Hi, I am having trouble understanding the workings of a cyclotron. Supposing the typical design with two D shaped objects, changing potential differences between the D's, and a source of charges in the center, I can see that the particles would move in arcs (circles) at a constant energy. I just don't quite understand why they end up gaining energy, moving in a spiral of increasing radius, rather than a circle. Can someone help me to understand this? Thanks.