Exacting Precision for the National Synchrotron Light Source II
Portable CMMs Enable Extreme Accuracy for Alignment, Measurement, and Assembly
An aerial view of Brookhaven National Laboratory (BNL) reveals an impressive half-mile ring structure for the National Synchrotron Light Source II (NSLS-II). Brookhaven’s first NSLS went online in 1982, and will be phased out of operation after more than 30 years of service. In contrast, the next-gen NSLS-II will produce x-rays more than 10,000 times brighter than the original light source. The facility will be outfitted with advanced instruments, optics, and detectors—all the tools needed to leverage the light source’s exceptional beam quality, high brightness and flux. Scheduled for completion in 2015, the national resource will support discovery-class research in the fields of engineering, biology, physics, chemical and materials sciences, and nanoscience.
Since 2010, BNL’s survey and alignment engineers have been delivering and aligning the magnet girders needed to focus the near-light-speed electron beam at NSLS-II. A maze of magnets and mirrors drive the light through many intersections, pulling and bending the beam through the 792-meter storage ring. The ring has 150 girders and as many as seven magnets are installed on a single girder.
Magnets are aligned to a very tight tolerance of 10 microns using a stretched wire technique to achieve sub-micron resolution. To locate the true center of the magnetic fields, the magnets are lined up on steel girders and a wire is stretched through their centers in space where the beam vacuum chamber resides. One magnet is powered with an AC current, and if it is misaligned, the wire will start to vibrate. As the engineers adjust the position of the magnet, the vibration lessens and stops as the magnets come into alignment.
This seemingly low-tech, yet highly effective procedure relies heavily on laser trackers to locate the wire relative to the magnets and the fiducials on the girders. A tracker uses a laser beam to accurately measure in a large radial volume, and is used to plot several points on a girder when the magnets are aligned. The procedure creates a 3D map for re-alignment of the magnets in the storage ring. Ready for installation, an aligned girder travels from the assembly room to the NSLS-II by truck, and its alignment can be disturbed. Once in the storage ring tunnel, engineers use laser trackers to survey the girder’s position, then proceed to restore the high-precision alignment to the 10-micron tolerance.
During the construction of the building, Leica industrial theodolites were utilized for measurement, layout, and verification, and to locate structural features such as monuments and window port holes. The survey team then moved to portable laser tracking systems to establish a global 3D coordinate network, measure the control data, and align components with high-accuracy requirements. To start, pre-survey and survey routines were performed on all the magnets for alignment within 100 microns. Using two Leica laser trackers, surveyors aligned 6 or 7 magnets on a girder, and then stationed the magnets relative to each other based on the acquired 3D measurements.
“Alignment is very critical to the performance of the machine,” states Frank Karl, senior project engineer, Collider Accelerator Department. Karl assembled the team responsible for aligning the accelerator and keeps the workflow moving toward completion. “The tolerance goals of building this machine demanded the use of laser tracking technology. The need was driven by physics, supported by engineering, and implemented by survey and alignment,” states Karl.
Today, all 150 girders are in place. The storage ring is fully populated with 843 magnets and the first phase of alignment is complete. The NSLS-II project also includes a booster accelerator that is 100 meters in circumference, and a linac (linear accelerator) serving as the injector for both the booster and storage ring. The survey and alignment team is working on the transport of those lines from the booster to the storage ring. They are also finalizing alignment of the girders …taking them from sub-millimeter to the 30-micron level.
Advances in Mobile Metrology
There are twelve members of the survey and alignment group, and nine laser trackers are used continuously for this project. Portability is a key CMM (Coordinate Measurement System) feature as the survey and alignment department measure components throughout the NSLS-II on a daily basis, but they also intermittently service the needs of other BNL facilities, such as the Relativistic Heavy Ion Collider (RHIC). Weighing less than 33 lbs., the Leica AT401 laser tracker (Hexagon Metrology) has a small footprint for easy transport and itcan be powered by its own internal battery. With the click of a button, predefined measurement modes adjust the tracker to different environmental conditions, while maintaining precision in the large work envelope.
“Measurement volume was indeed a consideration for our metrology needs at NSLS-II. We are currently measuring within 170 meters for our global control, and for some larger-scale parts, we use laser tracking for pre-survey to alignment,” states Matt Ilardo, Collider Accelerator Department, the surveyor responsible for keeping the team current with technology. He performed construction verification throughout the project, and oversees the integration of the outside control network and controls in the experimental area.
The team also leverages the advancements of AT401 laser tracker from its ability to sense gravity to environmental monitoring. The tracker’s PowerLock feature uses vision technology to detect a reflector and automatically lock the laser beam onto it, even when the target is moving. This is particularly helpful for surveyors working around cabling and other obstructions in the ring that disrupt the line of sight from target to tracker.
Chenghao Yu of the Photon Sciences Directorate is a data processor and analyst hailing from the Shanghai Lights Source. He maintains the use of laser tracking saves many hours of work based on its ability to capture horizontal and vertical measurements simultaneously. The laser tracker’s non-contact measurement capability is also used to evaluate the very precise concave and convex mirrors, relate the mirrors to outside fiducials, and verify their shape when the mirrors are bent or twisted.
Robust Software Adds Virtual Manpower
Spatial Analyzer metrology software (Hexagon Metrology) interfaces with the laser trackers to gather and record 3D data. The survey engineers liken the software to having another person on the team. All measured and processed data are stored on a SharePoint site, where reference files can be found for use later in the process. The team has written many measurement plans and scripts using the software, which was a primary driver in the characterization of the magnets.
“We use one script for every magnet type. The program instructs the user how to take the measurements, then it automatically compiles the data into one file at the end,” states Steve Seiler, whose initial job was to fiducialize the magnets going into the storage ring. This process locates reference points on the component relative to its magnetic center. He pre-aligned the magnets with laser trackers, then recorded their final positions using laser trackers again to produce a reference file for the girder installation process in the ring. “The data reside in the same format and is very usable. We have also written scripts to inspect and characterize parts so their properties are known relative to other control points associated with the piece, so we can align the component later.”
BNL engineers and designers performed the detailed design work on the magnets and girders. All components in the storage ring have a three-dimensional CAD model associated with it. Occasionally, a part needs to be inspected to evaluate its shape or surface features. The 3D model is imported into Spatial Analyzer, where points are measured and compared. The software maintains historical data from start to finish with 100% traceability from measurements to reporting.
The survey and alignment team is on their 6th epic measurement of the control network inside the storage ring, booster, and linac. This routine helps them monitor how much the building and the machine have settled since initial construction. Once the light source goes online, the facility will continue to settle somewhere in the millimeter range, and laser tracking technology will be used to review and correct vertical alignment for a few more years.
“The machine is behaving very well at this point in its construction life,” states Karl, a witness to many advancements in building accelerators with measurement technologies. “We have come a long way since building RHIC in 1979. We used optical tooling with telescopes mounted on tooling bars. In 1990, Leica-Wild T3000 theodolites and Leica’s Axyz software were used to accomplish our accuracy specifications. Seeking tighter tolerances, we then moved on to state-of-the-art laser trackers. Today, we can achieve the 10-micron tolerance requirement for NSLS-II through management’s commitment to hold the temperature in the storage ring to 78 degrees. It is a huge accomplishment.”
More Facts about Brookhaven National Laboratory and the NSLS-II
The National Synchrotron Light Source II’s sustainable design received accolades this year with a LEED gold rating from the U.S. Green Building Council. Nearly all of the steel in the building is recycled and much of the concrete uses recycled fly ash. Every aspect of green living was considered during its development—indoor air quality, green materials, heating and cooling, low-flow water fixtures in bathrooms, good insulation, automatic lighting, energy monitoring, and more. The smart design of this 24/7 facility will reduce its energy bill and shovel more dollars into science.
When completed in 2015, scientists will be able to observe fundamental properties with nanometer-scale resolution and atomic sensitivity at the NSLS-II. The medium-energy storage ring (3 billion electron-volts) will host major scientific endeavors expected to deliver new innovations in clean energy, molecular electronics, self-assembly of nanomaterials, and high-temperature superconductors.
Brookhaven National Laboratory, one of ten national laboratories under the U.S. Department of Energy’s Office of Science, is operated by Brookhaven Science Associates. For more than 60 years, the lab has served as a world leading multi-purpose research institution in the study of the basic nature of matter, including subatomic particles and the structure of the atom. Located on Long Island, NY, Brookhaven operates large-scale facilities for studies in physics, chemistry, biology, medicine, applied science, and advanced technology. Each year, nearly 3,000 scientists, engineers, and support staff are joined by more than 5,000 visiting researchers from around the world.
In 1946, representatives from nine major eastern universities — Columbia, Cornell, Harvard, Johns Hopkins, Massachusetts Institute of Technology, Princeton, University of Pennsylvania, University of Rochester, and Yale — formed a nonprofit corporation to establish a new nuclear-science facility, and they chose a surplus army base “way out on Long Island” as the site. Thus, Brookhaven National Laboratory was born. On March 21, 1947, the U.S. War Department transferred the site of Camp Upton on Long Island to the U.S. Atomic Energy Commission (AEC), the predecessor to the present U.S. Department of Energy (DOE). The AEC provided the initial funding for Brookhaven’s research into the peaceful uses of the atom, with the goal of improving public well-being.