Magnetic components of the HF field in a cyclotron can change the transverse beam size and the phase width of bunches during acceleration. As a consequence, the phase portraits of the beam at the entrance to the electrostatic deflector and thus the efficiency of the extraction system can change. Expressions are obtained for the components of the magnetic field from the HF system in the vicinity of the cyclotron median plane. The effect of this field on the beam parameters during acceleration and at the entrance to the electrostatic deflector is shown by numerical simulation. For the C235 proton cyclotron, the beam parameter variation is as large as 50%.
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