Future Light Sources Trip Report
Peter McIntosh, Andy Moss and Susan Smith from ASTeC joined 222 participants attending the 48th ICFA Advanced Beam Dynamics Workshop on Future Light Sources during March 1-5, 2010 at SLAC National Accelerator Laboratory, California. The workshop series on future light sources reviews and discusses modern accelerator-based light sources for wavelengths ranging from the Infrared to X-rays. Working groups were dedicated to critical issues of ERL, FEL, storage ring and novel light source concepts, as well as to essential technologies like linacs, undulators, synchronization, electron sources, beam diagnostics and numerical simulations.
ASTeC presented an update of the advances in accelerator science taking place in the UK at the ALICE R&D facility, including the latest commissioning results from UK’s first FEL. In a combined ERL/FEL session, new results from simulations done at Daresbury of a seeded FEL driven by the recirculation option for NLS were used to make a unique comparison between the challenging recirculation design and the more straight forward but costly single pass FEL design. The encouraging recirculation results fuelled a lively debate on the merits of the two options. Alan Gillespie from Dundee University presented state of the art R&D from a collaborative project with ASTeC which is pushing the forefront of electro-optic beam diagnostic techniques, critical to the operation of future short pulsed light source.
Other highlights from the meeting are summarised here:
- The science needs for the next generation of x-ray light sources for chemical reactions dictate 1 – 50 fs pulses, with a flexible bunch structure capability.
- Future storage ring R&D should focus on a diffraction limited, 5 GeV, with extremely low horizontal and vertical emittance of 10 pm, at ~1023 brightness.
- Challenges facing ERL based light sources include optimising for tighter orbit and return time tolerances, with effective impedance budget and energy spread management.
- Recirculating linac to save costs seems to survive CSR, with preserved beam quality.
- Primary issue is not cost of facility, but cost/beamline or cost/quality photon and lower charge configuration, even ~10 pC looks promising.
- Laser seeding offers best longitudinal coherence and stability, HHG may be advancing to get to keV on its own with research-level performance.
- In terms of 6D brightness, the injector beam quality requirements are similar for both low and high bunch charge machines – the required repetition rate dictating preferred technology choice.
- Challenge facing low repetition rate injector designs is to optimize RF structures for higher repetition rates without dramatically reduce the gradients.
- For high repetition rate designs, SRF guns must prove high gradient operation with high QE photocathodes at the fields required for emittance compensation.
- Significant progress has been made in the development of suitable multi-alkali antimonide photocathodes with high QE (>~ 1%) and photo-emitting in the visible.
- Timing distribution over stabilized fibers at <10 fs daily drift now available. Work needed to get <10 fs drift at the experiment – 1fs required!
The various contributions to the working groups and the group summaries can be found on the workshop web site http://www-conf.slac.stanford.edu/icfa2010/.
The photograph shows Susan, Peter and Andy visiting the LCLS Undulator Hall. The LCLS has the world's longest undulator; in order to drive the shortest-wavelength, highest-energy X-rays ever created by any laser, the LCLS team had to align the electron beam throughout the 132 m of undulator to less than a hairs breadth - an astounding feat of engineering and accelerator science.
"Moon Over Pigeon Point Lighthouse", © Tyler Westcott
48th ICFA Advanced Beam Dynamics Workshop on Future Light Sources
March 1-5, 2010
SLAC National Accelerator Laboratory
Menlo Park, California
SLAC's linac started up again last Wednesday after three weeks' downtime to install 21 of the 33 undulator magnets that will help generate X-rays in the Linac Coherent Light Source. The LCLS commissioning team completed the installation and initial alignment with impressive speed, taking less than half the time allotted for the work. If all continues to go well, the linac's electron beam could make its first test run through the new undulators as soon as April 4.
Resting on their support girders in the LCLS Undulator Hall, these first 21 magnets make the LCLS the world's longest undulator, at 84 meters (276 feet). By June, the remaining 12 will be added to the Undulator Hall, bringing the total length, including short breaks between magnets, to 132 meters—about the length of a football field plus an extra 24 yards. In order to generate powerful, free-electron light from this string of undulator magnets, the electron beam brightness must meet a set of challenging criteria—the focus of the successful accelerator commissioning up to this point, starting with the new radio-frequency photocathode gun in April 2007. (See "Have Gun, Will Travel (at Light Speed).") In addition, the magnets that guide the beam must be aligned to extraordinary tolerance levels. At its highest performance, the beam must not deviate from a straight line by more than about 5 microns per 5 meters. Much effort has gone into preparing for this primary challenge, from careful initial survey methods in the Undulator Hall through high-precision magnet tuning and creation of a novel beam-based alignment strategy.
Each 3.4-meter undulator magnet includes 224 alternating-polarity permanent magnets made of a neodymium-iron-boron alloy. These high-field magnets will force the linac's 14GeV electron beam to wiggle left-to-right along its path. The "wiggle" is very small, about 0.001 millimeters, but will be responsible for generating the extremely high-power coherent X-rays that make this new machine so revolutionary. The entire LCLS undulator system was designed, fabricated and tested at Argonne National Laboratory by a dedicated group in the lab's Advanced Photon Source division led by Argonne LCLS Project Director Geoff Pile.
At SLAC, each undulator magnet is measured and carefully shimmed to very high precision in SLAC's Magnetic Measurements Facility, led by Zack Wolf. MMF staff ensure that the "wiggle" amplitude is constant to one part in ten-thousand, and the beam trajectory after each magnet deviates by no more than 0.00005 degrees.
This measurement and shimming process requires at least one week of work in the MMF per magnet—but only after the magnet has been stored in the MMF long enough to reach its normal temperature, about 4 days. During measurements in the MMF, the undulator is intentionally set in the exact orientation it will take in the Undulator Hall, so that all measurements and corrections properly include the Earth's magnetic field, which would otherwise divert the electrons off their precise trajectory. Finally, just to be sure, the Earth's field is also attenuated by a wrapper of special "mu metal," a nickel-iron alloy that gives each undulator its shiny outer shell.
In addition to the undulator magnets, the full undulator beamline includes many other critical components that will guide the electron beam through the Undulator Hall. One focusing quadrupole magnet and one cavity-type beam position monitor, or BPM, sits between each pair of undulators. Each of more than 33 BPMs measures the beam position as it passes through. Designed and built by experts from ANL and the SLAC controls group, the BPMs must catch variations in beam position down to less than one-fiftieth the diameter of a human hair. Testing done in January demonstrated the BPMs can meet and even exceed that performance metric.
Once the beam position is captured by the BPMs, adjustment of the powerful quadrupole and undulator magnets can bring the beam into precise alignment. The beam data are automatically fed into a computer and the necessary girder position corrections are calculated and fed back to motors in the Undulator Hall. The entire girder that supports each undulator sits on top of five motorized cams, allowing remote control of position, pitch, yaw and roll angle for each magnet. In addition, two separate motors can move each undulator horizontally, independent of the girder, to effectively switch the magnet off for diagnostic work and adjustments. This horizontal movement also allows small corrections to the "wiggle" amplitude of each undulator, to optimize performance. The elaborate controls for the undulators were designed by the controls groups at ANL and SLAC, with much careful testing during the fall of 2008. Finally, a new beam-based alignment procedure is applied which can straighten the undulator path to a level not yet possible with standard survey techniques. It has already been successfully tested, and is possible due to the exceptional performance of the undulator BPMs.
With such tight tolerances in beam alignment, ground motion and temperature variations can affect beam positioning. To monitor these relatively-slow changes, a precision alignment diagnostics system, designed and built by the SLAC metrology department, will take a two-pronged attack, using stretched wires and hydrostatic levels. The wires reach over a distance of 140 meters, the full length of the 33 undulators. Four sensors at each undulator magnet monitor fields induced by a high-frequency alternating current run along the wire. Each sensor can detect position changes of the girder down to a remarkable level of 0.1 microns. Since gravity causes the wire to sag at its center, and this sag varies slightly with temperature, the vertical measurements require more finesse. To complement the wire system, four half-filled water vessels are stationed at each undulator magnet. Linked together with water piping across the Undulator Hall, the vessels monitor vertical alignment of each girder by sensing any changes in water level.
Installation of these precision diagnostics, focusing components, and now the first 21 undulator magnets allows the LCLS commissioning effort to press to the next and most exciting phase: the production and characterization of free-electron laser light. With the first 21 undulator magnets installed, there should be enough undulator length to generate FEL light at even the shortest X-ray wavelength (1.5 Angstroms), depending on the electron beam quality, trajectory, and magnet tuning—all of which appear to be within grasp. Then, the months of May and June will be spent commissioning the suite of X-ray diagnostics, collimators, mirrors and attenuators, designed and built at Lawrence Livermore National Laboratory, needed to characterize and optimize the FEL light. July of 2009 could see the introduction of X-rays into the Near Experimental Hall, in time to be tuned for the first user experiments in September.
Press Release Number: PR-SLAC-09-2-76G
Source: SLAC National Accelerator Laboratory
Date issued: April 21, 2009
Contact: Robert Brown, +1 (650) 926-8707, robbrown@slac.stanford.edu
New Era of Research Begins as World's First Hard X-ray Laser Achieves "First Light"
X-Ray laser pulses of unprecedented energy and brilliance produced at SLAC
Menlo Park, Calif.—The world's brightest X-ray source sprang to life last week at the U.S. Department of Energy's SLAC National Accelerator Laboratory. The Linac Coherent Light Source (LCLS) offers researchers the first-ever glimpse of high-energy or "hard" X-ray laser light produced in a laboratory.
When fine tuning is complete, the LCLS will provide the world's brightest, shortest pulses of laser X-rays for scientific study. It will give scientists an unprecedented tool for studying and understanding the arrangement of atoms in materials such as metals, semiconductors, ceramics, polymers, catalysts, plastics, and biological molecules, with wide-ranging impact on advanced energy research and other fields.
"This milestone establishes proof-of-concept for this incredible machine, the first of its kind," said SLAC Director Persis Drell. "The LCLS team overcame unprecedented technical challenges to make this happen, and their work will enable frontier research in a host of fields. For some disciplines, this tool will be as important to the future as the microscope has been to the past."
Image 1. Only 12 out of a total 33 LCLS undulator magnets were needed to create the first pulses of laser light. (Photo: Brad Plummer)
Even in these initial stages of operation, the LCLS X-ray beam is brighter than any other human-made source of short-pulse, hard X-rays. Initial tests produced laser light with a wavelength of 1.5 Angstroms, or 0.15 nanometers—the shortest-wavelength, highest-energy X-rays ever created by any laser. To generate that light, the team had to align the electron beam with extreme precision. The beam cannot deviate from a straight line by more than about 5 micrometers per 5 meters—an astounding feat of engineering.
"This is the most difficult lightsource that has ever been turned on," said LCLS Construction Project Director John Galayda. "It's on the boundary between the impossible and possible, and within two hours of start-up these guys had it right on."
The LCLS team is now honing the machine's performance to achieve the beam quality needed for the first scientific experiments, slated to begin in September. With its ultra-bright, ultrafast pulses, the LCLS will work much like a high-speed camera, capturing images of atoms and molecules in action. By stringing together many such images, researchers will create stop-motion movies that reveal the fundamental behavior of atoms and molecules on unprecedented timescales.
"The LCLS team saw a vision of a remarkable new tool for science that could be achieved by using the existing SLAC linear accelerator, and they delivered on that vision with remarkable speed and precision," said DOE Office of Science Acting Director Patricia Dehmer. "The science that will come from the LCLS will be as astounding and as unexpected as was the science that came from the lasers of a few decades ago. We do not yet know all that the LCLS will reveal about the world around us. But we can be sure that the new results will excite and energize the scientific communities that we serve."
For additional materials, please see the Fact Sheet, Image Gallery, and Video Interviews.
The LCLS project is a DOE Office of Science-funded collaboration among several DOE National Laboratories, including SLAC National Accelerator Laboratory and Argonne, Lawrence Berkeley, and Lawrence Livermore National Laboratories; and Cornell University and the University of California, Los Angeles. Pacific Northwest National Laboratory provided additional project management which helped make this project successful. SLAC National Accelerator Laboratory is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the U.S. Department of Energy Office of Science.
31 August 2010