With the space charge forces considered, simulations of the evolution of th
e beam bunch in the K70 AVF cyclotron, which is an injector of the K540 RIK
EN Ring Cyclotron (RRC), are carried out. The result shows that the deforma
tion of the bunch and the formation of beam halo also take place in a cyclo
tron. But the mechanism of the formation of the beam halo in a cyclotron is
different from that in a linear accelerator.