A charged particle moves perpendicular to a uniform magnetic field $B$ with speed $v$. The radius of its circular path is:

When $\vec{v} \perp \vec{B}$, the magnetic force $qvB$ provides the centripetal force: $\dfrac{mv^2}{r} = qvB \Rightarrow r = \dfrac{mv}{qB}$. This is the *cyclotron radius* (gyroradius). The corresponding angular frequency $\omega = \dfrac{qB}{m}$ is the cyclotron frequency, independent of speed — that's why cyclotron particle accelerators work: heavier or slower particles take longer paths but the period stays constant.