Berkeley Lab

ALS Accelerator Physics: More Information

ALS_from_Building71_700px Major accelerators are not built so much as rebuilt, better every time, as new ideas emerge and technology advancements make improvements possible. The Advanced Light Source — a synchrotron-light source at Berkeley Lab — exemplifies this principle. A host of upgrades over 20 years have greatly increased the quality and variety of photon beams provided to the users, as well as the short- and long-term dependability of a national user facility that serves more than 2000 scientists per year.

When commissioned in 1993, the ALS was in the vanguard of the third generation of synchrotron light sources: optimized for small beam size and emittance and for numerous “insertion devices,” magnetic arrays that “wiggle” or “undulate” the electron beam and thus cause it to produce light with laserlike qualities.

Twenty-plus years later, the ALS looks much the same on the outside, and is similar in overall function and configuration: a source of intense photon beams in, primarily, the vacuum-ultraviolet and soft-X-ray spectrum, based on an electron storage ring. In detail, however, it has evolved greatly, undergoing several large upgrades and dozens of small enhancements, with the result of providing higher-quality beams to its users.

These improvements are led by ATAP, through our dedicated program in accelerator physics for the ALS. (The facility was designed and built in the Accelerator and Fusion Research Division, predecessor of ATAP. The ALS became an LBNL Division in its own right in 1998, after which we continued leading the continuous efforts to understand and improve the machine, in addition to providing its operators.) Upon request our other programs and centers contribute to ALS improvement, and of course the Engineering Division and the ALS Division itself are integrally involved.

Because the ALS is a important Department of Energy user facility where some 2000 scientists come during the course of a year for photon-beam access, every upgrade or scheduled-maintenance project has to be carefully designed, planned, and scheduled. We are proud not only of the many improvements in beam quality we have made over the years, but also of short- and long-term dependability: ALS users receive about 95 percent of scheduled beamtime, with 36-48 hour spans of continuous operation and two-hour average recovery periods in between.

Here are a few themes of recent improvements.

Brightness Improvements

One of the characteristics of ALS light that users find most valuable is its brightness, a combination of beam size (the smaller the better) and spectral purity. A bright photon beam has to start with a high quality electron beam: small in diameter and low in “emittance,” the degree to which the electrons have a non-ideal energy and direction. Improving electron beams for brighter light have been an ongoing theme of many of our upgrades throughout the 20-year history of the ALS.

ALSbrightness

The quest to improve the electron beams, and thus the photon beams provided to the users, is as old as the ALS itself. Transverse and longitudinal feedback systems, superconducting bend magnets that provide harder x-rays, elliptically polarized undulators that give polarized light, simulations based on a mathematical technique called frequency mapping, Top-off injection that keeps the beam current at a continuous maximum, new operational modes such as pseudo-single-bunch operation, and a recent major campaign to increase beam brightness are among the major projects. Many other changes, ranging from an rf power system designed to ease maintenance to upgrades of the low-conductivity-water system for greater stability, increase the reliability of the facility.

Magnetic Lattice

The “lattice” — the repeating pattern of magnets that variously guide the electron beam around the storage ring, focus it, and correct various aberrations — is a key aspect of the performance of any accelerator, but particularly crucial in a synchrotron-light source, which is intentionally full of strong magnetic-field perturbations, just the thing that other accelerators would try hard to avoid.

In the 20-year history of the ALS, we have studied many potential detailed changes to the lattice (within a general framework called a triple-bend achromat), generally in search of higher brightness. Many of the most promising lattices, though, were beyond the capabilities of the existing sextupole magnets, so we designed new ones and had them built by colleagues at SINAP. The last of the 48 magnets were installed during the 2013 spring shutdown.

Bringing the ALS back up with the new lattice took only a few hours, and machine parameters were as expected, a testament to thorough, accurate study and to as-planned fabrication and installation. As a result, the horizontal emittance of the ALS electron beam was reduced from 6.3 to 2.0 nanometers. This resulted in a brightness increase of 3x for bend magnet beamlines and at least 2x for insertion-device beamlines.

As a result of this latest upgrade, the ALS continues its tradition of being among the brightest third-generation light sources in its energy range. Users are taking advantage of this improvement, and meanwhile we have proposed an ALS Upgrade for a “diffraction-limited” redesign that would deliver the ultimate performance possible from this type of machine.

SHDsextupole Left: One of the 48 stronger sextupole magnets (red) that were key to the latest upgrade in ALS brightness. Each one between two quadrupole focusing magnets (orange) in the lattice, or periodic array of magnets that guide the beam around the ring. (“SHD” refers to their position in the lattice.) The silver tube running across the picture is the vacuum chamber in which the hair-thin electron beam circulates.

Right: The yoke of one of the three superconducting bend magnets retrofitted to the ALS in 2002. An upgrade envisioned since the early days of the ALS, implemented in collaboration with ATAP’s Superconducting Magnet Program, Superbends gave hard-X-ray photons that proved especially attractive for the burgeoning field of protein crystallography.
Superbend Yoke

Click here for a technical paper about the upgrade: Christoph Steier et al., Completion of the Brightness Upgrade of the ALS, Journal of Physics: Conference Series 493 (2014) 012030.

RF Upgrades

Another major area of improvement, particularly relevant to reliability, has been the radiofrequency power system, which provides the acceleration power for the electrons. In the past two years, the klystron — the actual source of the RF power — and its high-voltage supply were replaced. This summer, the “crowbar” system that provided overvoltage protection for the storage-ring RF system was replaced with a new high-voltage switch, developed by LBNL electrical engineering staff. The new switch will serve the same purpose but with greater reliability.

The plans for next year, completing this $4.8 million dollar RF upgrade, center on a second klystron to go with the second RF accelerating cavity, along with a switch matrix so that if one klystron fails, the other can drive both RF cavities (albeit at lower power). This will mean that in the event of a klystron failure, the machine should be back “on the air” and serving users after one or two hours of straightforward work, rather than one or two days or more of technically tricky effort. The RF upgrades are highlighted in this behind-the-scenes article written by ALS Communications staff on the eve of the 2014 shutdown.