Pictures of the Future   Fall 2007

Solving the World’s Mysteries

The European Laboratory for Particle Physics (CERN) is building an accelerator that’s designed to solve some of the great mysteries of the universe. Components from Siemens will play a key role in ensuring that the accelerator’s superconducting magnets keep their cool at -271 °C.

When particle physicists go hunting, they take along big guns that fire invisible bullets. Next spring, they will open the hunting season deep below ground at the French-Swiss border near Geneva in a manner never seen before. More specifically, they will cause particles to collide in a 27-km tunnel ring at previously unattained energy levels in an attempt to solve some of the great mysteries of the universe. For example: Why do particles have a mass at all? And is the so-called Higgs boson responsible for this mass, as the Standard Model of particle physics predicts?

Scientists are working to complete the Large Hadron Collider (LHC) at a site 100 m below ground. The tunnel is dominated by a 1.2-m-thick steel pipe, which contains superconducting magnets and curves slightly as it leads off into the distance. Numerous cables and smaller pipes are mounted on the walls, while an adjoining tunnel houses a huge number of switch cabinets for high-voltage electronic systems and control systems for the ventilation units. Two pipes as thick as human arms run in parallel inside the large steel pipe. Inside the pipes, protons or lead ions will be accelerated to almost the speed of light. There are four separate areas in which the particle beams will collide head-on. These particle collisions—which will occur up to 600 million times per second—will enable the Large Hadron Collider to recreate the conditions that prevailed less than a billionth of a second after the Big Bang.

The LHC facility will be able to generate much higher energies than its predecessor, the LEP accelerator, was capable of producing. It will also create a beam with 100 times the particle density. Such high "luminosity" is very important, because it increases the probability of a collision and thus the chances of finding the Higgs boson, which must be at least 100 times heavier than a proton. Four large detectors placed at the points where the beams intersect will register the matter and the particle showers created by the collisions. Such experiments are expected to result in some 15 million gigabytes of data per year. The data will be analyzed by physicists in a new, dedicated computer network. In addition to finding the Higgs boson, CERN scientists hope the new accelerator will provide insights into the mysterious dark matter that constitutes around 25 % of the universe.

To keep the particle beams precisely on course, the LHC relies on superconducting magnets, which need to be cooled with superfluid helium to a temperature of -271 °C. "If we didn’t use these magnets, the facility would have to be 120 km in circumference and would require 30 times more energy," says Laurent Tavian, who is responsible for CERN’s cryogenic systems. He explains that while conventional magnets achieve a field strength of approximately two teslas, the superconducting magnetic coils reach eight teslas and can thus bend particle beams sharply. Nevertheless, over 1,600 ultra-cold magnets are required to achieve this result. "Basically, we’re building the world’s biggest refrigerator," Tavian jokes, adding that "Siemens is playing a major role in the project."

The biggest facility to date required 3,600 l of pressurized superfluid helium; the LHC will need about 600,000 l. It’s the first time that such a large amount of ultra-cold liquid will have to be transported over the large distances around the ring, while the temperature throughout the entire cooling system may not deviate by more than 0.1 °C. Such requirements place unique demands on the materials used. The 15-m-long magnet units, for instance, which are seamlessly connected to one another, will shrink by 4.5 cm due to the cooling. Special buffers ensure that the system remains sealed. Once achieved, the ultra-low temperatures will have to be maintained for months.

Special Controllers Keep things Cool. Helium distribution will be regulated by valves specially designed for use at the lowest temperatures. The system requires more than 1,000 elements with supply and return headers, which control the cooling of the magnets and other components. The valves will be moved by compressed-air driven units, whose position will be regulated by Siemens position controllers. "We can’t use the normal Sipart-PS2 controllers directly in the ring," says product manager Klaus-Peter Heer from Siemens Automation and Drives (A&D) in Karlsruhe, Germany. "That’s because the radiation is intense enough to affect or destroy the sensitive electronic systems."

To solve this problem, developers at A&D created a split version of the Sipart PS2 position controller, which has all of the microprocessors located in a separate radiation-proof tunnel nearby. "Before delivery, we thoroughly tested the split arrangement under the most realistic conditions possible," says Heer. The microprocessor circuit boards can be located up to one kilometer away from the position controller. "Siemens components are crucial for controlling the cooling process," says Tavian. "If one of the position controllers stops working, it might be possible in some cases to have others take over, but in most instances the entire cooling machinery would eventually fail."

In another part of the LHC facility, more precisely at Access Point 1, a narrow, brightly lit corridor ends at a blue steel door. Located behind this door is the ATLAS detector—a machine nearly 50 m long and 25 m high (about the height of a five-story building). On the walls of the 53,000-m³ room that houses this machine are ascending metal platforms that enable technicians to access the various levels of the detector, which consists of several million components, many of which need to fit together to within one hundredth of a millimeter.

The inner zone of the detector contains around ten billion transistors. The ATLAS detector is the biggest experimental component arrangement ever built by particle physicists. It is basically made up of three detector systems, each of which independently measures various properties of the particles derived from collisions. ATLAS also has eight superconducting magnets. "An additional 130 of our split-version position controllers will be used here as well," says Heer. Siemens has delivered 1,400 Sipart position controllers in the split version and 400 conventional ones.

The complex position controllers are not the only things Siemens has provided to the LHC or CERN. Over the last ten years, the company has also supplied numerous products such as Simatic control devices, power supply components, computers, and laptops. Other CERN and LHC suppliers also rely on such Siemens products as mobile operator panels and hidden electronic control systems. Siemens alone has received orders worth around €30 million—but that’s only a fraction of the €6 billion that will have been spent over 15 years to design and build the LHC and its detectors when the facility is completed. As the project’s conclusion draws near, thousands of scientists worldwide can hardly wait for the facility to be switched on in May 2008. Laurent Tavian is one of them. "One thing’s for sure," he says. "If there is a Higgs particle, we’ll find it very quickly." And if there is no Higgs boson? "That’s when things will really get exciting. We could end up finding something unexpected that could change the face of particle physics as we know it."
                                                                                              Norbert Aschenbrenner