from the Laboratory’s main technical area, but those seven miles encompass two security checkpoints, an elk-filled ponderosa pine forest, and a view from the DARHT parking lot that stretches east across the entire Rio Grande Valley to the peaks of the Sangre de Cristo mountains some 30 miles away. In other words, the facility feels quite remote.

That feeling doesn’t go away inside DARHT. The building itself is an explosives bunker: thick walls, no windows, no personal cell phones. On the day of an experiment, once you’re in, you’re in. Most of the people who arrived there early on the morning of May 24 were there for more than 10 hours.

Which is why there was so much food. Chips, salsa, hummus, popcorn, cheesecake, and cupcakes covered three folding tables pushed together in the kitchenette area. Well before noon, someone started passing around Spam sushi, and by 2 p.m. a couple mechanical engineers were chopping ingredients for their much-anticipated mango guacamole.

The food was a necessary distraction from the holding pattern everyone at DARHT was caught in that day. This particular test was a “hot shot,” meaning that the test device was heated at three degrees a minute to 74 degrees Celsius (165 degrees Fahrenheit). That temperature then had to be maintained for 24 hours, which meant the experiment (aka the shot) wouldn’t happen until after 4:30 p.m. During a brief update that afternoon, Priestley reminded the crowd that this experiment had been in the works for 18 long months; a few more hours of waiting was nothing in the grand scheme of things.

While folks dove into the guacamole, Wooten explained the reasoning behind a hot shot. “Have you ever been on an airplane, sitting on the runway, during the summer?” he asked, alluding to the fact that the bombers and fighter planes that could deploy a B61 might stew on the tarmac in places such as Guam or Saudi Arabia. “This test is designed to measure any potential changes to the primary implosion resulting from LEP-designed components at the upper temperature extremes at which the B61 is required to perform.”

Scientists must also consider how heat might affect the weapon as its temperature increases—will it twist? Expand? To make sure the final radiographs account for any physical changes to the weapon’s shape and capture exactly what the scientists want them to capture, several dry runs— radiographs taken without an explosion—were performed as the team waited for the actual detonation.

“We have to guarantee performance across the temperature range,” says Wooten, noting that back in August 2017, a “cold shot” was performed at DARHT to assess how components of a B61 perform at minus 54 degrees Celsius (minus 65 Fahrenheit)—the approximate temperature at 30,000 feet above sea level and the height at which a B-52 bomber might fly with a B61 exposed on an underwing pylon.

HYDRO TEST 3682

By 4:30 p.m., the vessel had been “soaked” at the appropriate temperature for 24 hours and people began making final preparations for the experiment—and the radiographs and other data that would accompany it.

In a DARHT experiment, scientists detonate a mock nuclear weapon—essentially a weapon that, instead of having a (nuclear) plutonium pit at its core, contains a non-nuclear metal. The explosion that follows is not nuclear but can be used to understand how that weapon would work if armed with a real pit.

The heat and pressure created by the implosion cause the weapon’s mock non-nuclear pit to melt and flow like water. This change from solid metal to liquid is why the experiment is considered “hydrodynamic” and often called a “hydro test,” or more simply, a “hydro.” Each hydro is given a number; this hot shot of a mock B61 was hydro 3682.

At DARHT, multiple suites of diagnostic cables are attached to a weapon to gather data as it explodes, but the most important data come in the form of five radiographic (x-ray) images that are taken as the weapon detonates. The radiographs are used to better understand the implosion—specifically the implosion symmetry—and this understanding, in turn, influences the computer simulations that predict how well a real nuclear weapon will perform. “Image quality at DARHT is amazing in terms of contrast and resolution,” Wooten says. “Without underground testing, our codes need to be predictive, and experiments like 3682 help us continually improve our understanding and modeling of multiphysics phenomena.”

Turns out, taking radiographs during a detonation is a lot more complicated than taking radiographs at, say, the dentist’s office. For starters, the button at the dentist’s office isn’t labeled “fire,” and you don’t have to have a security clearance to see the images. And, rather than providing one image of a fixed target, DARHT must produce five high-resolution images of phenomena moving thousands of miles per hour. 

DARHT is the only facility in the world that can do this; in fact, DARHT is the world’s fastest and most powerful x-ray machine. To acquire high-resolution images of some of the densest metals known to humankind (that are even more compressed during an implosion) requires x-rays that are about 20,000 times more intense than a medical x-ray.

The DARHT facility comprises two perpendicular wings: Axis 1 and Axis 2. The wings stop just short of meeting one another at a right angle; at the would-be intersection, called the firing point, a six-foot spherical confinement vessel contains the mock nuclear weapon.

Each axis contains a massive linear accelerator that is focused on a tantalum target outside the confinement vessel. Each accelerator generates an intense high-energy beam of electrons that hits this target at nearly the speed of light. The electrons are yanked off course by the strong electrostatic pull of the positively charged nuclei in the tantalum atoms. Their sudden change in direction causes them to give off high-energy x-rays. The x-ray beams penetrate the confinement vessel, which holds the weapon.

AXIS 1

Axis 1 takes one radiograph per hydro test by producing one short electron pulse (60 nanoseconds, or billionths of a second) of extreme intensity(1.9 kiloamperes) with an energy of 19.5 megaelectronvolts. The beam is focused to a 1.3-millimeter-diameter spot on the 1-millimeter-thick tantalum target which, at the time of Axis1s construction in 1999, was the smallest spot size and shortest pulse length achieved at that intensity. As a result (and also because new electronic detector technology developed at Los Alamos replaced film), the image quality was much higher than that at any other hydro test facility, providing the clearest single views ever made of the inside of a hydro test object. The views helped validate new descriptions of implosion physics used in computer simulations of a weapons performance.

AXIS 2

Initially, Axis 2 was to be an exact replica of Axis 1, but when the moratorium on nuclear testing began in 1992, scientists realized the need for more images more data from each experiment. Los Alamos, Lawrence Livermore, and Lawrence Berkeley National Laboratories collaborated on a design that produces four images, making Axis 2 the only accelerator of its type in the world.

Fully assembled in 2008, Axis 2 produces four short electron pulses sliced from a 1,600-nanosecond-long beam of extreme intensity (2.1 kiloamperes) with an energy of 16.5 megaelectronvolts. The beam is focused to a 1.3-millimeter-diameter spot on the tantalum target to produce four x-ray pulses. The tantalum target for Axis 2 is thicker than that of Axis 1 because it has to withstand four pulses without debris and particles from the target upsetting the incoming beam and without being significantly eroded from pulse to pulse.

Depending on when scientists want to take radiographs, the four pulses are independently adjusted from 35 nanoseconds to more than 100 nanoseconds in duration. Information on the symmetry of the implosion is obtained when a pulse from Axis 2 is simultaneous with the pulse from Axis 1.

IMAGE CONTROL

To produce the highest-quality images, recorded x-rays must originate at the target x-ray spot and not come from other sources, such as scattered X-rays from the beam stop(which absorbs spent electrons after they exit the target). DARHT reduces these unwanted x-rays by using metal collimators that focus the x-ray beam between the target and the weapon.

Another type of collimator, called a Potter-Bucky grid, sits on the other side of the weapon, between the weapon and the detector (camera) system. Los Alamos pioneered these grids, which use 137,000 thin tubular lines of sight in a 40-centimeter-thick block of x-ray-absorbing material. These tubular lines point back to the x-ray target spot so that only rays from the target spot pass through. By rejecting 99 percent of scattered x-rays, the grid has significantly improved the contrast of DARHT radiographs. After passing through the target, x-rays are converted into visible light with a scintillator. The light is recorded on the most sensitive optical recording devices available: astronomy-grade charge-coupled devices that are cooled to reduce electronic noise.The DARHT camera systems are more than 100 times more sensitive than  lm and 40 times more efficient at absorbing x-rays.

On Axis 2, four images are recorded at a rate of 2 million frames per second. Because data cannot be transferred off the chip at this high rate, the information for each frame must be stored locally on each pixel and slowly read o after the experiment ends.

CONFINEMENT VESSEL

When DARHT was first built in 1999, test devices were detonated outside the bunker, producing fiery explosions that took weeks to clean up. In the early 2000s, growing concerns about environmental a contamination and the health impacts of materials such as beryllium and depleted uranium prompted scientists to consider enclosing explosions in confinement vessels. The first fully contained hydro was executed in 2007. More than 50 hydros have been executed in confinement vessels since then (DARHT averages six to eight hydro tests per year).

Today, all tests occur inside two-layer confinement vessels, which not only contain all hazardous waste but also reduce cleanup time at the firing site. The cylindrical outer vessel provides mechanical support to the spherical inner vessel, which is six-feet wide and made from 6.25-centimeter- thick steel. The inner vessel, which contains overlapping aluminum shielding plates around the device to protect the vessel from shrapnel damage, can handle up to 18 kilograms of explosives and can be cleaned up and reused for other hydro tests.

THE COUNTDOWN

As 4:30 approached, people gathered in the accelerator control room, huddled around monitors showing the live feed of the confinement vessel. "5, 4, 3, 2, 1," said a voice on the loudspeaker. In the adjacent test control room, Trevor Sanders, the DARHT Detection Chamber Operator, pressed the "fire" button. On the monitors, nothing happened a sign of a well-contained explosion.

Wooten and his colleagues examined the radiographs on screens in the test control room, and word began to spread around the facility that the test was a success. Scientists and analysts shook hands and patted one another on the back, saying "Congratulations, we nailed it!"

Less than an hour later, people were allowed to leave the building, and a line of cars slowly moved west toward the Jemez Mountains and back to civilization.

A few days after the test, Wooten confirmed that he was "very pleased" with the images and the 100 percent data return from all the diagnostics. Next steps involve looking for differences between images from this hydro and images from prior related shots. More quantitative detailed analyses will follow in the future, he says, noting that the data from this hydro will help validate codes used for computer simulations and also provide feedback on his pre-shot prediction and the choices he made as he developed the model for this experiment.

Wooten also confirmed that he "slept hard" in the nights following the test, which was the culmination of years of work. This being my first hydro test, the stress was largely self-imposed, he laughs. "As a new designer, being responsible for an experiment that is so expensive and that so many people across the Lab have worked on is daunting."

"It's also one of the most significant privileges one can have at this Laboratory, and I'm humbled to be able to play a role," he continues. "To get to work on something that will enter the stockpile, thereby helping to further continue the security of this great country it's really beyond words." ★