Stochastic cooling is a form of particle-beam cooling. 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, and later the Super Proton Synchrotron (SPS), at CERN in Geneva, Switzerland, by Simon van der Meer, 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 30 September 2011, Fermi National Accelerator Laboratory used stochastic cooling in its antiproton source. 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, operationally used beginning in 2006. Since 2012, RHIC has 3D operational stochastic cooling, that is, cooling of the horizontal, vertical, and longitudinal planes.
Technical details Stochastic cooling uses the electrical signals produced by individual particles in a group of particles (called a "bunch" of particles) to drive an electromagnetic 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 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 bunch 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 a bunch can be damped (reduced) using standard radio-frequency equipment, 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 positions 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.
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