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A New Era of Synchrotron Science at SLAC—PEP-X

Science at SLAC stands at the edge of an evolutionary leap. After a half-century of high-energy physics, SLAC's scientific focus is shifting, with the decommissioning of the BaBar detector and PEP accelerator and the approaching completion of the Linac Coherent Light Source (LCLS). Among the wide diversity of scientific pursuits underway at SLAC, photon science—using very bright, energetic X-rays to probe the properties of matter—will soon constitute the major work of the laboratory.

That evolution is made possible by SLAC's long history of learning from the past and building on its achievements. In 2009, the LCLS—the world's first hard X-ray free-electron laser, which uses a portion of the historic two-mile linear accelerator as its backbone—will turn on, generating ultra-fast pulses of X-rays a billion times brighter than any source in the world.

Now, following that tradition of building on the past, scientists at SLAC are envisioning the future beyond the LCLS. By capitalizing on hardware and infrastructure already in place—namely, the recently decommissioned PEP-II facility—a new synchrotron storage ring project, "PEP-X," would catapult SLAC even further beyond the research capabilities available at existing photon science laboratories.

A History of Light Source Science

More than 30 years ago, SLAC played a founding role in the genesis of light source science. At the time, the original SPEAR storage ring accelerator served primarily the high-energy physics research community, but its power as an X-ray source soon became apparent. The SPEAR machine evolved through subsequent generations, and for nearly two decades it has operated as a dedicated tool serving the photon science community. What began as a small project running on the coattails of SLAC's high-energy physics program now represents the primary tool of photon scientists around the world. Over 60 such “synchrotron” light sources based on storage rings are currently operating globally.

In 2004, the Stanford Synchrotron Radiation Laboratory (SSRL) at SLAC commissioned the SPEAR3 storage ring, a "third-generation" light source optimized specifically for creating highly focused beams of X-rays useful for research in a vast range of disciplines. Thousands of scientists from around the world use SPEAR3 for research in a host of fields—chemistry, physics, environmental science, medicine and biology, to name a few.

Next year, the completion of the LCLS will mark a major milestone in SLAC's evolution into a world-leading photon science laboratory. Together, the two light sources—the SPEAR3 synchrotron facility and the LCLS—will provide X-ray scientists worldwide with a suite of scientific capabilities unavailable anywhere else. That expertise will be complemented by SLAC's photon science research centers, PULSE and SIMES.

But ensuring that trajectory continues well into the future requires thinking beyond the LCLS. Although SPEAR3 remains one of the finest synchrotron light sources in the world, technical and scientific advances and the construction of competing facilities will, in a few years, eclipse the capabilities available at SPEAR3. A series of future upgrades for the LCLS are already in the works, but owing to a number of physical limitations of the SPEAR3 machine, similar upgrades to the storage ring are impractical.

Over the next decade, keeping pace with the world of photon science will require an entirely new synchrotron light source. But the good news is—owing to its legacy of high-energy physics—SLAC is already halfway there.

The recently decommissioned PEP accelerator was built nearly 20 years ago to store beams of electrons, again for SLAC's high-energy physics program. In the late 1990s, the PEP accelerator was upgraded with more powerful systems to store electron–positron beams for the BaBar collaboration. Now, beneath the hills of SLAC, the PEP-II accelerator sits, in more than a mile of existing tunnel, unused and waiting for the next chapter in its already illustrious life.

The Next Synchrotron Light Source—PEP-X

The PEP-II accelerator facility is the ideal location for the next synchrotron light source at SLAC after SPEAR3. The PEP-X project would capitalize on the existing infrastructure, such as the accelerator tunnel, the high-power radio frequency accelerating system, and support utilities such as electrical and cooling networks. And because of its size—over a mile in circumference—PEP-X would create a steady stream of high average brightness X-rays unequaled at any other light source, a thousand times brighter than SPEAR3. PEP-X truly represents a leap in scientific capability at SLAC, and, by utilizing the existing infrastructure, one that could be completed at a savings of tens of millions of dollars.

Why Two Light Sources?

The LCLS, based on SLAC’s linear accelerator, represents an unprecedented leap away from conventional ring-like synchrotron sources. It opens completely new territory—but it does not replace the broad scientific capability of a synchrotron storage ring.

Although together the two light sources will serve complimentary purposes, a synchrotron light source such as PEP-X is a very different machine from the LCLS. The LCLS, prized for its "peak brightness," packs unparalleled X-ray energy into very short, single pulses used, for example, to probe matter on very short time scales. One exciting aspect of the LCLS is the uncertainty as to how such unusual ultra-short and ultra-strong x-ray flashes will interact with matter. At present, scientists envision observing chemical bonds forming and breaking, or recording snapshots of materials transforming from one physical state to another—all of which occur within unimaginably short slivers of time.

A synchrotron storage ring, by contrast, provides a virtually continuous stream of X-rays. It is this feature that, despite much lower peak brightness, gives synchrotron light sources very high "average brightness." In contrast to the intense LCLS X-ray pulses, synchrotron X-rays gently probe or continuously “tickle” a sample without disturbing its natural state. Synchrotrons, for instance, are used to determine the chemical and structural make-up of materials, for example, how atoms and molecules are arranged in proteins, or how elements are distributed in soil contaminants.

Together, two light sources offer the complementary qualities of peak flashes of brightness and continuous brightness. This enables scientists to study what materials are made of, and how their atoms are arranged and bonded; the electrons and their distribution, and how unusual phenomena arise from the mysterious property of electrons called the spin; and how the interactions between the three ingredients—atoms, electrons and their spins—vary with time, all down to the shortest timescales.

PEP-X Wants to Hear from You—Help us Envision the Future

This is the first in a series of articles aimed at sharing the excitement and potential of the PEP-X project. As a light source PEP-X will be unlike any other storage ring, with a higher brightness and smaller emittance by far than any facility existing or under construction. But to move forward in a way that best serves the scientific community, the project needs your help. If you are a potential user, the PEP-X team wants to hear from you. You can become a part of this exciting process. Share with us your ideas of how PEP-X would be of greatest use to you by using the contacts below:

Hard X-rays, Uwe Bergmann (

Soft X-rays, Donghui Lu (

Structural Biology, Hiro Tsuruta (

Brad Plummer, SLAC Today, July 2, 2008