Physicists planned the Intersecting Storage Rings (ISR) in the 1960s because they knew that smashing two particle beams head on would give much higher collisions energies that when a single beam of particles collides with a fixed target. The ISR started operation in 1971 and ran until 1984. During that time, it changed the views of many people in the physics community who had doubted the usefulness of a hadron collider.
The ISR was composed of two interlaced rings with diameters of 150 metres. Each ring contained a beam pipe surrounded by magnets that directed the circulating particles. Protons circulated in opposite directions and collided with a maximum centre-of-mass energy of 62 GeV, equivalent to a 2000 GeV beam hitting a stationary target. The Proton Synchrotron, which is still in operation, fed proton beams into the ISR. Until 2004, the ISR held the luminosity record for hadron colliders.
The ISR performed the first-ever proton-proton and proton-antiproton collisions. It was also where stochastic cooling was first developed, which reduces both the transverse dimension of the beam and the spread in the energy of the particles. This technique was subsequently used, with great success, to allow proton-antiproton collisions in the Super Proton Synchrotron, and is still in use at the Antiproton Decelerator. While discoveries of new particles was left to later colliders, the ISR helped provide hints that protons are composed of smaller parts, now known to be quarks and gluons.
About 15% of the research done at the ISR was on the machine itself. The physicists and engineers continuously implemented improvements, so that the collider grew increasingly sophisticated during its 13 years of operation. Projects like the Large Hadron Collider would not have been possible without the ISR and the detector expertise gained during its operation.
After the ISR shut down in 1984, CERN’s focus shifted to the planned Large Electron-Positron Collider. The ISR tunnel is now used for storage and magnet work.