Top-up operation has substantially improved both the intensity and the quality of the x-ray beam for experiments. The method is now used in most third-generation light sources worldwide and is integral to the design of new sources, such as the Diamond Light Source and the National Synchrotron Light Source-II.
"It has fundamentally changed the operational paradigm for all the world's synchrotron facilities," said Moncton." Facilities can now run at their maximum x-ray output all the time, significantly increasing the production of x-rays around the world. Perhaps more important, beam stability and operational flexibility have been greatly enhanced as well," he added.
The key issue that top-up addresses is the inevitable loss of stored electron beam, or current, in the storage ring. In the original APS operating modes, current was added, or injected, every 12 hours, and the current decreased 35% between injections, causing the x-ray intensity to decrease as well. This paradigm posed problems for users: X-rays were not available during injection, and the large drop in intensity was very problematic for many experiments because it introduced thermal errors and intensity variations and often limited the length of an experiment.
With top-up, injection is done every 2 minutes. The current drops only 1%, thermal and intensity transients are minimal, and x-ray production is never interrupted. The result is radically improved operation. Top-up provides higher average photon flux and better beam stability, while also stabilizing heat loads on storage ring magnets and beamline optics.
Furthermore, top-up also allowed changes to the storage ring that significantly reduced x-ray beam size (and thus increased brightness) and allowed new modes that support time-resolved studies. These changes—small emittance, small coupling, and fill patterns with few bunches—significantly decrease the lifetime of the electron beam in the storage ring, but because top-up continually compensates for gradual loss of beam, these much shorter beam lifetimes became practical. Thus, top-up opened the door to many new kinds of experiments that benefit from increased brightness and different time structures in the x-ray beam.
Two advances were crucial to the adoption of top-up. The first was to prove that current could be injected safely while the shutters that separate the storage ring from the beamlines remained open. There was concern that a failure of a storage ring magnet could send the electron beam into the x-ray beam pipe and thus into the x-ray optics, which did not have sufficient shielding. Borland and Emery conducted exhaustive studies to demonstrate conclusively that this could not happen.
The second advance was to minimize disruption to user experiments. Many users were initially highly skeptical, fearing glitches in their data every couple of minutes. "The team did an excellent job of minimizing such transients and documenting their magnitude to the users," said Mark Rivers of GSECARS/University of Chicago.
In their roles as director of the APS and director of the APS Accelerator Systems Division (ASD), respectively, Moncton and Galayda identified the critical developments for implementing and verifying top-up operation and advocated for it in the face of significant resistance from users. Borland, a member of ASD, carried out a massive tracking study of the injection process, co-opting many employee workstations after hours. Emery, also a member of ASD, did analytical studies of top-up safety as an independent confirmation, and led the technical effort to implement quiet, automated top-up, overseeing major improvements to many APS accelerator systems. Borland and Emery also collaborated to reduce the electron beam emittance (a measure of beam size) from about 8 nm to about 3 nm, employing top-up to compensate for the attendant lifetime reduction.
The effort began in September 1996, around the time APS began full user operations, and the first tests with shutters open were done in June 1999. In February and March of 2000, sympathetic users made their first tests during machine studies periods, then from June 2000 to June 2001, a week or two of top-up was offered to users during each run. Finally, in October 2001, top-up became the default mode, offered for 75% of user time.
The impact of top-up has been important across the spectrum of APS experiments. For example, protein crystallography and extended x-ray absorption fine structure (EXAFS) experiments have particularly benefited from stable beam intensity. “Top-up had a major impact on the quality of x-ray crystallography data, especially in the most challenging experiments," according to Andrzej Joachimiak of Argonne National Laboratory. For example, data collected soon after the implementation of top-up mode provided crucial information for solving ribosomal structures, work that was recognized with the Nobel Prize in Chemistry in 2009. The reduction of systematic errors has been important for low-count-rate experiments, such as inelastic and magnetic scattering. Finally, top-up allows APS to offer unique operating modes (such as 24-bunch and hybrid modes) that would be impossible without the improvements made to the injection process. "I can now do experiments which I could not do before," said Mark Sutton of McGill University. In sum, top-up has permitted remarkable scientific achievements and pushed synchrotron radiation sources to new levels of performance.
by Jane Marie Andrew
About the recipients
David E. Moncton was Associate Laboratory Director for the APS from 1987 to 2001; he is now a professor of physics and director of the Nuclear Reactor Laboratory at MIT.
John N. Galayda was director of the APS Accelerator Systems Division (ASD) from 1990 to 1999 and then deputy associate director of APS. In 2001 he left the APS to head the Linac Coherent Light Source (LCLS) Project; he is now directing the development of its successor, LCLS-II.
Michael Borland is the associate division director of the APS Accelerator Systems Division.
Louis Emery is an Argonne senior scientist and a group leader in the APS Accelerator Systems Division.
About the award
The Arthur H. Compton award was established in 1995 by the APS Users Organization (APSUO) to recognize an important scientific or technical accomplishment at the Advanced Photon Source. Compton was an American physicist who won the Nobel Prize for Physics in 1927 for discovering and explaining changes in x-ray wavelengths resulting from x-ray collisions with electrons, the so-called Compton effect. This important discovery in 1922 confirmed the dual nature (wave and particle) of electromagnetic radiation. A Ph.D. from Princeton University, Compton held many prominent positions including professor of physics at The University of Chicago and chairman of the committee of the National Academy of Sciences that studied the military potential of atomic energy. His position on that committee made Compton instrumental in initiating the Manhattan Project, which created the first atomic bomb.
The Advanced Photon Source at Argonne National Laboratory is one of five national synchrotron radiation light sources supported by the U.S. Department of Energy’s Office of Science to carry out applied and basic research to understand, predict, and ultimately control matter and energy at the electronic, atomic, and molecular levels, provide the foundations for new energy technologies, and support DOE missions in energy, environment, and national security. To learn more about the Office of Science x-ray user facilities, visit http://science.energy.gov/user-facilities/basic-energy-sciences/.
Argonne National Laboratory seeks solutions to pressing national problems in science and technology. The nation's first national laboratory, Argonne conducts leading-edge basic and applied scientific research in virtually every scientific discipline. Argonne researchers work closely with researchers from hundreds of companies, universities, and federal, state and municipal agencies to help them solve their specific problems, advance America's scientific leadership and prepare the nation for a better future. With employees from more than 60 nations, Argonne is managed by UChicago Argonne, LLC for the U.S. Department of Energy's Office of Science.