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Cool Running for SSRL

(photo - Beamline 7-2 Monochromator) As SSRL prepares to turn up the juice to 500 mA, engineers are busily upgrading components to handle the increased power. Key among these components are the monochromators—devices that allow researchers to fine-tune the x-ray beams to the needs of the experiment.

Much of the research conducted at a synchrotron lab requires x-rays of a specific energy. Using specially prepared silicon crystals, a monochromator filters the x-ray beam much like a prism splits sunlight into a spectrum of colors. In this way researchers can select the specific x-ray energy of interest to them.

The new monochromators employed on SSRL's most intense x-ray beamlines are refined from earlier designs in several ways. Chief among these improvements is how the diffraction crystals handle the intense heat from 500 mA of current, which can distort the crystals and degrade the beam. Unlike lower power designs, which use water to cool the crystals, the new monochromators are flooded with liquid nitrogen to regulate their temperature. Using a high-pressure, closed-loop system that recycles the coolant, these new devices circulate up to 700 liters (about 185 gallons) of liquid nitrogen an hour.

As is typical of the close-knit synchrotron community, the design of the optics was influenced by developments at other synchrotron labs around the world. In particular, liquid nitrogen monochromators originated at the Cornell High Energy Synchrotron Source and was further developed at the European Synchrotron Radiation Facility in Grenoble and the Advanced Photon Source outside Chicago.

"Like everyone else in this game, we stand on the shoulders of our predecessors," said beamline development group leader Tom Rabedeau.

Though they derive from those earlier monochromators, the SSRL liquid nitrogen cooled monochromators were specifically engineered to handle the unique challenges of the high power SPEAR3 wiggler beamlines operating at 500mA current.

The new liquid nitrogen monochromators are assembled at SLAC. When the SPEAR3 beamline upgrade is complete in 2007 there will be 11 such monochromators in operation at SSRL.

—Brad Plummer
   SLAC Today, June 22, 2006

Image: A liquid-nitrogen cooled monochromator, located at SSRL's Beamline 7-2. (Click on image for larger version.)