et’s travel back in time to September 10, 2008, at 10:28 a.m. At that moment, CERN became known to the general public when the first beam was successfully steered around the world’s most powerful particle accelerator—the Large Hadron Collider (LHC). This historic event marked the beginning of a new era of scientific discovery.
CERN is the European Organization for Nuclear Research, with headquarters in Geneva. Recognized as the world’s leading laboratory for particle physics, CERN is located 50 to 150 meters below ground under the city’s surroundings, and crosses the Swiss border with France. The LHC is installed in a tunnel 27 kilometers in circumference and provides collisions at the highest energy levels ever achieved in laboratory conditions. CERN physicists can observe these collisions via four huge detectors, exploring new territory in matter, energy, space, and time.
Hollywood and More at CERN
Inside the accelerator, subatomic particles called “hadrons”—either protons or lead ions—travel at close to the speed of light with very high energies before colliding with one another. The beams travel in opposite directions in separate beam pipes—two tubes kept at ultrahigh vacuum. They are guided around the accelerator ring by a strong magnetic field, created by huge superconducting electromagnets. These are built from coils of special electric cable that operate in a superconducting state, efficiently conducting electricity with almost no resistance or loss of energy. This requires chilling the magnets to almost absolute zero, about –271°C—a temperature colder than outer space. For this reason, much of the accelerator is connected to a distribution system of liquid helium, which cools the magnets, as well as to other supply services.
Thousands of magnets of different varieties and sizes are used to direct the beams around the accelerator. These include 1,232 dipole magnets 15 meters long, which are used to bend the beams, and 392 quadrupole magnets, each five to seven meters long, to focus the beams. Just prior to collision, another type of magnet is used to “squeeze” the particles closer together to increase the chances of collisions. The particles are so tiny that the task of making them collide is akin to firing two needles at each other from more than six miles apart with such precision that they meet point to point at the halfway mark.
The efficiency of the LHC is primarily based on its extremely intense beam (i.e., its densification to energy and its fineness). The challenge lies in keeping this high beam intensity within the accelerator and the storage ring all the way to the collision points where the scientific experiment are being conducted—all this despite the enormous energy levels achieved in the particle rays. To guarantee protection from any kind of path deviation and dimensional variation, CERN planned to install a total of 125 collimators in two of the most radioactive areas of the accelerator ring. No quality, high-capacity beam and, consequently, no usable scientific results are possible without extremely robust collimation.
CERN approached Service de Mécanique Nucléaire (SMN) from the Society for the Development and Production of Nuclear Fuels (CERCA), a part of the AREVA group, with the task of producing the 125 collimators. SMN has its headquarters in Romans, France, and employs 850 people. The company specializes in manufacturing sophisticated electromechanical components in the field of nuclear physics, covering applications ranging from research and development to industrial production. A second field of work involves electron-beam welding machines and laser welding, as well as performing installations in nuclear facilities. SMN is a recognized leader in research and development, as well as for manufacturing a small series of sheathed nuclear fuels.
Under strong international competition, the decision to go with CERCA was in part based on positive experiences during the 1990s, when CERCA delivered parts for the super-conducting accelerator for the large electron positron collider ring. CERCA was entrusted with the job of artisan serial production of an array of high-tech prototypes.
“Collimators are similar to energy-absorbing joints that purify (or collimate) particle beams, thus controlling the power output of the accelerator by capturing particle clusters with the superconducting magnets,” explains Pierre Maccioni, director of the nuclear mechanics department at CERCA. “The beam hits parts made of carbon composite materials, an idea that came from the development of fusion receptacles and the divergent cones of rocket propellers. The collimator holders are made of aluminum-oxide-enriched copper and exhibit extreme mechanical and thermal resilience. The assembly is cooled by a water flow running at 20 bar and is then set into motion in a stainless steel container by means of a highly developed, highly precise drivetrain.”
The technical specifications for the collimators designated by CERN require rigorous inspection for accuracy. Therefore, a solution had to be found to conduct part inspections that took into consideration the needs of both the physicists involved and the realities of industrial manufacturing. A portable articulating arm quickly emerged as the ideal instrument for this important task, setting itself above traditional measurement systems that are far less flexible and require much higher investments of capital.
To fulfill the exacting CERN accuracy requirements, two articulating arms were installed at the site. ROMER, a Hexagon Metrology Co., manufactures the ROMER seven-axis articulated arms (a popular type of portable coordinate measuring machine), made of advanced carbon fiber and aluminum. The arm has a 4-foot to 12-foot hemispherical measuring envelope and operates much like a human arm. Holding the probe in hand, the operator can gather 3-D data from a surface of a part, the motion being similar to holding a pencil and touching its point to a piece of paper.
“The decision to go with ROMER arms was unanimous,” says Alain Morin, director of nondestructive control methods at CERCA. “We went with them because they were able to offer us a measurement instrument with the best performance-to-cost ratio. In addition, CERN already owns several ROMER articulating arms.”
From start to finish, CERCA used the articulated arms for the complete inspection and adjustment of 125 collimators. Today, CERN technicians continually use two arms with a measurement volume of 2.2 meters, and the included measurement software for fine-tuning the collimators. CERN specifications make it necessary for the technicians to wear protective gloves at all times to avoid introducing even the slightest contamination with organic materials that would influence the scientific experiments once the parts are hermetically closed and heated in a vacuum.
CERN’s inspection routine itself is not merely for performing a 3-D check. A collimator consists of a complex array of many parts that have to be perfectly aligned to one another. After each measurement, the technicians perform an adjustment based on a highly specific process designed for each step. The accuracy requirement for different parts is about 20 µm.
ROMER also provided skilled technicians who were in charge of designing a measurement procedure for successfully adjusting the collimators. By developing a repeatable, nearly automated measurement procedure that was totally compatible with the macros in the control module, a significant time savings was realized. The inspection time for a single collimator was reduced to two working days.
For more than 20 years, CERN has closely collaborated with various measurement companies. During this time, the highly skilled team of metrology engineers and technicians at CERN relied on a variety of instruments for surveying and metrology—from optical and digital levels; Leica theodolites, total stations, and laser scanners; to Leica laser trackers, and ROMER articulated arms, and associated software.
Surveying in the LHC tunnel using a Leica HDS 3000 high-density, large-scale laser scanner ensured that all services and infrastructure were installed in accordance with specifications. The process ensured that there was always adequate space for successive installations of equipment and accelerator elements. Precision laser trackers were used for dimensional control of the dipole and quadrupole magnets, and levels and theodolites were used to accurately align the magnets along the curved path of the LHC. In addition to the articulating arms, CERCA also utilizes several gauging cylinders made by TESA.