News — Counteracting this miniscule effect showcases the sensitivity and precision of the facility’s electron beam alignment system.
Twice a day, interactions between the Earth, moon and sun cause the ground beneath us to rise and fall by a small amount. This rhythmic effect on Earth’s crust, called the Earth tide, is driven by the same forces of gravity behind the ocean tides.
Since water is less rigid than land, it flows and pools more dramatically in response to those forces, resulting in the rise and fall of water levels we see along the coasts. The effects of the Earth tide, however, are too subtle for most of us to notice at all — unless you’re a physicist operating an extremely sensitive X-ray synchrotron.
Louis Emery is part of a team of physicists who operate the Advanced Photon Source (APS), a and U.S. Department of Energy (DOE) Office of Science user facility at DOE’s Argonne National Laboratory. Twenty-five years ago, Emery went looking for the mark of an earthquake within his team’s data, which tracks certain disturbances to the facility’s operation. During his search, he discovered the faint but continuous influence of the Earth tide on the APS.
“When the beam’s orbit changes, it affects everything else. We have systems that can be tuned to prevent the beam from moving too much.” — Argonne Physicist Louis Emery
“I wasn’t expecting to see it, but the data was clear,” said Emery, who still tracks the effect today. 鈥�“I consider it a fun curiosity that we can measure.”
At the APS, a beam of charged particles called electrons travel around a large ring at near the speed of light. At bends along its path, the electron beam releases intense X-rays, which are delivered to researchers conducting experiments at stations around the ring.
Throughout the day, tidal forces from the moon and sun stretch and compress Earth’s crust around the APS. This causes the ring, which is about two-thirds of a mile around, to grow and shrink by around 30 microns, or the thickness of a human hair.
As the ground moves, everything attached to it moves, too, including buildings, trees, the ring and the experimental stations. The electron beam, however, travels through the ring inside a vacuum chamber, so it is not attached to anything at all. Also, the electric and magnetic forces within the ring have a much stronger influence on the electrons than any gravitational forces they experience.
This means that the beam’s orbit doesn’t change in proportion to the ring. As the ring is stretched and compressed by the tide, the beam’s relative position shifts inside, causing changes to its trajectory unless the discrepancy is corrected.
“When the beam’s orbit changes, it affects everything else,” Emery said. 鈥�“We have systems that can be tuned to prevent the beam from moving too much. We also have 500 beam position monitors that tell us where the beam is located in the ring down to the micron, or one ten-thousandth of a centimeter.”
Emery and his team are responsible for precisely aligning the electron beam at the APS. They maintain a feedback system that keeps the beam as centered in the ring as possible, ensuring the X-rays reach the experimental stations as intended. This system continually tracks and corrects for complex and unpredictable influences that would otherwise compromise the facility’s operation.
A crucial part of this feedback system are 12 sections around the ring called radio frequency cavities. When the beam passes through one of these cavities, it gets a boost of energy from an oscillating electric field. These boosts prevent energy loss over time, since one bunch of electrons might orbit the ring for up to two hours.
The cavities oscillate at a frequency that fixes the length of the beam’s path to match the circumference of the ring. This forces the beam to return to the same place at the same time every revolution, which is crucial for producing dependable, high-quality X-rays for experiments.
But if the ring’s circumference changes and the radio frequency stays the same, the beam is forced to compensate by finding a new trajectory through the ring. For example, if the ring gets smaller, the beam will find places to wiggle along its path in order to maintain its longer path length while keeping the duration of each revolution the same.
“It’s forced to seek an equilibrium, so in some places in the ring, the beam will pop out from its regular orbit if its path length is too long or too short,” Emery said. 鈥�“These are very slight movements — around one micron, which is even smaller than the width of the beam.”
The beam position monitors pick up these wiggles in real time, prompting adjustments to the radio frequency to account for changes in the ring’s circumference. The feedback system performs this cycle every second, making corrections as needed and archiving the changes.
In 2000, five years after the APS began its operation, Emery discovered that the feedback system adjusted the beam’s path length in the same way every 12 hours. 鈥�“I’m a bit of an astronomer, so I immediately thought of the tides,” he said.
The gravitational field of the moon distorts Earth’s crust, causing two bulges to form on opposite sides of the planet. As the moon moves with respect to Earth’s surface, the bulges move with it. This means that any one location on Earth rises and falls twice per day — once when the moon is overhead, and once when it is beneath the Earth.
The sun also exerts a tidal force on the Earth. Although the sun is 27 million times more massive than the moon, it’s also much farther away, so its tidal effect is half as strong. As the sun and moon move in relation to the Earth, they can add to — or detract from — each other’s tidal effects depending on how they are positioned.
“We can see all of this in our data. On a new moon or a full moon, the sun and moon are aligned, so the corrections to the radio frequency are the greatest during those periods,” Emery said. 鈥�“On top of that signal, there are drifts due to the seasons. We see a smaller ring circumference in the winter, and we see the opposite effect come spring. The seasonal change is around one millimeter — less than the thickness of a dime.”
The feedback system can also detect compression waves caused by earthquakes that occur around the world. Emery did find that earthquake he was looking for 25 years ago, and he’s identified several more since.
By itself, the Earth tide’s effect is too small to influence the X-ray beams enough to have a noticeable impact on APS experiments, even if it were left uncorrected. Other disturbances accounted for by the feedback system, such as changing magnetic fields in the ring or the inevitable movement of the ring’s components over time, would more strongly affect the quality of the X-ray beams and experiments.
It’s a great sign, however, that the APS can detect and correct for influences as small as the Earth tide. The subtle traces of the moon and sun within the data showcase the impressive sensitivity and precision of the electron beam alignment system, which enables thousands of researchers to conduct successful experiments every year.
About the Advanced Photon Source
The U. S. Department of Energy Office of Science’s Advanced Photon Source (APS) at Argonne National Laboratory is one of the world’s most productive X-ray light source facilities. The APS provides high-brightness X-ray beams to a diverse community of researchers in materials science, chemistry, condensed matter physics, the life and environmental sciences, and applied research. These X-rays are ideally suited for explorations of materials and biological structures; elemental distribution; chemical, magnetic, electronic states; and a wide range of technologically important engineering systems from to fuel injector sprays, all of which are the foundations of our nation’s economic, technological, and physical well-being. Each year, more than 5,000 researchers use the APS to produce over 2,000 publications detailing impactful discoveries, and solve more vital biological protein structures than users of any other X-ray light source research facility. APS scientists and engineers innovate technology that is at the heart of advancing accelerator and light-source operations. This includes the insertion devices that produce extreme-brightness X-rays prized by researchers, lenses that focus the X-rays down to a few nanometers, instrumentation that maximizes the way the X-rays interact with samples being studied, and software that gathers and manages the massive quantity of data resulting from discovery research at the APS.
This research used resources of the Advanced Photon Source, a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.
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