Accelerator - Fermilab's Tevatron
The Tevatron is the most powerful proton-antiproton accelerator in the world. It accelerates beams of protons and antiprotons to 99.99999954 percent of the speed of light around a four-mile circumference. The two beams collide at the centers of two 5,000-ton detectors positioned around the beam pipe at two different locations. The collisions reproduce conditions in the early universe and probe 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 below grade, underneath an earthen berm. In the Tevatron, beams of particles travel through a vacuum pipe mostly surrounded by superconducting electromagnets. The magnets bend the beam in a large circle.
The Tevatron has more than 1,000 superconducting magnets, which produce much stronger magnetic fields than conventional magnets. Operating at negative 450 degrees Fahrenheit, the cable inside the magnets can conduct large amounts of electric current without resistance. The extra strength allows for the acceleration of particles to higher energy.
Components
Upper Magnets: The upper section of magnets operates 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 is part of the Tevatron Collider. Protons travel through its beam pipe from left to right, while antiprotons travel the other way. An acceleration section speeds up the particles as they circle the tunnel 47,000 times per second.
Beam pipes: Particles travel through a vacuum pipe located inside a string of magnets.
Dipole magnets: Dipole magnets bend the particle beam so that it stays within the slightly curved beam pipe as it moves around its orbit.
Quadrupole magnets: Quadrupole magnets have four poles: north-south-north-south. They focus particle beams, similar to the way that lenses focus a beam of light. This narrows the beam into a thin line, confining it inside the beam pipe for millions of turns around the accelerator.
Correction magnets: Correction magnets allow 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 insulates them and keeps them at the same temperature as the other superconducting magnets.
Magnet cooling: Stainless steel and copper pipes supply water to cool the coils in the conventional magnets in the upper beam line. Vacuum-insulated, stainless steel transfer lines cool the superconducting magnets in the Tevatron with liquid helium.
Power: Large power supplies provide 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 uses a series of accelerators. Starting with hydrogen gas, scientists create proton beams. They divert a portion of the proton beams to create antiprotons. Once they have accumulated enough antiprotons, they load them into the Tevatron, where they collide at the CDF and DZero detectors with protons traveling in the opposite direction.
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. It has three primary functions that support the Tevatron Collider: It accelerates protons and antiprotons for injection into the Tevatron; it delivers protons for antiproton production; and it transfers antiprotons between antiproton storage rings and from the antiproton storage rings to the Tevatron.
Antiproton Source: To produce antiprotons, physicists steer proton beams onto a nickel target. The collisions produce a wide range of secondary particles, including many antiprotons. The aniprotons enter a beamline where beam operators capture and focus them before injecting them into a storage ring, where they are accumulated and cooled. Cooling the antiproton beam reduces its size and makes it very bright. After accumulating a sufficient number of antiprotons, beam operators send them to the Recycler for additional cooling and accumulation before they inject 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 test detectors and carry out fixed-target experiments not involving neutrinos. Placing various samples of materials into the beam lines, physicists study 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 use in the Tevatron tunnel to observe collisions between protons and antiprotons. As large as a three-story house, each detector contains many detection subsystems that identify the different types of particles emerging from collisions at almost the speed of light. Analyzing the “debris,” scientists explore 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 is one of two detectors that physicists use to study collisions produced in the Tevatron. Proton-antiproton collisions create 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 record each particle’s flight path, energy, momentum and electric charge. Working in shifts, physicists monitor the functioning of the detectors 24 hours a day. In 1995, physicists from both experiments observed the first top quarks ever produced by accelerators.