Exquisite Photon Beams, Dependable User Service
Major accelerators are not built so much as rebuilt, better every time. The Advanced Light Source — a synchrotron-light source at Berkeley Lab — exemplifies this principle.
When commissioned in 1993, the ALS was among the bellwethers 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.
Twenty-plus years later, the ALS looks much the same on the outside, and is similar in overall function and configuration: a source of light 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 ALS 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 their continuous efforts to understand and improve the machine, in addition to providing its operators.) Upon request our other programs and centers contribute to these efforts, 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.
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.
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 as we continue to think about a “diffraction-limited” ALS: a future redesign that would deliver the ultimate performance possible from this type of machine.
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.
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.
In general, the corresponding or principal author is listed first. Indicating the broad collaborative nature of our work, some entries (noted in italics) have a corresponding author outside LBNL.
S. de Santis, W. Barry, S. Kwiatkowski, T.H. Luo, G.C. Pappas, L.R. Reginato, D. Robin, C. Steier, C. Sun, H. Tarawneh, and W.L. Waldron, “Injection/extraction kicker for the ALS-U Project,” in unrefereed Proceedings of IPAC ’14, the 5th International Particle Accelerator Conference (Dresden, Germany, 15-20 June 2014), JACoW, contribution WEPRO016, p. 1977.
Madur, A., D. Arbelaez, B. Bailey, A. Biocca, A. Black, P. Casey, C. Chun, D. Colomb, D. Humphries, N. Li, S. Marks, H. Nishimura, C. Pappas, K.V. Petermann, S.O. Prestemon, A.W. Rawlins, D. Robin, T. Scarvie, R. Schlueter, C. Steier, S. Troy, W. Wan, E. Wlliams, Y. Lixin, Q. Zhou, J. Jin, J. Zhang, C. Chen, Y. Wen, and J. Wu, “The installation and commissioning of the Advanced Light Source combined-function harmonic sextupoles for the Low Emittance Upgrade,” in Proceedings of MT-23, the 23rd International Conference on Magnet Technology (Boston, Massachusetts, US, 14-19 July 2013), IEEE Transactions on Applied Superconductivity 24, 3 (June 2014), 4100404 (2014).
G.C. Pappas, S. De Santis, J.E. Galvin, L.R. Reginato, C. Steier, C. Sun, H. Tarawneh, and W.L. Waldron, “Fast kicker systems for ALS-U,” in unrefereed Proceedings of IPAC ’14, the 5th International Particle Accelerator Conference (Dresden, Germany, 15-20 June 2014), JACoW, contribution MOPME083, p. 564.
C. Steier et al., “Proposal for a soft-x-ray diffraction limited upgrade of the ALS,” in unrefereed Proceedings of IPAC ’14, the 5th International Particle Accelerator Conference (Dresden, Germany, 15-20 June 2014), JACoW, contribution MOPME084, p. 567.
- ATAP Special Projects Section (Free-Electron-Laser R&D; APEX; LCLS-II) - These papers are presented here because one or more authors is a member of our group and in many cases because of subject matter that is also pertinent to the ALS. D. Filippetto, C.W. Cork, S. De Santis, L.R. Doolittle, G. Huang, R. Huang, W.E. Norum, C. F. Papadopoulos, G.J. Portmann, H.J. Qian, F. Sannibale, […]
- Accelerator Physics for the Advanced Light Source - A. Madur; Steier, C.; Arbelaez, D.; Marks, S.; Prestemon, S.; and Schlueter, R., “Magnetic design of the Advanced Light Source harmonic sextupole,” in Proceedings of MT-22, the 22nd International Conference on Magnet Technology (Marseilles, France, 12-16 September 2012), IEEE Transactions on Applied Superconductivity 22, 3 (June 2012), 6164236. C. Steier, “Lattices for low emittance light […]