Faster-than-adiabatic cooling of non-neutral plasmas

Non-neutral plasmas can be confined for long times in electromagnetic traps. Adiabatic cooling of such plasmas can be achieved by slowly decreasing the effective trap stiffness and letting the plasma occupy a larger volume. Here, we propose an optimal control strategy to cool an antiproton plasma in a time much shorter than that required by an adiabatic process. The optimal cooling protocol is found by minimizing an appropriate cost functional that measures the distance of the system from the desired "cold" state, while keeping the stiffness amplitude within prescribed limits. The proposed optimal protocol is shown to outperform other expansion-based cooling methods in terms of stability of the plasma, duration of the process, and volume occupied by the final plasma.