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From the Director of the Stanford Synchrotron Radiation Lightsource: Timeless SSRL

(Photo - Joe Storh)

In my last SLAC Today column, I wrote about the history of the Stanford Synchrotron Radiation Laboratory and how it fits into the bigger model of "one lab." In the meantime, SSRL changed its name from "Laboratory," which now appears in the SLAC name, to the more appropriate "Lightsource." Today I want to update you on how this 35-plus-year-old lightsource manages to keep up with the best in the world. No doubt, SSRL has stiff competition: the Web site now lists nearly 70 lightsources worldwide. Here is the story of how through the years SSRL has kept its place in the elite club of facilities that can count themselves as "top notch" in terms of brilliant X-rays, number of users and scientific achievements.

In terms of spectral brightness, a metric that best characterizes the quality of an X-ray source, SSRL's SPEAR3 is an excellent source; yet newer ones just commissioned or under construction have edged ahead with time. Fortunately, however, source brightness is only part of the story in lightsource quality. Given that the performance of our source is sufficiently close to the best, other factors can move us ahead on the path to scientific excellence. The secret of SSRL's success has been to squeeze the most out of what is there. Tight operations budgets and understaffing, especially over the last two years, have been overcome by the ability of SSRL staff to tighten their belts, stretch their responsibilities and go the extra mile in support of the user program. Another factor is the quality of the SSRL scientific staff, which always receives special mention during external reviews. Many of our scientists carry out their own scientific programs, performing experiments not only at SSRL but if needed also at other synchrotron radiation facilities. In the end, they bring back to SSRL what they have seen and learned, and this is incorporated into the SSRL program.

Since rebuilding SPEAR3 in 2004, SSRL has used beamline upgrade funds supplied by the Department of Energy and National Institutes of Health to make the beamlines compatible with the new source. SPEAR3 offers better X-ray quality, for example X-ray collimation, expressed by a parameter called emittance, as well as X-ray quantity, or number of X-rays which simply scale with the electron current in the ring. These enhanced properties of SPEAR3 lead to more X-ray power and power density falling on the components in the beamlines, such as X-ray optics that manipulate the energy and the spot size of the X-ray beam.

SSRL's Piero Pianetta and Jacqueline Kerlegan examine the X-ray microscope at Beamline 6. (Photo by Peter Ginter. Click for larger image.)

Now, SSRL's most precious beamlines, based on so-called wiggler and undulator sources in SPEAR3, are ready for the next big step: increase of the electron beam current from 100 to 500 mA. However, the increased X-ray power load on the optics and its change as the beam decays, or when beamline stoppers are closed for injection of new electrons, causes drifts of the X-ray beam position or spot size in the experimental stations. Therefore, like some other leading X-ray facilities, SSRL will be implementing "top-off" operation of SPEAR3, wherein electrons are injected frequently with beamline stoppers open to keep the current and heat load as constant as possible. SSRL staff are in the final stages of working with SLAC radiation physics to implement these changes and expect to run in 500 mA "top-off" mode in 2010.

Of the four major U.S. synchrotron radiation facilities in operation—including the Advanced Light Source at Berkeley, the Advanced Photon Source at Argonne, and the National Synchrotron Light Source at Brookhaven—SSRL has the smallest annual operations budget. Therefore SSRL has to carefully pick and choose its X-ray menu. This requires balancing its user program between general "bread-and-butter" capabilities and a few selected specialized facilities which are best-in-class. In the latter category, we have aggressively moved into X-ray imaging, in response to the growth and importance of the field of nanoscience. Nanoscale objects are too small to be seen with our eyes or conventional microscopes, but they play crucial roles in many fields of science. For example, they are the cutting-edge tiny building blocks present in all high-tech devices, for example, computers. One of SSRL's latest microscopes, funded by the NIH's National Institute of Biomedical Imaging and BioEngineering, would be more appropriately named "nanoscope" (see photo, above left). It was built in close cooperation with a local company Xradia and is one of the leading instruments worldwide.

My next article will address how SSRL prepares to rejuvenate itself once again in years to come, drawing on SLAC's core strength in accelerator science.

—Joachim Stöhr
SLAC Today, April 3, 2009