After coming into operation last year, CERN’s Antiproton Decelerator (AD) has got up to speed for physics this year. Changes to the AD since its debut mean that the three experiments – ASACUSA, ATHENA and ATRAP – now enjoy more intense antiproton beams.
The AD is a unique machine. Its job is to decelerate, not accelerate, antiparticle beams, and it has to handle energies that decrease by an unprecedented factor of 35 from the injection ceiling at 3.57 GeV to the ejection floor at 100 MeV.
In its first incarnation in 1987 as a collector of antiprotons, precooling the particles before they passed to the accumulator ring for CERN’s proton-antiproton collider, it was designed to operate at fixed energy, so this factor of 35 presented a big challenge.
The AD team’s design goal was to hang onto a quarter of the injected antiprotons through their vertiginous fall in energy, and to repeat the deceleration cycle once a minute. Recent AD improvements have put the team well on the way to reaching this target.
One important new feature is in the electron cooling system, adapted from that used for CERN’s LEAR low energy antiproton ring, which closed in 1996. Electron cooling gives the antiproton beam a final “cold shower” after the initial stochastic cooling, keeping the antiprotons tightly bunched at the lowest energies. Improvements have also been made to the radiofrequency deceleration system. This summer the AD succeeded in decelerating an injected antiproton beam without losing a single precious particle.
Meanwhile the three AD experiments are getting to the heart of the antimatter (see Weighing the antiproton).
