Accelerator

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Fermilab is home to the Tevatron, once the most powerful particle accelerator in the United States and the second most powerful particle accelerator in the world.

The Tevatron was the second most powerful proton-antiproton accelerator in the world before it shut down on Sept. 29, 2011. It accelerated beams of protons and antiprotons to 99.999954 percent of the speed of light around a four-mile circumference. The two beams collided at the centers of two 5,000-ton detectors positioned around the beam pipe at two different locations. The collisions reproduced conditions in the early universe and probed the structure of matter at a very small scale.

Scientists at Fermilab also study particle collisions by directing beams into stationary targets to produce neutrino beams.

The Tevatron tunnel is buried 25 feet belowground, underneath an earthen berm. In the Tevatron, beams of particles traveled through a vacuum pipe mostly surrounded by superconducting electromagnets. The magnets bent the beam in a large circle.

The Tevatron had more than 1,000 superconducting magnets, which produced much stronger magnetic fields than conventional magnets. Operating at negative 450 degrees Fahrenheit, the cable inside the magnets could conduct large amounts of electric current without resistance. The extra strength allowed for the acceleration of particles to higher energy.

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Components

Upper Magnets: The upper section of magnets operated at room temperature. They were used to transfer particles from one part of the Fermilab accelerator complex to another.

Lower Magnets: The lower section of magnets was part of the Tevatron Collider. Protons traveled through its beam pipe from left to right, while antiprotons traveled the other way. An acceleration section sped up the particles as they circled the tunnel 47,000 times per second.

Beam pipes: Particles traveled through a vacuum pipe located inside a string of magnets.

Dipole magnets: Dipole magnets bent the particle beam so that it stays within the slightly curved beam pipe as it moved around its orbit.

Quadrupole magnets: Quadrupole magnets have four poles: north-south-north-south. They focused particle beams, similar to the way that lenses focus a beam of light. This narrowed the beam into a thin line, confining it inside the beam pipe for millions of turns around the accelerator.

Correction magnets: Correction magnets allowed for the fine-tuning of beam orbits, providing extra focus or horizontal and vertical steering of the beam.

Spoolpiece: The correction magnets for the Tevatron Collider are located in the spoolpiece, a vacuum container that insulated them and kept them at the same temperature as the other superconducting magnets.

Magnet cooling: Stainless steel and copper pipes supplied water to cool the coils in the conventional magnets in the upper beam line. Vacuum-insulated, stainless steel transfer lines cooled the superconducting magnets in the Tevatron with liquid helium.

Power: Large power supplies provided more than 4,000 amps of current to the magnets through heavy copper rods called bus bars.

The Accelerator Chain

To create some of the world’s most powerful particle beams, Fermilab used a series of accelerators. Starting with hydrogen gas, scientists created proton beams. They diverted a portion of the proton beams to create antiprotons. Once they accumulated enough antiprotons, they loaded them into the Tevatron, where they collided at the CDF and DZero detectors with protons traveling in the opposite direction.

Fermilab's Accelerator Chain

Linear Accelerator: Producing negatively charged hydrogen ions is the first step in creating proton and antiproton beams. The Linac, approximately 500 feet long, accelerates the negatively charged ions to 400 million electron volts, MeV, or about 70 percent of the speed of light. Just after they enter the next accelerator, the ions pass through a carbon foil, which removes electrons from the hydrogen ions, creating positively charged protons.

Booster: The Booster, located about 20 feet below ground, is a circular accelerator that uses magnets to bend beams of protons in a circular path. The protons coming from the Linac travel around the Booster about 20,000 times. They experience an accelerating force from an electric field in a radio-frequency cavity during each revolution. This boosts the protons’ energy up to 8 billion electron volts (GeV) by the end of the acceleration cycle.

Main Injector: The Booster sends protons to the Main Injector. The Main Injector, completed in 1999, has become the center ring of Fermilab’s accelerator complex. Before the Tevatron shut down, it had three primary functions that supported the Collider: It accelerated protons and antiprotons for injection into the Tevatron; it delivered protons for antiproton production; and it transfered antiprotons between antiproton storage rings and from the antiproton storage rings to the Tevatron.

Antiproton Source: To produce antiprotons, physicists steered proton beams onto a nickel target. The collisions produced a wide range of secondary particles, including many antiprotons. The aniprotons entered a beamline where beam operators captured and focused them before injecting them into a storage ring, where they were accumulated and cooled. Cooling the antiproton beam reduced its size and made it very bright. After accumulating a sufficient number of antiprotons, beam operators sent them to the Recycler for additional cooling and accumulation before they injected them into the Tevatron.

Fixed Target Area: Three beam lines, buried under earthen berms, allow the delivery of protons from the Main Injector to the neutrino targets. Beams in this area also tested detectors and carried out fixed-target experiments not involving neutrinos. Placing various samples of materials into the beam lines, physicists studied different types of particles and their interactions. Using these facilities, physicists discovered the bottom quark in 1977 and the tau neutrino in 2000.

CDF Detector: CDF is one of two detectors that physicists used in the Tevatron tunnel to observe collisions between protons and antiprotons. As large as a three-story house, each detector contained many detection subsystems that identified the different types of particles emerging from collisions at almost the speed of light. Analyzing the “debris,” scientists explored the structure of matter, space and time. In 1995, physicists from both experiments observed the first top quarks ever produced by accelerators.

DZero Detector: DZero was one of two detectors that physicists used to study collisions produced in the Tevatron. Proton-antiproton collisions created showers of new particles at the center of both CDF and DZero detectors more than 2 million times a second. For interesting events, the detectors recorded each particle’s flight path, energy, momentum and electric charge. Working in shifts, physicists monitored the functioning of the detectors 24 hours a day. In 1995, physicists from both experiments observed the first top quarks ever produced by accelerators.

Tevatron shut down plan and legacy