Art Robinson
Lori Tamura
David Robin
Constrained in size by its site on a hillside above the University of California, Berkeley campus, the Advanced Light Source (ALS) at the Lawrence Berkeley National Lab is based on an electron storage ring with a 198-m circumference and a maximum beam energy of 1.9 GeV. The relatively low energy of the ALS, as compared to that of other so-called "third-generation" sources of synchrotron radiation, means that the facility generates the greatest flux and brightness at soft x-ray photon energies.
While the ALS has turned out to be a world leader in providing beams of soft x-rays (indeed, furnishing these beams remains its core mission), there has nonetheless been a steadily growing demand from synchrotron radiation users for harder x-rays with higher photon energies. The clamor has been strongest from protein crystallographers whose seemingly insatiable appetite for solving structures of biological macromolecules could not be satisfied by the number of crystallography beamlines available worldwide.
But how to provide these x-rays in a cost-effective way without disrupting the thriving research programs of the existing ALS users was the problem. One way for the ALS to respond to the demand for higher photon energies would have been to use some of its scarce straight sections for high-field, multipole wigglers. Late in 1997, the ALS did in fact install one such wiggler, a device that provides hard x-rays for an extremely productive protein crystallography beamline (Beamline 5.0.2) operated by the Berkeley Center for Structural Biology. The drawback of the wiggler route was immediately obvious: many wigglers would limit the number of high-brightness undulators that give the ALS its state-of-the-art soft-x-ray performance and justified its construction in the first place.
Superconducting bend magnets (superbends) provided the answer for the ALS, which adopted a proposal, originally made in 1993 by Alan Jackson (Lawrence Berkeley National Lab) and Werner Joho (Paul Scherrer Institute), to replace some of the normal combined-function (gradient) magnets in the curved arcs of the storage ring with superconducting dipoles that could generate higher magnetic fields and thus synchrotron light with a higher critical energy. Early on, David Robin (ALS Accelerator Physics Group) conducted modeling studies showing that three superbends with fields of 5 T (compared to the 1.3 T of the original combined-function magnets), deflecting the electron beam through 10 degrees each, could in fact be successfully incorporated into the storage ring without destroying the symmetry of the lattice.
Beginning in 1995, Clyde Taylor of Lawrence Berkeley National Lab's Accelerator and Fusion Research Division (AFRD) led a project to design and build a superbend prototype. By 1998, the collaboration (which included the ALS Accelerator Physics Group, the AFRD Superconducting Magnet Program, and Wang NMR, Inc.) produced a robust magnet that reached the design current and field without quenching. 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.
ALS Director Daniel Chemla made the final commitment to follow through with production in mid-1998. Superbends were to replace the center combined-function (gradient) magnets in Sectors 4, 8, and 12 of the ALS triple-bend achromat storage-ring lattice. The Superbend Project Team, now including members of Berkeley's Engineering Division, 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.
Subsequently, the success of wiggler Beamline 5.0.2 combined with some pioneering work on normal bend-magnet beamlines by Howard Padmore and members of his ALS Experimental Systems Group led to the formation of user groups (participating research teams) from the University of California, the Howard Hughes Medical Institute, and elsewhere willing to help finance superbend beamlines, further adding to the momentum of the project.
Representing the first time that superconducting magnets would be retrofitted into the magnet lattice of an already operating synchrotron light source, the superbend installation and commissioning had to proceed quickly and transparently to users of the ALS. Unlike wigglers and undulators in straight sections, superbends would be an integral part of the storage-ring lattice in a large multiuser facility and could not simply be turned off in case of failure or malfunction. So, the stakes were very high: the payoff would be an expanded spectrum of photons to offer users, but the risks included the possibility of ruining a perfectly good light source or, at the very least, causing unacceptable down time.
For the next three years, the superbend team worked toward making the ALS storage ring the best understood such ring in the world, and then it was time to begin the installation. After some preparatory work during previous shutdowns, installation of the superbends began in August 2001. The goal was to commission the ALS with superbends and return the beam to users by October 4. This schedule allowed the month of September to commission the ring. Superbend Project leaders were bracing for a commissioning period of up to six weeks. Instead, it took less than two weeks after installation began before the machine was ramped up to full strength.
In early October, the ALS re-opened for users with an extended spectral range to 40 keV for hard x-ray experiments. Since then, the ALS has not experienced any major bumps or glitches that might be associated with such a major change. Overall, the ALS has made good on its promises to users of installing and commissioning the superbends without disrupting or delaying their research programs and operating them with no adverse effects on performance in the bread-and-butter soft-x-ray spectral region. Moreover, the first of 12 possible superbend beamlines are already taking data and more are under construction or planned.
Three protein-crystallography beamlines are running and users are solving structures. Three more crystallography beamlines are on the way. Noncrystallography beamlines currently in the works include one for tomography and one for high-pressure research with diamond-anvil cells, two areas for which superbends are even more advantageous than they are for protein crystallography, because they more fully exploit the higher photon energies that superbends can generate. Many other areas, including microfocus diffraction and spectroscopy, would also benefit enormously through use of the superbend sources.
In sum, a new era at the ALS is under way.
Photos from "Superbends at the ALS: A Perfect Fit," by Lori Tamura in ALS News, Vol. 185 (with permission).
Figure 1. One of three superbends being lifted over the shielding wall just before installation in the storage ring.
Figure 2. 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).
Figure 3. Iron C-shaped yoke, with oval poles visible. A liquid helium vessel is on top.
Figure 4. Superbend enclosed in cryostat.
Figure 5. Layout of Sector 8 showing the University of California, Berkeley and San Francisco (UCB/UCSF), and Howard Hughes Medical Institute (HHMI) protein crystallography beamlines and their corresponding endstations.