After catching antiprotons from the Antiproton Decelerator, they are still too energetic and hot (around 30 million Kelvin) to be used for antihydrogen formation, as we want the antihydrogen we produce to be as cold as possible so it is easier to trap. In order to cool them, they are mixed with electrons, which cool via emission of cyclotron radiation. In a Penning trap, charged particles are constantly travelling in small circles called cyclotron orbits. A circular path means the particles must be accelerating, and an accelerating charged particle emits radiation (and therefore loses energy) according to the Larmor formula.

In a Penning trap, the energy emitted in this way is inversely proportional to the square of the particle’s mass, which means that heavier particles cool via cyclotron emission much more slowly than lighter particles. Where a cloud of electrons might reach thermal equilibrium with the trap in tens of seconds due to their low mass, a cloud of antiprotons would take years. Since waiting years for our antiprotons to cool is impractical, we instead mix the two species together such that they can both benefit from the efficient cyclotron cooling of the electrons. This allows us to cool our antiprotons down to a few hundred Kelvin.