Antiproton Source: Transition ahead

Wide-lens photo of the Debuncher and Accumulator rings in the Antiproton Source. Photo: Reidar Hahn, Fermilab.

The Antiproton Source at Fermilab has been doing yeoman service for more than two decades. Even after the Tevatron has been switched off, scientists think that the Antiproton Source’s equipment could be a valuable asset for future experiments.

Built in the early 1980s, the Antiproton Source began operation in 1985. It neatly solved a key problem: how to send two beams of particles through the Tevatron in opposite directions with only one ring of magnets. The solution: use particles with opposite electric charge. The Tevatron magnets guide the positively charged protons clockwise through the ring of magnets, while the negatively charged antiprotons travel counterclockwise through the same ring.

This technique was based on the success of a similar system at CERN’s SPS accelerator. (The Large Hadron Collider, which accelerates two proton beams, uses two sets of magnets.)

Gathering enough antiprotons for the Tevatron is quite a trick. Since they’re not found readily in nature, every antiproton must be manufactured, collected and prepared for transfer into the Tevatron. That’s where the Antiproton Source comes in. Every 2.2 seconds, a beam of protons from the Main Injector strikes a nickel target, and a lithium lens focuses the emerging antiprotons so that they can be guided into the Debuncher ring, which is where the delicate process of cooling starts. The cooling process reduces the antiprotons’ spread in direction and speed so that the particles can form a compact beam. The beam then is transferred to the Accumulator ring, where more cooling takes place. Since 2005, the final cooling occurs in the Recycler ring with the aid of an intense electron beam.

The collection of antiprotons goes on day and night, seven days a week. Back in 1985, Fermilab accumulated several billion antiprotons per hour. Today, thanks to the Main Injector, electron cooling and other upgrades, we collect nearly 300 billion per hour. We transfer 3 to 4 trillion antiprotons to the Tevatron about once per day, which makes it the world’s most productive antiproton source, an order of magnitude better than the planned Facility for Antiproton and Ion Research (FAIR) at the GSI research laboratory in Darmstadt, Germany. At CERN, a facility known as the Antiproton Decelerator produces antiprotons at much lower intensity and energy to explore the properties of antihydrogen.

Besides providing beam for the CDF and DZero collider experiments, the Antiproton Source has supported other Fermilab experiments during its lifetime: study of charmonium (E760/E835), antiproton lifetime (APEX) and antihydrogen observation (E862).

When the Tevatron shuts down later this year, Fermilab plans to re-purpose the equipment of the Antiproton Source for two muon experiments, first muon g-2, then, in the longer term, Mu2e. Two proposals to continue making antiprotons for low-energy experiments are currently being updated. One group of scientists is revisiting the original proposal for The Antiproton Annihilation Spectrometer (TAPAS), which had been rejected, and plans to resubmit a strengthened proposal to Fermilab. Another group is conducting R&D at the University of Texas at Austin to advance the proposal for the Antimatter Gravity Experiment (AGE).

The Antiproton Source has enabled a lot of important research in the last couple of decades. Its equipment and infrastructure has the potential to be a great resource for future experiments as well.

— Keith Gollwitzer and Dan Kaplan