After nearly 40 years of service and 4,726 firings, Sandia National Laboratories has completed the first phase of a top-to-bottom refresh of its Saturn accelerator, one of the nation's most intense hard X-ray generators. The machine, which began operating in 1987, was shut down from January to October 2024 and has since been fine-tuned to bring its new X-ray source to full performance.

Saturn produces intense, hard X-rays primarily to test and qualify national security electronics, subsystems and materials against X-ray radiation. Its reproducibility had degraded over decades of use, according to Ben Ulmen, who led development of the reengineered X-ray source — a direct concern for any program relying on the facility for radiation qualification data.

"The goal was to figure out what needed to be right, where the big knobs are, to improve the reliability and performance of Saturn," said Mark Savage, an electrical engineer and technical lead for the refurbishment. "A lot of things have to go right for a really good shot on any big machine. It doesn't take many things going wrong to affect it seriously."

Synchronization at the nanosecond scale

From above, Saturn resembles a wagon wheel with 36 spokes feeding a center hub; the machine would just fit within two side-by-side college basketball courts. The hub is a 6-foot-diameter vacuum chamber where the X-rays are generated. Each spoke stores electricity and releases it in a fraction of a second, and shot-to-shot reproducibility depends on how well the 36 spokes synchronize — a variance of one nanosecond matters.

The refresh replaced Saturn's gas switches, the components that synchronize the spokes. "The gas switches were replaced because we wanted to improve the accuracy of their timing," said Savage, who designed the new switches. "The gas switches were a hard problem because they're switching millions of volts and hundreds of kiloamps with nanosecond precision."

The redesign also changes maintenance workflow. Previously, divers had to remove a switch from the water tank to adjust its timing, a process that slowed troubleshooting significantly. Timing can now be adjusted in place. The new switches were also designed to last longer and to handle higher voltages, giving operators headroom if they decide to raise Saturn's operating voltage in the future — a projection, not a committed upgrade.

A new core

Sandia replaced Saturn's hub — the vacuum insulator stack — to improve diagnostic monitoring of energy flow toward the center. According to Savage, this helps operators diagnose what happened when a shot fails to produce the expected X-ray output, shortening the feedback loop between an anomalous result and its cause.

The magnetically insulated transmission lines, which carry millions of volts from the spokes into the vacuum-insulated hub and X-ray source, were replaced as well. The originals were flexibly mounted — reasonable for a first-of-its-kind machine — but hard to install and align consistently. The new lines are less adjustable, which makes alignment easier and improves performance, and they feature better connections between components to reduce electricity-wasting arcing.

The updated design borrows from the transmission lines on the Z-machine, the world's most powerful pulsed-power machine and a descendant of Saturn. Before any parts were ordered, Sandia physicists Peggy Christenson and David Sirajuddin ran computer modeling of power flow through the new system.

Supply chain management proved to be its own engineering problem. The team worked with specialized machine shops across the country to fabricate one-of-a-kind parts for the core section and other redesigned components.

"We had some early issues where we weren't receiving exactly what we needed, so we ended up putting a team together to travel out to the machine shops and catch issues before the parts were shipped," said Alex Nash, a technologist involved in both phases of the refurbishment.

Reengineering the X-ray source

The new X-ray source addresses the consistency drift directly. It includes design elements that let technicians partially preassemble it on a benchtop rather than overhead inside the machine, and it is easier to align — changes that reduce turnaround time between shots.

The source itself is a vacuum diode with an anode, a cathode and a small vacuum gap. When the high-voltage pulse from the spokes reaches the diode, it arcs across the gap and strikes a thin metal foil that converts the electricity into X-rays.

"Saturn works a lot like a dental X-ray, except it is billions of times more powerful and its x-ray pulse is about a million times faster," Ulmen said.

Phase two already in planning

Planning and design for the second phase is underway. That work targets the triggering system for the Marx bank generators — the electricity-storing parts of each spoke that produce the high-voltage burst — to push timing accuracy further. Phase two will also redesign five systems on each spoke to compress and conduct the electrical pulse better. Savage expects that phase to be completed in the next three to four years.

"This was a huge project," said Jeff Tunell, the team lead for Saturn operations who was involved in the refurbishment. "It took many people from different organizations to make this work. Everyone coming together to solve problems, modify parts, put them into the machine and then prove it could work as designed was a huge accomplishment."