There is no change in the energy of a charged particle moving in a magnetic field although a magnetic force is acting on it .

Explanation

The role of magnetic field in a cyclotron is that it essentially keeps the charged particle in the electric field for a longer period of time, so that it can experience a larger change in speed from the electric field, by forcing it in a spiral path. Without the magnetic field, the particle would have to move in a straight line through an electric field over a distance that is very large compared to the size of the cyclotron to attain the same speed.