When a charged particle is accelerated it emits energy in the form of electromagnetic waves. In a Penning trap, charged particles are on circular orbits, known as cyclotron orbits, around the magnetic fields lines. Because they are undergoing circular motion, they are constantly accelerating and therefore losing energy by emitting electromagnetic waves. This process is known as cyclotron cooling.
The characteristic cyclotron cooling time for electrons or positrons in a 1 T field is roughly 4 seconds, so this is a quick and effective cooling mechanism. Unfortunately, due to a much larger mass the characteristic cooling time of antiprotons (or protons) is hundreds of years so we must rely on other cooling mechanisms such as sympathetic cooling and evaporative cooling.