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Spallation Neutron Source on the Right Track(er)
Laser Tracking Refines Construction of Spallation Neutron Source at Oak Ridge National Laboratory
by Belinda Jones
The Spallation Neutron Source (SNS), a $1.4 billion research facility at Oak Ridge National Laboratory (ORNL), is a feat of precision engineering, execution, and control. Each component of the SNS machine requires exactness in location and assembly only achieved through the use of laser tracking systems. Year by year, piece by piece, the planet's most powerful accelerator-based neutron source has slowly and methodically come alive. Its first target station will operate with 24 beam lines running at a frequency of 60 Hertz (Hz), then eventually ramp up to 1.4 megawatts with 8 times more beam power than any existing pulsed neutron source. 
Survey and Alignment Puts Jigsaw Puzzle Together
Operation of the SNS involves experts from various fields including electrical engineering, mechanical engineering, diagnostics, and more. The Survey & Alignment group essentially puts the giant jigsaw puzzle together, and is responsible for interfacing with all elements of the operations team. One of their main tasks is drawing integration.
The construction of the SNS has been a collaborative effort amongst six DOE laboratories: Argonne, Brookhaven, Jefferson, Lawrence Berkeley, Los Alamos, and Oak Ridge. The Survey & Alignment group receives engineering drawings from partnering laboratories, and integrates them into one common 3D coordinate system. This universal blueprint enables all parties to know exactly where their part fits into the overall scheme. When components start arriving at the SNS loading dock, the Survey & Alignment team fiducializes each component, pinpoints their location within the interior network, then sets and aligns them to a very tight tolerance.
With their accurate roadmap, the team started working inside the SNS tunnels using four LTD500 laser tracking systems from Leica Geosystems (Miamisburg, OH) to build the interior network of magnets and other components. The LTD500 consists of a high-speed tracking 3D laser interferometer and precision angular encoders. This portable instrument has a measuring rate of up to 1,000 points per second, with a measurement range up to 35 meters. The group also added an LTD640 laser tracker with a measuring rate of up to 3,000 points per second, with a measurement range of up to 40m (131ft). This technology enables scientists and manufacturers to capture 3D coordinate data on-demand, validate designs, build-and-inspect, confirm close tolerance work, perform alignments and part mating, and more.
When Component A needs to be installed at a particular position in the SNS, a laser tracker is rolled into the tunnel. The laser tracker is used to locate a number of monuments, then uses these measurements to orient itself within the 3D coordinate system. A Survey & Alignment engineer will perform the necessary calculations, and pass the data onto a Survey & Alignment technician who sets the component in place. Throughout the tunnel network, there are roughly 704 monuments designed to hold corner cube reflectors for the laser trackers. There are a similar number of wall monuments. Laser trackers are also used to locate all sorts of equipment for other SNS groups, such as wiring,  conduit boxes, waterlines, phones ¾all positioned to semi-precise or precise locations.
Over a period of five years, all of the main SNS components have been installed and aligned. These components produce accelerated pulses of protons that strike a mercury target at a power of approximately 1.4 million Joules per second, or about the equivalent of one stick of dynamite per second. This action generates intense pulses of neutrons within the target. The neutrons released from the target are cooled in a set of moderators and guided into a number of individual beam lines. The Survey & Alignment group will continue to align components and targets for a number of years until all 24 beam lines are operational.
Fiducialization Perfected
Laser trackers are also used for the fiducialization process or determining the locations of reference points on the component relative to its magnetic center. To give a simple example, the Survey & Alignment group has created a 3D lattice showing the center of every component throughout the machine. While the mantra of the group is "the component must be accurate on paper," the reality of getting it installed is another matter. Imagine a magnet the size of your kitchen oven. Ideally, a beam line would go directly to magnet's center in a perfect world. But that is not always the case. During the fiducialization process, a number of holes are drilled on the outside of the magnet to accommodate holders for the laser tracker’s half-inch corner cube reflectors. A coil is inserted into the magnet's center, and electronic testing equipment establishes where the strongest signal will be generated, usually a miniscule distance off the mechanical center. Once the magnetic center is identified, a laser tracker is used to map the point within the coordinate system, and the magnet is set to a tighter tolerance than ever before.Pushing the Tracker Envelope
Used daily, ORNL's laser trackers are normally kept "on", ready to go, and acclimated to the environment. When the trackers are transported from one work area to the next, backup UPS systems provide a half hour runtime for a typical move of 15 minutes or less. After nearly seven years of uninterrupted usage, the Survey & Alignment group is just now sending their laser trackers in for routine maintenance during a downtime period.
"Modeling components within a single coordinate system is the way of the future for building networks," states Joseph Error, Group Leader of SNS Survey & Alignment. "The concept is not new, but what is really new is the tolerances in which we work. We can globally identify any point to an accuracy of a few millimeters. However, this accuracy is nowhere near good enough for component alignment in the SNS tunnel. In addition to overall accuracy, we are most concerned with component-to-component accuracy. With the use of laser trackers, our goal is to position them so any component adjacent to the next component will be aligned within 50 microns or less with respect to beam."
The 945 meter SNS accelerator system is comprised of components residing in a series of connecting tunnels, including the 360-meter linac tunnel. A prime example of their refined alignment technique involves a drift tube linac built by Los Alamos Laboratory. The entire drift tube linac assembly is roughly the size of a 55-gallon drum in diameter and 36.5 meters long. Inside these tanks are many quadrupole magnets about the size of a fist. A laser tracker was used first to fiducialize 147 quadrupole magnets, then utilized to position the magnets within the six drift tube tank assemblies forming the 36.5-meter length. The drift tube tank exteriors were then fiducialized. Finally, the entire structure was globally positioned in the linac tunnel with all measurement, assembly, and alignment aspects relying on the laser tracker. At the end of the process, the beam went right through the linac on the first take. "This may well be one of the most successful alignment endeavors in our field particularly on this large a scale," concludes Error. "As each segment of the SNS was made operational, the quality of the beam was exceptional. A machine this size consists of hundreds of magnets of all sizes and shapes ¾ some the size of your fist, some as large as an automobile, and some weigh in excess of 20 tons each. A number of these magnets are referred to as 'correctors' as they help steer the beam and compensate for alignment deficiencies. As each section of the machine was commissioned, corrector magnets were not initially used. This was an amazing feat."Metrology Steps in
Traditionally, the Survey & Alignment group starts with a coordinate network provided by site construction surveyors. On the SNS project, they used a Leica Geosystems Industrial Total Station to precisely build their own exterior control network. This coordinate system enables them to identify a corner of a building or locate any piece of equipment globally to an overall accuracy of about 4 millimeters in an area over one mile in length and a half-mile wide. Again, this accuracy is not appropriate for the SNS component alignment, but it provided the group with an ideal starting point.
Dozens of land surveyors were hired to help construct the buildings, and they built a typical global control network consisting of wooden stakes and rebar monuments. Later in the construction process, the Survey & Alignment team arrived to construct their own outside control network. They implemented large, substantial concrete monuments and other sophisticated adaptations to leverage their Leica Total Stations in the setup of the exterior network. Coordinating with the construction site surveyors, the Survey & Alignment group told them to use their existing monuments. But they also urged the surveyors to use a point in their control network if a third or fourth point was needed to orient their instrumentation. One month later, the site surveyors had completely abandoned their monuments, and moved over to the Survey & Alignment network as it far exceeded anything they had used before.
To the End and Back again
During maintenance periods, the beam lines are shut down and every SNS group has a number of items to repair or update. The Survey & Alignment group will take a section of the machine and verify the alignment of the beam line components. Two laser trackers will be setup at different locations in the 945-meter long tunnel to map all monuments and perform component fiducials at the same time. Once complete, the group will map and rebuild the entire network in about two weeks. This exercise brings everything back to optimum working level. The alignment verification process will be conducted for as long as the SNS is operational.
Ground settlement, changes in atmospheric conditions, or engineering changes also require re-verification of component positions. As the floor changes, monuments change. Alignment of an accelerator this large is somewhat like painting the Golden Gate bridge. When the painters finally get done, it is time to go back to the beginning and start again. As for the Survey & Alignment team, they will drive the SNS toward more power, with more control than ever before.
Why is Neutron Scattering Useful to Researchers?
Neutron scattering is a useful source of information about the positions, motions, and magnetic properties of solids. When a beam of neutrons is aimed at a sample, many neutrons will pass through the material. But some will interact directly with atomic nuclei and "bounce" away at an angle, like colliding balls in a game of pool. This behavior is called neutron diffraction, or neutron scattering.Using detectors, scientists can count scattered neutrons, measure their energies and the angles at which they scatter, and map their final position (shown as a diffraction pattern of dots with varying intensities). In this way, scientists can glean details about the nature of materials ranging from liquid crystals to superconducting ceramics, from proteins to plastics, and from metals to micelles to metallic glass magnets.The importance of neutron scattering to the scientific community was recognized by the awarding of the 1994 Nobel Prize for Physics to Clifford Shull and Bertram Brockhouse. Shull pioneered the use of neutron scattering at Oak Ridge to decipher the structure of materials, and Brockhouse found ways to use it in his Canadian laboratory to learn about the motions of atoms in materials.
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