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Dorfan Today: SSRL

The SPEAR3 synchrotron storage ring at SSRL, now well into its third year of operation, is a world-class lightsource by any measure. Each year nearly two thousand users take advantage of the experimental facilities at SSRL, resulting in an average of more than 400 scientific papers annually. Improvements and upgrades continue to expand the operational capabilities of SPEAR3, as we have seen most recently at the dedication of Beamline 12, the Molecular Observatory for Structural Molecular Biology.

This exciting evolution continues with the recent commissioning of a new experimental station currently at Beamline 5-2, which received its first users this past April. This unique facility offers users a range of experimental capabilities for probing the structure and dynamics of materials at the nanoscale. "Nano" objects have a length scale which is one-billionth of a meter or roughly a ten-thousandth of the thickness of human hair.

Two principal techniques available at the new beamline that take advantage of SPEAR3's exceptional brightness are x-ray photon correlation spectroscopy and "lensless imaging." Neither would have been possible with the earlier generation SPEAR2. Beamline 5-2 uses a very special magnet—called an elliptically polarizing undulator—to convert the stored electrons to soft x-rays. Special properties of the magnetic optics downstream of the undulator cause these soft x-rays to become "coherent." Coherence is a property of electromagnetic radiation (of which light or soft x-rays are two examples) which implies an orderliness of the parameters of the soft x-rays, rather than a randomness. This orderliness, much like that exhibited by laser light, distinguishes the soft x-rays produced at this new experimental station from other experimental stations at SPEAR3 in powerful ways. This opens up investigations into fluctuating, dynamic states of matter such as magnetism and electrical properties that are otherwise not possible at SPEAR3.

In many instances, the samples placed in the x-ray beams at SPEAR3 are crystallized. This has the advantage of presenting a regular, periodic pattern of molecules to the incoming beam, but the disadvantage that motion of the molecules is frozen in place. Coherent x-rays, however, enable researchers to probe samples that do not have regularly arranged, periodic structures—such as molecules that resist crystallization—and can identify individual elements and their dynamic states within a single sample. One notable recent example is the research conducted by SSRL staff scientist Hendrik Ohldag and colleagues that conclusively demonstrated that elemental carbon can be magnetized.  Ohldag's data were collected over the previous several years using facilities at Berkeley Lab's Advanced Lightsource. With the new facilities now available at Beamline 5-2, such research can soon be undertaken right here at SLAC.

During the upcoming annual shutdown this fall, the experimental station at Beamline 5-2 will be relocated to its permanent home on the new Beamline 13. A new undulator on that beamline will offer an even brighter source of coherent x-rays, and new control systems will provide an operational platform especially optimized to take full advantage of the potential of SPEAR3. And once SPEAR3 begins operating at the design current of 500 mA (as is planned to begin partially during the next year's run), experiments at this station will take one-fifth the time as with the 100 mA in use today.

This new suite of tools also shares many similarities with the LCLS. The overlap is such that much work has been accomplished toward understanding how to best capitalize on the unique imaging capabilities of the LCLS. SPEAR3's continuing evolution is opening new doors at SSRL, both in the type of science available to our users now, and, in the case of the LCLS, the way we will approach doing science at SLAC in the future.

—Jonathan Dorfan, SLAC Today, May 21, 2007