xyht20

CERN

“The actual state of our knowledge is always provisional and … there must be, beyond what is actually known, immense new regions to discover” – Louis de Broglie

“Conseil Européen pour la Recherche Nucléaire” (European Council for Nuclear Research), the celebrated research center in Geneva, Switzerland and home to the Large Hadron Collider (LHC), holds a special place in the hearts and minds of the scientific community. With recent, high-profile experiments at CERN becoming immortalized in the press and popular culture, CERN stands as an idealized “geek Mecca” that is igniting the world’s imagination—and importantly, that of the world’s youth.

Sub-atomic particles are not the only things that collide at CERN; theoretical and experimental physics also collide—this is where scientists must put their “money where their muons are.” The stakes are high. Millions (if not billions) of dollars can be spent on experiments to prove something that was previously purely theoretical, like the search for the Higgs boson (see “The Little Bang” sidebar). Designing and executing experiments at CERN often require some of the most sophisticated feats of engineering and construction ever attempted, and, indeed, the number of engineers, technicians, and, yes, surveyors outnumber the physicists at CERN.

Imagine trying to get a pair of small proton clusters to circumnavigate a 27km track at near-lightspeed in opposite directions and to have them collide precisely inside a series of detection chambers. How does one align such infrastructure? Who performs these alignments, and what instrumentation and methods do they employ?

After nearly two years of preparation and permission-seeking, I was granted the honor of being able to accompany a survey crew down into the hallowed LHC. This team has, in many ways, stretched the frontiers of surveying as we know it. Surprisingly, some of the solutions have roots in surveying methods going back through the ages, while other methods border on science fiction. I tried to read up on white papers describing this unique segment of the field of surveying, called Large Scale Metrology, but found that I was wholly unprepared for the contrasts in grand scale and dizzying precisions. Mind blown.

Going In

Dr. Helene Mainaud Durand was the gracious host for the visit. She and surveying/metrology section lead Dominique Missiaen and Andreas Herty, a colleague from the Micro Technology and Instrumentation Unit she leads were very patient with my rudimentary questions—there are few folks who perform the type of work they do. The “SU” or Surveying and Large Scale Metrology Unit of CERN usually has 21 scientists on staff, plus 10 students/fellows, an additional three to four contractors when the LHC is running, and about 15 contractors when the system is shut down for maintenance and upgrades. The SU has also supported facilities management and mapping, but now that has split into a separate group that maintains a GIS, map apps, and standard facilities support.

Before I entered the underground world of CERN, Dr. Durand escorted me to the biometrics and security facility to get my eyes “scanned.” As Herty noted, “The only thing they got right about CERN in that book [Angels & Demons] was the retina scans to open doors.” Security is tight, but not for the reasons you might think. Certainly, there are few major “secrets” at CERN, as much of what is discovered is made readily available to a worldwide network of scientists and the press. The primary concerns are safety and protecting the valuable infrastructure and experiments—it would be tragic if some looky-loo tripped over critical wiring, messing up a decade’s work for a researcher. Tours of the above ground facilities are encouraged, and there are many outreach opportunities for students of all ages, but underground tours are limited.

Before you head below, you are handed a helmet and lantern, a small air pack, and safety instructions. One safety instruction in particular goes against conventional wisdom: in case of an emergency you are urged to use the elevators instead of the stairs, though the environmental systems can clear air quality hazards in under 30 minutes.

There are eight major access shafts evenly spaced around the LHC’s 27km circumference. The depth of the main tunnel is around 100m, with much larger chambers, as much as ten stories high, at each of the four experiments: LHCb, ALICE, ATLAS, and CMS. Entry at each shaft is by retina scan and a rapid elevator down. You find bicycles parked at each landing, and there are small, motorized carts for gear.
Part of the LHC that might rarely be on tour itineraries for visiting scientists is the metrology tunnels that run parallel to and slightly above some of the measurement-critical sections of the LHC (like at the four experiments); these tunnels are infrastructure specifically for surveying. In these tunnels the hydrostatic leveling systems and stretched wire systems reside. There are geodetic control points throughout the LHC, but they have seldom been used since the original construction and retrofits. There are fold-down instrument mounts for total stations and scanners, but these are used mainly for asset-integration purposes; for alignment of components during and after installation/maintenance/retrofits, the team uses conventional instruments, but they also employ photogrammetry using, for instance, a Nikon D200 camera.

There is little conventional surveying instrumentation in use when it comes to the alignment of the actual collider. Many of the supporting systems have been custom-engineered/designed/built; in many ways the unique systems represent world-class scientific research endeavors in their own right. The hydrostatic and wire systems are the primary systems, and, ironically, both of these solutions have their roots in old-school surveying from centuries and millennia past. The history of the LHC provides insight into how this came about.

Fellowship of the Ring

Dominique Missiaen began his career at CERN when the Large Electron Positron (LEP) accelerator (27km in circumference and recently re-tasked as the LHC) was being constructed in the 1980s. “There are eight [entrances] 3.3km apart. There were gyroscopes [and conventional instruments]; it was calibrate in the morning, measure all day, calibrate again at night,” says Missiaen, “and the two boring machines coming from the shafts would meet to 1cm to 2cm.” Dr. Durand showed me the separate smaller shafts that were used to transfer the geodetic control down into the tunnel. Plumb lines, inverted plumb lines, nadir cameras, and theodolites were used at these shafts. But this was not a simple tunnel to build and align; the entire circumference has a 1.4º tilt to keep a consistent depth as the terrain rises with the hills of France to the west.

Missiaen gave an idea of achieved accuracies: “In the LHC from one shaft to another, 3.3km, max error was 4mm absolute in the horizontal plane. Critical is the 0.1m across individual 150m sections. Across the entire LHC, 4mm [horizontal], but vertical is 1mm – 14 milliradian.” The tunnel slope, temperature, humidity, and airflow are all taken into account for digital leveling.

The team said that, ironically, some of the digital level runs in the LEP era were very consistent, but lately they have had anomalies. New runs are underway to determine if this is due to the varied airflows in and around the LHC experiments, much larger than those of the old LEP. Leveling runs are done with standard (but high-end) digital levels. And, as with any other method or tool used, the team performs rigorous tests before choosing an instrument. (They noted that, for the first time, they might even be switching the brand of level based on such tests.) Illuminated rods fit into custom floor monuments with brass receptacles.

A local geoid was developed and is updated periodically. Closely-spaced gravimetric observations are taken along 500m baselines, and zenith cameras were used. A zenith camera is an astrogeodetic instrument, a lens pointing up with a CCD taking very short exposures and GPS for a rough location; they are often mounted on instrument carts. By checking alignment to multiple celestial bodies, the deviations from the vertical can be precisely determined.

The LEP was so well-aligned that when the tunnel was to be converted to its current LHC configuration, it was more of an as-built situation. “The [conventional surveying] was very complete for the LEP,” said Missiaen. “There were measurements on the floor and bolt locations on magnets that could be used [perfectly well] for the LHC.” The alignment of the magnets for the accelerator, and especially the final focus magnets entering the experiment chambers, is another story.

Unconventional

Consider the effects of a mass like the ATLAS experiment detector array, roughly six stories high, 44m long, and weighing as much as two Eiffel Towers, and it gets slid out of its cradle for maintenance periodically. Both the chambers and the final focus magnet arrays at both ends of each have sensitive hydrostatic leveling systems (HLS) running along them. There are also stretched wire systems above the final focus magnets, tied together by invar rods. Liquid levels have been in use for millennia, but these take that concept way out there … In the Atlas detector, Dr. Durand showed me the HLS above and below the array, also showing me the data from 2004 when the Aceh earthquake in distant Malaysia produced spikes.

The stretched wire systems work in much the same way as in legacy chaining with a steel tape—record the temperature, pull the spring, apply the correction—but using much more sophisticated modeling. Dr. Durand and Herty have done a lot of unique research in this field and have published numerous papers. Three points define the height offsets to a very precisely specified and manufactured wire. With the temperature and climate monitored and controlled, the resonance in the wires is also taken into account during constant modeling. There are also wire position sensors at different key positions on the wires. The effect of humidity and temperature is not negligible; neither are measured frequencies and oscillations. The precision stretching devices are considered the key components of the system.

I was shown a section of the metrology tunnel at the top of the ATLAS chamber where the alloy enclosure for the wires diverges from the HLS along the length of the chamber by the 1.4º slope of the tunnel. Noting that the critical components are so rigorously monitored by these unconventional methods, I asked what role conventional methods have in the alignment of the rest of the 27km of tunnel (where the accelerator magnet sections reside). “There are stretched wires in short segments in [the rest of the tunnel],” explained Herty, and “the wires have little [eyelets] that can [signal] when the wire touches them,” indicating some change the shape of the collider.

“But, this is really not necessary,” adds Missiaen. “The [scientists] will let us know if it is out of alignment; if there is something wrong, the data will tell us where.” So, in a way, the LHC is the largest surveying instrument in the world—self-checking, except in certain critical areas.

That is not to say that the work of the team will be slowing down; an extended shutdown is scheduled that will increase their work. One can only speculate what new experimental infrastructure will be installed or upgraded (to top the recent Higgs boson experiments) that they may be called upon to support. The team will also take this opportunity to “re-measure everything,” as Missiaen put it, and this work is already under way.

Accelerating Forward

The team has already performed the calculations for several massive proposed projects. The Compact Linear Collider (CLIC) could be as long as 50km—it may extend far under nearby Lake Geneva. Currently, the longest linear accelerator is at the Stanford Linear Accelerator Center (SLAC) at 3.2km. Durand and Herty are quite confident in being able to meet the specifications. The relative precision between the components of the accelerator is the most critical: 10-20 microns over 200m segments. There have even been preliminary studies for an accelerator that could be as long as 90km in circumference.

Geoid, geodetic control, hydrostatic, and wire systems would be deployed for these proposed projects, and, if something else is needed, this team will develop it from scratch. Missiaen noted that if such projects come to fruition, the massive scale would require consideration of even more automation. The concept of a “metrology train” has been explored—a robotic instrumentation toolset that would ride along a track and stop at each segment, sampling data and even possibly calibrating instruments as needed.

Large Scale Metrology is not limited to scientific research facilities. Offshoots of this “brain surgery” field of surveying employ many surveyors in structural integrity monitoring and large, complex structure and plant construction, such as the International Thermonuclear Experimental Reactor in France under way to become the world’s first fusion reactor to generate electricity for the consumer grid.
There is also the boom in intelligent transportation, high-speed and maglev
rail, wind generation, and more. C’mon surveyors, get inspired!

This article originally appeared in Professional Surveyor Magazine.

Leave a Reply

Your email address will not be published. Required fields are marked *