Beamline 8.3.1, at left, is the first of the superbend protein crystallography beamlines, constructed by the ALS Experimental Systems Group in collaboration with the ALS Engineering Group for a UC Berkeley-UC San Francisco consortium.

The ALS is a synchrotron-type particle accelerator designed to accelerate electrons to energies as high as 1.9 billion electron volts (GeV), focus them into a tight beam, and send this beam around the curved path of a storage ring for several hours. While in this storage ring, intense beams of photons are extracted from the electron beam through use of either bending, wiggler, or undulator magnetic devices.

Originally, the ALS was optimized to extract ultraviolet and x-ray photons in the 10 to 1,500 electron-volt range, but with the installation of the new superbends it can now produce x-rays at energies up to 40,000 electron volts. This is because the "superbends" are superconducting magnets with a field strength of 5 Tesla, which is about four times stronger than the field strength of the conventional bend magnets they replaced.

Replacing three of the 36 conventional bend magnets with superbends posed major technical challenges for the superbend project team, which was under the leadership of accelerator physicist David Robin and included, from Berkeley Lab, members of the ALS Accelerator Physics group, the Superconducting Magnet group of the Accelerator and Fusion Research Division, the Engineering Division, and the ALS Project Management group. Compounding every hurdle that the superbend team faced was the need to not compromise in any way the ALS's outstanding capabilities, already established, in the extreme ultraviolet and soft x-ray regions.

	The superbend project team stands next to one of three superbend magnets. The team included members from the Berkeley Lab ALS Accelerator Physics group, the Superconducting Magnet group of the Accelerator and Fusion Research Division, the Engineering Division, and the ALS Project Management group.

"What this meant was that our team was tasked with transitioning in a relatively short period (six weeks for installation and commissioning) to superbend operation with no impact on beam lifetime, photon brightness, reliability, or stability," Robin says. "This was especially tough because the superbend systems are an essential component of the ALS. A superbend failure would mean failure for the entire facility."

The idea of retrofitting the ALS with superconducting bend magnets was conceived in the early 1990s, and a prototype magnet coil was built and tested in 1997. Based on the success of this prototype, the superbend study became an offical project with Robin at the helm in 1998. Knowing that once they began the actual installation, work would have to proceed quickly with little margin for error, the ALS superbend team planned very carefully, studying every aspect of the proposed retrofit, from beam dynamics to the cryosystem, to the spatial restrictions of the ring, to the timing of the installation shutdown.

The careful planning paid off, as Robin explains. "It was major surgery of the machine. Everything needed to work right away and it did. For ALS users of soft x-rays, the transition from pre- to post- superbend operation has been transparent. However, for ALS users who take advantage of the superbends, the impact has been tremendous."

The primary reason for retrofitting the ALS with superbends was to help meet the huge demand for protein crystallography. Sending an intense beam of x-rays through a protein crystal creates a set of diffraction patterns that can be translated by computer into 3-D images with atomic-scale resolution of the protein's structure. Of the thousand-plus users of ALS photons, over a fourth do their research at the Macromolecular Crystallography Facility, one of the top experimental facilities in the world for determining protein crystal structures. Yet the waiting line for would-be users continues to swell.

Consequently, six of the 12 experimental beamlines that will eventually be served by the three new superbend magnets are slated to be used for protein crystallography. Three of these, one funded by a collaboration of the University of California campuses at Berkeley and San Francisco, and two by the Howard Hughes Medical Institute, are already up and running and making contributions.

Of the remaining superbend beamlines, one will be used in conjunction with diamond anvil cells to do high-pressure studies of materials that have been condensed to nanometer sizes, and the other will most likely be devoted to tomography for obtaining high-resolution 3-D images of the internal structures of nanoscale objects. For the final beamline, application possibilities include microfocus diffraction and spectroscopy.

This is the first-ever retrofit of superbends into the storage ring of an operating synchrotron radiation source. To date, against all odds, there has no been no noticeable degradation in the performance of the ALS, nor any facility down-time as a result of the retrofit.

Says project leader Robin, "I am extremely proud of the accomplishments of the superbend team members. It was really an extraordinary achievement."

-- Lynn Yarris