Physics-poton acceleration

The main points of a linac are:
1. When the proton is in the each tube, it is kept at +ve potnetial by the alternating voltage cycle, so that the proton travels forward inside the tube at constant speed since the potential is uniform within the tube ;
2. When the proton leaves a tube and is in the gap between 2 tubes, the upcoming tube will be at -ve potential, thus accelerating the proton.
3. When the proton enters the tube, the alternating cycle will turn the tube to +ve potential again, thus repeating the process.
4. Hence, the proton is accelerated when it is at each gap between the tubes.
As the speed of the proton gets faster and faster, the distance it can travel between successive cycle of the alternating voltage become longer and longer. Thus tubes at the later part of the linac have to be made longer and longer.