Cooling antiprotons in an ion trap

The measurement of the inertial mass of the antiproton and proposed antihydrogen formation experiments require antiprotons stored in ion traps, cooled to very low (4K) temperatures. Techniques to cool the trapped antiprotons from energies around 1 keV are discussed. Coupling to an external circuit produces cooling times of order 10³ s, which may be reduced somewhat with negative feedback. Adiabatic reduction of the trapping potential produces significant cooling when the particle energies are substantially less than the well depth. Most promising is cooling via energy-transferring collisions to a cooled cloud of electrons simultaneously trapped with the antiprotons. Electron cooling times are of order 1 s, and strongly depend on electron number and density.