drift tube
Inside a drift tube linac, the particles are subjected to a regularly oscillating electrical field. They move through a series of tubes. Electric fields accelerate them as they move from one tube to the next. Each time the field changes direction, they move into the next tube. Protected in the tube from the change of field, they drift until the field oscillates once again. The length of each tube depends on the speed at which the particles travel through it and on the oscillation frequency. Thus, in the final stages the tubes are longer, as the particles are moving faster.