During the first half of each alternating field cycle, the particles in the gaps between the drift tubes experience an accelerating field since they are repelled by the similarly charged tube they are leaving, and are attracted by the oppositely charged one they are approaching.
During the second half of the cycle, when the field is in the opposite direction and would decelerate them, the particles travel (or drift) through the centre of the tubes and are shielded from its effect. In this way the particles only ever experience accelerating electric fields.
As the particles travel down the linac, the drift tubes increase in length to directly compensate for the increase in speed as they must spend the same length of time drifting through each tube.
The main advantage of linear accelerators is that the particles are able to reach very high energies without the need for extremely high voltages.
The main disadvantage is that, because the particles travel in a straight line, each accelerating segment is used only once. This means that the only way of achieving particle beams with even higher energy is to undertake the expense of adding segments to the length of the linac.
At ISIS, a 50 metre linear accelerator is used to accelerate H– ions with an initial energy of 665 keV to an energy of 70 MeV.