Antiproton Accumulator

The Antiproton Accumulator (AA) was the core item of the CERN Antiproton Project, aimed at the discovery of the intermediate bosons in high-energy proton-antiproton collisions. The AA was built in 1979 and 1980, for the production and accumulation of antiprotons. Antiprotons were produced by directing an intense proton beam at a momentum of 26 GeV/c from the Proton Synchrotron (PS) onto a target, after which they were focused, injected into the AA at a momentum of 3.5 GeV/c, and accumulated, using the method of stochastic cooling, invented by S. van der Meer. Bunches of antiprotons were delivered from the AA to the PS for acceleration to 26 GeV/c and then transferred to the Super Proton Synchrotron (SPS). The SPS had been modified from a proton accelerator, so as to a function as a Proton-Antiproton Collider, in which the antiprotons and protons were first accelerated to an energy of 270 GeV and then made to collide (from July 1981 on) at a c.m. energy of 540 GeV (later raised to 630 GeV and finally, in a pulsed mode, to 900 GeV). The concept of the project was developed and promoted by C. Rubbia, as a way to find the intermediate bosons, W+, W−, and Z0. These were indeed discovered in 1983 by the huge underground detectors (UA1, UA2), and C. Rubbia and S. van der Meer were awarded the Nobel prize in 1984. From the beginning of the project, the potential of physics with low-energy antiprotons was recognized. A Low Energy Antiproton Ring (LEAR) was built and received antiprotons from the AA from 1983 on, for deceleration to as low as 100 MeV/c. The first artificially created antimatter, in the form of anti-Hydrogen, was created in a trapping experiment at LEAR in 1995.

However, the first client for antiprotons from the AA had been the Intersecting Storage Rings (ISR), where proton-antiproton collisions were achieved early in 1981. To satisfy the need for more antiprotons, the ACOL project was conceived in 1983 and implemented in 1986 and 1987. The antiproton production (target and target area) was upgraded; the Antiproton Collector (AC), with an acceptance in transverse and longitudinal phase-space much larger than that of the AA, was built tightly around the AA; and the AA was consequently modified. The AA accumulation rate, previously typically 10¹¹ antiprotons per day, was thus raised by an order of magnitude, to typically 10¹². AC and AA together were referred to as the Antiproton Accumulation Complex (AAC). The AAC was one of the most highly automated complex of accelerators of its time.

After the last SPS collider run, in 1991, LEAR remained the sole client of the AAC, and a simpler way to serve low-energy physics was sought. LEAR was converted to become the Low Energy Ion Ring (LEIR), the AA was dismantled, and the AC was converted to become the Antiproton Decelerator (AD).