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Stochastic cooling is a form of particle beam cooling. [1] It is used in some particle accelerators and storage rings to control the emittance of the particle beams in the machine. This process uses the electrical signals that the individual charged particles generate in a feedback loop to reduce the tendency of individual particles to move away from the other particles in the beam.
The technique was invented and applied at the Intersecting Storage Rings, [2] and later the Super Proton Synchrotron (SPS), at CERN in Geneva, Switzerland, by Simon van der Meer, [3] a physicist from the Netherlands. It was used to collect and cool antiprotons—these particles were injected into the Proton-Antiproton Collider, a modification of the SPS, with counter-rotating protons and collided at a particle physics experiment. For this work, van der Meer was awarded the Nobel Prize in Physics in 1984. He shared this prize with Carlo Rubbia of Italy, who proposed the Proton-Antiproton Collider. This experiment discovered the W and Z bosons, fundamental particles that carry the weak nuclear force.
Before the shutdown of the Tevatron on the 30th of September 2011, [4] Fermi National Accelerator Laboratory used stochastic cooling in its antiproton source. [5] The accumulated antiprotons were sent to the Tevatron to collide with protons at two collision points: the CDF and the D0 experiment.
Stochastic cooling in the Tevatron at Fermilab was attempted, but was not fully successful. The equipment was subsequently transferred to Brookhaven National Laboratory, where it was successfully used in a longitudinal cooling system in RHIC, [6] operationally used beginning in 2006. Since 2012 RHIC has 3D operational stochastic cooling, [7] i.e. cooling the horizontal, vertical, and longitudinal planes.
This section may be too technical for most readers to understand.(November 2007) |
Stochastic cooling uses the electrical signals produced by individual particles in a group of particles (called a "bunch" of particles) to drive an electro-magnet device, usually an electric kicker, that will kick the bunch of particles to reduce the wayward momentum of that one particle. These individual kicks are applied continuously and over an extended time, the average tendency of the particles to have wayward momenta is reduced. These cooling times range from a second to several minutes, depending on the depth of the cooling that is required.
Stochastic cooling is used to narrow the transverse momentum distribution within a bunch of charged particles in a storage ring by detecting fluctuations in the momentum of the bunches and applying a correction (a "steering pulse" or "kick"). This is an application of negative feedback. This is known as "cooling," as the kinetic energy of particles is related to their internal temperature: the faster the particles are moving, the higher the temperature. If the average momentum of the bunch were to be subtracted from the momentum of each particle, then the charged particles would appear to move randomly, much like the molecules in a gas.
The charged particles travel in bunches in potential wells that keep them stable. While the overall motion of the bunch can be damped (reduced) using standard radio frequency, the internal momentum distribution of each bunch cannot. This can instead be accomplished by stochastic cooling, which aims to slow down individual particles within each bunch using electromagnetic radiation.
The bunches pass through a wideband optical scanner, which detects the position of the individual particles. In a synchrotron, the transverse motion of the particles can be easily damped by synchrotron radiation, which has a short pulse length and covers a broad range of frequencies, but the longitudinal (forward and backward) motion requires other devices, such as a free-electron laser. To achieve cooling, the position information is fed-back into the particle bunches (using, for example, a fast kicker magnet), producing a negative feedback loop that stabilizes their motion.
The bunches are focused through a small hole between the electrode structure, so that the devices have access to the near-field of the radiation. Additionally the current impinging on the electrode is measured and based on this information the electrodes are centered on the beam and moved together while the beams cools and gets smaller.
The word “stochastic” in the title stems from the fact that usually only some of the particles can unambiguously be addressed at once. Instead, small groups of particles are addressed within each bunch, and the adjustment or kick applies to the average momentum of each group. Thus they cannot be cooled down all at once but instead it requires multiple steps. The smaller the group of particles which can be detected and adjusted at once (requiring higher bandwidth), the faster the cooling.
As the particles in the storage ring travel at nearly the speed of light, the feedback loop, in general, has to wait until the bunch returns to make the correction. The detector and the kicker can be placed on different positions on the ring with appropriately chosen delays to match the eigenfrequencies of the ring.
The cooling is more efficient for long bunches, as the position spread between particles is longer. Optimally bunches are as short as possible in the accelerators of the ring and as long as possible in the coolers. Devices which do this are intuitively called stretcher, compressor, or buncher, debuncher. (The links point to the equivalent devices for light pulses, so please note that the prisms in the link are functionally replaced by dipole magnets in a particle accelerator.)
In low energy rings the bunches can be overlapped with freshly created and thus cool (1000 K) electron bunches from a linac. This is a direct coupling to a lower temperature bath, which also cools the beam. Afterwards the electrons can also be analyzed and stochastic cooling applied.
While stochastic cooling has been very successful, its application is limited to beams with a low number of particles per bunch. Optical stochastic cooling (OSC) was proposed in 1993 to increase the cooling bandwidth. [8] By using visible wavelengths instead of microwave wavelengths, OSC promises 4-orders of magnitude increase in cooling bandwidth from that of stochastic cooling. In transit-time OSC, developed in 1994, a particle first produces a wave-packet in a “pickup undulator” (“PU”). [9] The wave-packet and particle are separately transported to a downstream “kicker undulator” (“KU”). Here, the wave-packet is used to give a corrective energy kick back to the particle. The sign and magnitude of the energy kick depends on the relative arrival times of the particle and the wave-packet. The light and particle paths must be tuned such that the reference particle is not kicked.
In August 2022 optical stochastic cooling was demonstrated for the first time at Fermilab [10] [11]
Carlo Rubbia is an Italian particle physicist and inventor who shared the Nobel Prize in Physics in 1984 with Simon van der Meer for work leading to the discovery of the W and Z particles at CERN.
The Tevatron was a circular particle accelerator in the United States, at the Fermi National Accelerator Laboratory, east of Batavia, Illinois, and was the highest energy particle collider until the Large Hadron Collider (LHC) of the European Organization for Nuclear Research (CERN) was built near Geneva, Switzerland. The Tevatron was a synchrotron that accelerated protons and antiprotons in a 6.28 km (3.90 mi) circumference ring to energies of up to 1 TeV, hence its name. The Tevatron was completed in 1983 at a cost of $120 million and significant upgrade investments were made during its active years of 1983–2011.
ISABELLE was a 200+200 GeV proton–proton colliding beam particle accelerator partially built by the United States government at Brookhaven National Laboratory in Upton, New York, before it was cancelled in July, 1983.
The Large Electron–Positron Collider (LEP) was one of the largest particle accelerators ever constructed. It was built at CERN, a multi-national centre for research in nuclear and particle physics near Geneva, Switzerland.
A collider is a type of particle accelerator that brings two opposing particle beams together such that the particles collide. Compared to other particle accelerators in which the moving particles collide with a stationary matter target, colliders can achieve higher collision energies. Colliders may either be ring accelerators or linear accelerators.
A synchrotron is a particular type of cyclic particle accelerator, descended from the cyclotron, in which the accelerating particle beam travels around a fixed closed-loop path. The magnetic field which bends the particle beam into its closed path increases with time during the accelerating process, being synchronized to the increasing kinetic energy of the particles.
Simon van der Meer was a Dutch particle accelerator physicist who shared the Nobel Prize in Physics in 1984 with Carlo Rubbia for contributions to the CERN project which led to the discovery of the W and Z particles, the two fundamental communicators of the weak interaction.
The Underground Area 2 (UA2) experiment was a high-energy physics experiment at the Proton-Antiproton Collider — a modification of the Super Proton Synchrotron (SPS) — at CERN. The experiment ran from 1981 until 1990, and its main objective was to discover the W and Z bosons. UA2, together with the UA1 experiment, succeeded in discovering these particles in 1983, leading to the 1984 Nobel Prize in Physics being awarded to Carlo Rubbia and Simon van der Meer. The UA2 experiment also observed the first evidence for jet production in hadron collisions in 1981, and was involved in the searches of the top quark and of supersymmetric particles. Pierre Darriulat was the spokesperson of UA2 from 1981 to 1986, followed by Luigi Di Lella from 1986 to 1990.
The Collider Detector at Fermilab (CDF) experimental collaboration studies high energy particle collisions from the Tevatron, the world's former highest-energy particle accelerator. The goal is to discover the identity and properties of the particles that make up the universe and to understand the forces and interactions between those particles.
The ISR was a particle accelerator at CERN. It was the world's first hadron collider, and ran from 1971 to 1984, with a maximum center of mass energy of 62 GeV. From its initial startup, the collider itself had the capability to produce particles like the J/ψ and the upsilon, as well as observable jet structure; however, the particle detector experiments were not configured to observe events with large momentum transverse to the beamline, leaving these discoveries to be made at other experiments in the mid-1970s. Nevertheless, the construction of the ISR involved many advances in accelerator physics, including the first use of stochastic cooling, and it held the record for luminosity at a hadron collider until surpassed by the Tevatron in 2004.
Electron cooling is a method to shrink the emittance of a charged particle beam without removing particles from the beam. Since the number of particles remains unchanged and the space coordinates and their derivatives (angles) are reduced, this means that the phase space occupied by the stored particles is compressed. It is equivalent to reducing the temperature of the beam. See also stochastic cooling.
Swapan Chattopadhyay CorrFRSE is an Indian American physicist. Chattopadhyay completed his PhD from the University of California (Berkeley) in 1982.
A hadron collider is a very large particle accelerator built to test the predictions of various theories in particle physics, high-energy physics or nuclear physics by colliding hadrons. A hadron collider uses tunnels to accelerate, store, and collide two particle beams.
A particle accelerator is a machine that uses electromagnetic fields to propel charged particles to very high speeds and energies to contain them in well-defined beams. Small accelerators are used for fundamental research in particle physics. Accelerators are also used as synchrotron light sources for the study of condensed matter physics. Smaller particle accelerators are used in a wide variety of applications, including particle therapy for oncological purposes, radioisotope production for medical diagnostics, ion implanters for the manufacture of semiconductors, and accelerator mass spectrometers for measurements of rare isotopes such as radiocarbon.
A storage ring is a type of circular particle accelerator in which a continuous or pulsed particle beam may be kept circulating, typically for many hours. Storage of a particular particle depends upon the mass, momentum, and usually the charge of the particle to be stored. Storage rings most commonly store electrons, positrons, or protons.
The Low Energy Anti-Proton Ring (LEAR) was a particle accelerator at CERN which operated from 1982 until 1996. The ring was designed to decelerate and store antiprotons, to study the properties of antimatter and to create atoms of antihydrogen. Antiprotons for the ring were created by the CERN Proton Synchrotron via the Antiproton Collector and the Antiproton Accumulator (AA). The creation of at least nine atoms of antihydrogen were confirmed by the PS210 experiment in 1995.
The Antiproton Collector (AC) was part of the antiparticle factory at CERN designed to decelerate and store antimatter, to study the properties of antimatter and to create atoms of antihydrogen. It was built in 1986 around the existing Antiproton Accumulator (AA) to improve the antiproton production by a factor of 10. Together, the Antiproton Collector and the Antiproton Accumulator formed the so-called Antiproton Accumulator Complex (AAC).
The Antiproton Accumulator (AA) was an infrastructure connected to the Proton–Antiproton Collider – a modification of the Super Proton Synchrotron (SPS) – at CERN. The AA was built in 1979 and 1980, for the production and accumulation of antiprotons. In the SppS the antiprotons were made to collide with protons, achieving collisions at a center of mass energy of app. 540 GeV. Several experiments recorded data from the collisions, most notably the UA1 and UA2 experiment, where the W and Z bosons were discovered in 1983.
Vinod Chandrasinh Chohan was a Tanzanian-born accelerator specialist and engineer. He was a Senior Staff Member at CERN for nearly 40 years.
The Super Proton–Antiproton Synchrotron was a particle accelerator that operated at CERN from 1981 to 1991. To operate as a proton-antiproton collider the Super Proton Synchrotron (SPS) underwent substantial modifications, altering it from a one beam synchrotron to a two-beam collider. The main experiments at the accelerator were UA1 and UA2, where the W and Z bosons were discovered in 1983. Carlo Rubbia and Simon van der Meer received the 1984 Nobel Prize in Physics for their contributions to the SppS-project, which led to the discovery of the W and Z bosons. Other experiments conducted at the SppS were UA4, UA5 and UA8.