For the first time since the Spallation Neutron Source (SNS) began operations in April 2006, civil construction is taking place there to upgrade the SNS’s linear accelerator, or linac, at the US Department of Energy’s (DOE’s) Oak Ridge National Laboratory (ORNL).

A planned seven-year project, the Proton Power Upgrade (PPU) will double the power of the linac’s proton beam, from 1.4 to 2.8 megawatts. This will permit a broader range of pulsed neutron beam experiments at the First Target Station (FTS) that complement the continuous neutron beam experiments at ORNL’s High Flux Isotope Reactor (HFIR). Eventually the upgraded linac will also power world-leading neutron research at the Second Target Station (STS) that will be built at the SNS site.

“The PPU is the next step in keeping the US at the forefront of the neutron sciences and in making discoveries that can lead to  new and improved processes and products for industry and consumers,” said John Galambos, project director for the PPU. “We’re extremely pleased to see the first stage of the PPU project under construction after years of planning and technology development.”

Specifically, the upgrade project will provide up to 2.0 of the 2.8 megawatts to the FTS (with the balance reserved to eventually power the STS). Delivering more neutrons for faster experiments will help accelerate the pace of scientific discovery across a wide range of materials and technologies—with the ability to study smaller and less-concentrated samples in more extreme environments.

Key PPU improvements include adding new superconducting cryomodules and supporting radiofrequency (RF) power systems, upgrading existing RF power systems, installing new injection and extraction magnets in the accumulator ring, and upgrading conventional accelerator components and equipment.

The project also includes installing at the FTS a 2.0 megawatt-capable mercury target that incorporates gas injection and jet flow technologies for improved performance and longer operating life. An extended outage of the accelerator and FTS is planned during 2023 to allow workers to perform the upgrades required in “rad” (radiation) areas, including adding a short section of new beamline, called the “stub,” that will later allow the STS to be connected to the accelerator beamline without interrupting operations at the FTS.

Currently in the development stages, the STS will meet the demands of the scientific community for a neutron source that complements ORNL’s FTS and HFIR (the highest flux, reactor-based source of neutrons for research in the United States) by filling key gaps in our nation’s materials research capabilities. STS will deliver intense, cold (longer wavelength) neutrons to as many as 22 additional, next-generation instruments that are optimized for exploring broader length and time scales.

The SNS linac operates by accelerating bunches of negatively charged hydrogen ions (each ion having two electrons and one proton) to nearly the speed of light using a series of oscillating electric fields along a 300-yard-long underground beamline.

After acceleration, the ions pass through a thin diamond foil that strips off their electrons, leaving just the protons. The protons are further bunched into discrete pulses in an accumulator ring before impacting 60 times per second on a steel target full of liquid mercury. Each proton in a pulse causes 20-30 neutrons to break away, or “spall,” from the mercury atoms.

Finally, the spalled neutrons are moderated to lower energies and then guided down individual beamlines to different types of experimental instruments where they penetrate sample materials and interact with their atoms. The interactions cause the neutrons to scatter in unique patterns that are recorded by digital detectors and studied by scientists to understand the atomic structures and activities of the materials.

For many years, the SNS has provided the most intense, pulsed accelerator-based neutron beams in the world. An even more powerful SNS linear accelerator will help ensure the US maintains this capability and enable our scientists to make new types of scientific discoveries that are essential to sustain our nation’s global leadership in the industrial, military, national security, medical, and consumer sectors, among many others.

Previous neutron research at ORNL has contributed to innovations and improvements in a wide range of technologies, including mobile phones, medicines and medical devices, automobile engines, batteries, spacecraft propulsion, transportation security, and aerospace components.

The overall PPU project is expected to be completed sometime in 2024, with an ongoing power ramp-up implemented before then as various stages of the upgrade are completed.

Together the FTS, HFIR and STS facilities will contribute to the understanding and design of new and more complex materials while forming an unmatched research facility that will maintain US global leadership in the neutron sciences.

For details about the Proton Power Upgrade, visit

SNS and HFIR are DOE Office of Science User Facilities. ORNL is managed by UT-Battelle LLC for DOE’s Office of Science, the single largest supporter of basic research in the physical sciences in the United States. DOE’s Office of Science is working to address some of the most pressing challenges of our time. For more information, visit– Paul Boisvert