ALS Superbends


One by one, the pieces fell into place. Slowly but surely, the story lines converged. The development of superconducting bend magnets ("superbends"), intended to expand the capabilities of the ALS in general, dovetailed neatly with the extraordinary growth of protein crystallography research in recent years. The superbends will allow up to 12 new beamlines of intermediate energy (from 7 to 40 keV) without sacrificing the quality or quantity of light available at the lower energies. This will be more than enough to accommodate the fast-growing protein crystallography community and to provide complementary diffraction, spectroscopy, and imaging capability for materials science in the higher energy range. Superbends, in other words, are tailor-made for the future of the ALS. When the superbend-enhanced ALS starts up for user operations this week, it will mark the beginning of a new era in its history. It will be a testament to the vision, ingenuity, and dedication of the multitude of people who contributed over the course of many years to this resounding success story.


 

Superbend installation
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.

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.

What's the Big Deal?

 

 

Lattice changes

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 yoke and helium vessel

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

Superbend cryostat
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.

  Frequency map
Frequency map analysis of an electron bunch. Stable electrons near the center of the bunch are represented by blue dots in the upper right; less stable electrons are represented by red dots at the lower left.

Meanwhile, on the beamline end, Alastair MacDowell, Richard Celestre, and Padmore of the ALS Experimental Systems Group and Carl Cork of the MCF had demonstrated, at Beamline 7.3.3, the feasibility of doing protein crystallography easily and cheaply at a normal bend-magnet beamline. On the strength of this demonstration, users Tom Alber and James Berger of the Univ. of California, Berkeley (UCB) with David Agard of the Univ. of California, San Francisco (UCSF) agreed to build "Beamline 9.1," a normal bend-magnet beamline for protein crystallography. Fortunately, it was soon recognized that, right next door in Sector 8, a superbend would become available that would be an even better source. The UCB/UCSF participating research team (PRT) decided to take the plunge and committed to building the first-ever superbend beamline (Beamline 8.3.1). The detailed plans that were developed for this beamline were subsequently instrumental in convincing representatives of the Howard Hughes Medical Institute (HHMI), which was interested in investing in a West Coast facility for its protein crystallography investigators, to fund two more superbend beamlines in Sector 8.

Sector 8 layout

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.

ALSNews Vol. 185, October 3, 2001