One of three superbends being lifted over the shielding wall just before installation in the storage ring.

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).

Then, beginning in 1995, Clyde Taylor led a Laboratory Directed Research and Development (LDRD) project to design and build a superbend prototype. By 1998, the collaboration (which included the ALS Accelerator Physics Group, the Superconducting Magnet Program of Berkeley Lab's Accelerator and Fusion Research Division, and Wang NMR, Inc.) produced a robust magnet that reached the design current and field without quenching (i.e., loss of superconductivity). The basic design, which has remained unchanged through the production phase, includes a C-shaped iron yoke with two oval-shaped poles protruding into the gap. The superconducting material consists of wire made of niobium-titanium alloy in a copper matrix, over a mile long, wound over 2000 times around each pole. The operating temperature is about 4 K.

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

Superbend enclosed in cryostat.