Lower beam emittance means brighter X-rays for research across many scientific fields

A new set of electron beam measurements puts the upgraded Advanced Photon Source (APS) at the top of the list of the world’s synchrotron X-ray research facilities.

The APS, a U.S. Department of Energy (DOE) Office of Science user facility at DOE’s Argonne National Laboratory, is one of the most productive X-ray light sources in the world. Since April 2023, the more than 5,000 scientists who make use of its ultrabright X-ray beams for research each year have waited patiently as the APS underwent a comprehensive upgrade. Central to that upgrade was  replacing the over 25-year-old electron storage ring that is used to generate those X-ray beams. A brand new one now sits at the heart of the facility.

The APS works by circulating electrons at nearly the speed of light around that storage ring. At special locations in the ring, arrays of alternating magnetic fields cause the electron beam to emit intense X-ray beams which are delivered to experiment stations around the facility. Scientists then use that light to see deep into materials. Those experiments lay the groundwork for potential breakthroughs in battery and solar cell technology and more efficient microelectronics, to name a few.

“It’s exciting not just for our team, who worked hard to imagine, design, engineer, build and commission the new storage ring, but also for the entire light source community and the scientists who will make use of the brighter beams for decades to come to positively impact science and society.” — Laurent Chapon, director of the APS

The brightness of that light is determined, in part, by the emittance of the electron beam. Emittance is a measurement of the size and angular spread of the electron beam, and a lower-emittance beam essentially means that the particles are packed into a smaller space. The more electrons you can pack in a smaller region, the brighter the X-ray beams you can generate with those electrons.

“The new APS electron storage ring was designed to deliver the lowest possible emittance for a facility of this size,” said Michael Borland, associate director of the Accelerator Systems Division at the APS and one of the visionaries behind the upgraded design. ​“It relies on several never-before-used ideas: reverse-bending magnets and a novel method of replenishing electrons in the ring.”

The design and implementation of the new ring has resulted in an emittance measurement that is comfortably the best (meaning lowest) in the world for synchrotron X-ray facilities. The previous record, held by the Extremely Brilliant Source (EBS) at the European Synchrotron Radiation Facility (ESRF), is 134 picometers radians (pm.rad).

The APS measurement, conducted at 50 milliamps of beam current, leads to an emittance of 45 pm.rad. For certain configurations of the APS, such as round beam mode, the emittance is as low as 28 pm.rad.

“This is a significant achievement,” said Laurent Chapon, Argonne associate laboratory director for Photon Sciences and director of the APS. “ We have a beautiful source, as one of our scientists told me right after the measurement. It’s exciting not just for our team, who worked hard to imagine, design, engineer, build and commission the new storage ring, but also for the entire light source community and the scientists who will make use of the brighter beams for decades to come to positively impact science and society.”

With electron source sizes this small it can be challenging to directly measure the beam emittance. Two separate measurements were performed at beamlines 29-ID and 3-ID-B to determine the electron beam emittance by using sophisticated equipment to measure the characteristics of the X-ray beam. These measurements were performed during normal beam operations at the APS.

The APS team is in the process of bringing beamlines into operation after the year-long shutdown and has already hosted its first experiments by outside users. Research is expected to resume in force later this year, and the new storage ring will continue to be ramped up to its full design current.

“It’s thrilling to see it all come together,” said Borland. ​“To go from an idea to a design to a fully functioning storage ring, and then to see a world-leading emittance measurement, is simply unbeatable. We’re all looking forward to what the scientific community will do with these remarkable new capabilities.”