A new technique for cooling antiprotons has been tested at CERN’s Antiproton Decelerator (AD), yielding 50 times more trapped antiprotons per cycle than ever before. Storing and cooling large samples of antiprotons is an important step towards achieving the physics goals of the experiments at the AD, which require the synthesis of exotic atoms such as antihydrogen (pbar e+) and protonium (p pbar).
Atoms, including exotic ones like these, can be efficiently synthesized only at chemical-energy scales (a few electron-volts or lower). This is many orders of magnitude below the energy scales needed for the production of antiprotons using an accelerator (a few giga-electron-volts). The AD reduces this gap by decelerating the 3 GeV antiprotons generated when the proton beam from the Proton Synchrotron hits an iridium target down to an energy of 5.3 MeV. This is still too high, however, for electromagnetic traps that can only capture antiprotons at the 10 keV range. Until now, thin “degrader” foils were used to slow antiprotons further, but the efficiency of such a system is low as many antiprotons stop and annihilate within the foils. Indeed, out of the 3 x 107 antiprotons ejected every 2 min in a 90 ns pulse (or “shot”) of the AD, only about 25,000 were retained.
Now a team from the Atomic Spectroscopy And Collisions Using Slow Antiprotons (ASACUSA) experiment and CERN have replaced these foils by a radio-frequency quadrupole decelerator (RFQD). This 4 m-long device can decelerate antiprotons to 10-120 keV. In the tests, the antiprotons passed from the RFQD into a standard multi-ring trap (MRT), as was the case with the earlier work with degrader foils. The trap is filled with an electron gas that helps to cool the antiprotons through thermal exchange, as the electron gas dissipates energy through the emission of synchrotron radiation.
During the tests, around 1.2 x 106 antiprotons per AD shot were stored in the MRT for 10 min or more. This is 50 times higher than the previous best values obtained with degrader foils and corresponds to an antiproton trapping efficiency of about 4%.