A beam of charged particles (e.g., an electron beam) from a charged particle source can be selectively applied to a pair of electrodes. For example, the charged particles can be electrons that are directed toward a first electrode when the charge difference between the electrodes is in one state and directed toward the second electrode when the charge difference between the electrodes is in another state. The electrodes are configured so that the beam of charged particles oscillates between the first and second electrodes.