Antiprotons have been a highlight of close-of-the-century physics. CERN’s Antiproton Decelerator will continue this tradition into the 21st century.
CERN is best known for pushing the high-energy frontier of physics, but, with its new Antiproton Decelerator, the low-energy frontier is about to resume its place at the heart of the laboratory’s experimental programme.
The Antiproton Decelerator (AD) is scheduled to switch on for physics this month, and an important milestone was reached this summer when, for the first time, the AD team decelerated a beam of protons to the AD’s target momentum of just 100 MeV/c.
It might seem strange that milestones for the AD are measured in terms of achievements with protons, but, as Flemming Pedersen of the AD team explained, “We already know how to make antiprotons. The real challenge is low fields.”
With an AD beam momentum of just 100 MeV/c, the magnetic bending fields, which hold the beam in orbit, are so low that even the magnetic field of the Earth must be taken into account. Protons are used instead of antiprotons in the setting-up phase because higher intensities can be achieved, which make for easier diagnostics.
Decelerating the beam
Beam particles enter the AD with a momentum of 3.5 GeV/c, which is about 35 times as high as when they leave it. The beam is rapidly decelerated to 2000 MeV/c before undergoing stochastic cooling. Reducing the energy further is a delicate task, owing to the origin of the AD’s components. The AD is not a purpose-built machine it has been assembled using components from the Antiproton Collector, the job of which was to collect antiprotons at 3.5 GeV for CERN’s historic protonantiproton collider project of the 1980s.
Bending magnets that are designed for constant 3.5 GeV operation are not ideal for the AD, where the field is constantly cycled. In particular,eddy currents, provoked by changing the magnetic field in the AD, can become large, and these can disturb the beam. To avoid this problem the momentum is reduced from 550 MeV very slowly.
When the beam reaches 300 MeV/c there is another pause. This time the technique of electron cooling, better adapted to very low-energy beams, is used. Like the magnets,the electron cooler is recycled. It came from CERN’s previous low-energy antiproton facility, LEAR, which was the world’s second application of the technique pioneered by Gersh Budker at Novosibirsk in the late 1960s.
For the final approach to 100 MeV/c, the beam is slowed down again. Here the Earth’s magnetic field has to be considered, along with remanent fields induced in the AD’s metallic components.
The main challenge in reaching 100 MeV/c was to produce very stable power supplies that would control eddy currents in the magnets. Soon after the 100 MeV/c challenge had been met, decelerating to 100 MeV/c had become routine and the AD was shut down to allow physicists to install the three experiments that will start taking data with the new machine.
When work resumed in September, the first task was to consolidate what had already been achieved with protons and to add a further electron-cooling stage at 100 MeV/c before the beams are extracted for delivery to experiments. Next on the agenda was reversing the polarity of the bending magnets to handle antiprotons. Some further setting up is expected because, as Pedersen pointed out, “We can’t reverse the polarity of the Earth’s magnetic field!”
