Relativistic Electron Cooling

Relativistic electron cooling of an antiproton beam has been demonstrated at Fermilab.

Increasing the density of antiprotons by reducing the spread in longitudinal speeds leads to a larger collision rate in particle colliders, producing more sought-after scattering events that contain rare particles and decays.

Antiprotons, made artificially by smashing protons into a metal target, must be collected on the fly and focused before they can be accelerated and collided with opposite-moving batches of protons; such proton-antiproton smashups are the premier activity at Fermilab's Tevatron facility.

The more compact and tightly focused the two beams are, the more desirable high-energy collision there will be. The degree of focus and beam density is expressed in a parameter called luminosity. To achieve interesting results it is desirable to have both high collision energy and high luminosity. Taming swarming antiprotons, however, is difficult. One would like all the antiprotons to be co-moving at the same velocity, but because of the way they're made in the first place, they will be flying at high speeds through a beam pipe with a variety of motions, both longitudinal and lateral. The lateral motions can be largely suppressed by a process called stochastic cooling, in which electric signals are dispatched to various electrodes stationed around the Fermilab's three antiproton storage rings; the electrodes offer minor kicks which serve to lower the lateral "temperature" of the swarm.

Reducing the spread in longitudinal speeds has been harder to accomplish, until now. In the new Fermilab process a continuous beam of electrons at an energy of 4.8 MeV is made to overlap with a beam of 8.9 GeV antiprotons which, because of their higher mass, move at the same speed as the electrons. The electron beam -- in effect an electrical current of 0.5 ampere and 2 megawatts -- removes some of the unwanted longitudinal velocity spread, increasing thereby the luminosity by a factor of 30 percent. Electron cooling of this kind has been used before but only with much lower-energy particle beams.

Nagaitsev et al., Physical Review Letters, 3 February 2006
Contact Sergei Nagaitsev, nsergei@fnal.gov
See Fermilab's Electron Cooling Web site