Question

what is the purpose of using magnetic field in cyclotron

Answer 1

Using perpendicular magnetic field and moderate electric field makes the charge particle to rotate..

Answer 2

Magnetic fields are used to apply a force a right angle to the direction of motion of the charged particle, thus giving it a centripetal force so travels in a circular paths at constant speed. Use r=p/bq. as speed goes up so does momentum, b and q are constant so radius goes up, hence spiralling outwardly