The first discussions on incorporating superbends into the ALS took place in 1993, between Alan Jackson, who was the ALS Accelerator Physics Group Leader at the time, and Werner Joho, who was here on sabbatical from the Paul Scherrer Institute in Switzerland. The ALS, somewhat constrained by its available acreage, was originally designed to be a 1- to 2-GeV third-generation light source, whose straight sections were optimized to serve the vacuum-ultraviolet (VUV) and soft x-ray (SXR) communities. Since then, however, light sources have been trending upwards in energy. One way for the ALS to follow this trend would have been to use some of its scarce straight sections for higher-energy wiggler insertion devices. A less costly alternative, proposed by Jackson and Joho, was to replace the ALS's normal dipole bend magnets with superconducting dipoles that could generate higher magnetic fields within the available space.

What's the Big Deal?

Because the superbends are responsible for directing the paths of the electrons circulating in the storage ring, it is essential that they work properly and continuously. Unlike straight-section insertion devices such as wigglers and undulators, superbends cannot simply be turned off in case of failure or malfunction. No other third-generation synchrotron light source has been retrofitted in this fundamental way.

The stakes were very high: the payoff would be an expanded spectrum of photons to offer users; the risks included the possibility of ruining a perfectly good light source or, at the very least, causing unacceptable down time. Needless to say, the superbends had to work right, and they had to work right away.

Superbend project leaders were bracing for up to a six-week adjustment period. Instead, thanks to extensive modeling and planning beforehand, it took less than two weeks after installation began before the machine was ramped up to full strength. Superbend Project Team Leader David Robin describes it this way: "It's as if you performed major surgery and the patient immediately got up and walked away."

In 1993, newly hired accelerator physicist David Robin was assigned the task of performing preliminary modeling studies to see how superbends could fit into the storage ring's magnetic lattice and to determine whether the lattice symmetry would be broken as a result. He concluded that three 5-Tesla superbends (compared to the 1.3-Tesla normal bend magnets), deflecting the electron beam through 10 degrees each, could indeed be successfully incorporated into the storage ring.

Changes to be made to the ALS lattice in a typical superbend sector. One normal-conducting bend magnet (B2, top) was replaced by a superconducting magnet and two quadrupole magnets (B2, QDA1, QDA2, bottom).

Iron C-shaped yoke, with oval poles visible. A liquid helium vessel is on top.

Superbend enclosed in cryostat.

By this time, wiggler Beamline 5.0.2 of the Macromolecular Crystallography Facility had already debuted in 1997 with spectacular success, and protein crystallographers were soon clamoring for more beamtime. Howard Padmore, head of the ALS Experimental Systems Group (ESG), developed a "figure of merit" to get a handle on how well superbends would meet the needs of the protein crystallography community. He concluded that a superbend would be an optimal x-ray source for most protein crystallography projects, similar to the performance of the existing wiggler beamline. Furthermore, the ALS had undergone the upheaval generated by the Department of Energy's Birgeneau review in 1997, which asserted (controversially) that "important scientific issues which require UV radiation have decreased in number compared to those which require hard x-rays." The subsequent ALS Workshop on Scientific Directions supported the development of superbends as a way to provide higher-energy photons without diminishing support for the vital and active core VUV/SXR community. This direction was also endorsed by the ALS Science Policy Board and the ALS Scientific Advisory Committee. Against this backdrop and with the strong support of Berkeley Lab Director Charles Shank, ALS Director Brian Kincaid made the decision to proceed with the superbend upgrade, and his successor, Daniel Chemla, made the commitment to follow through.

The Superbend Project Team held a kickoff meeting in September 1998, with David Robin as Project Leader, Jim Krupnick as Project Manager, and Ross Schlueter as Lead Engineer. Christoph Steier came aboard a year later as Lead Physicist. Over the next three years, the team worked toward making the ALS storage ring the best understood such ring in the world. In every dimension of the project, from beam dynamics to the cryosystem, from the physical layout inside the ring to the timing of the shutdowns, there was very little margin for error.

To study the beam dynamics, the accelerator physicists adapted an analytical technique used in astronomy called frequency mapping. This provided a way to "experiment" with the superbends' effect on beam dynamics without actually requiring the use of the storage ring. Another technical challenge was to design a reliable, efficient, and economical cryosystem capable of maintaining a 1.5-ton cold mass at 4 K with a heat leakage of less than a watt. Wang NMR was contracted to construct the superbend systems (three plus one spare). Because so much was at stake, the storage ring was studied and modeled down to the level of individual bolts and screws to ensure a smooth, problem-free installation into the very confined space within the storage ring.

Layout of Sector 8 showing the UCB/UCSF and HHMI protein crystallography beamlines and their corresponding endstations.

The UCB/UCSF and HHMI beamlines provided the necessary momentum for other groups to follow suit: additional proposals were submitted and construction of beamlines was begun even before a single superbend had been installed. The Molecular Biology Consortium (MBC, affiliated with the Univ. of Chicago) and a PRT from The Scripps Research Institute have also committed to building superbend beamlines. Non-crystallography beamlines currently in the works include one for tomography and one for high-pressure research, two areas for which superbends are even more advantageous than they are for protein crystallography, because they more fully exploit the higher energies that superbends can generate. Many other areas, including microfocus diffraction and spectroscopy, would also benefit enormously through use of the superbend sources. In addition to paying for their beamlines, each PRT contributes funds to help offset the cost of the superbends (estimated at $4.5 million). The PRTs will get 75% of the beamtime on their respective beamlines, with 25% of the beamtime allocated to independent investigators.

Eight years in the making, with a large supporting cast of physicists, engineers, technicians, and others too numerous to list, the remarkably successful installation and commissioning of the superbends these past weeks marks—not the end of the story—but the beginning of a new chapter in the history of the ALS. Well-deserved thanks go to all the Superbend Project Team members, all of whom assumed the full measure of their responsibilities in ensuring the success of the project. Their technical achievement of integrating three superbends into the ALS storage ring will permit this facility to achieve balanced growth in many areas of science, well into the future.