What is the trajectory in your mind if there is an electron inside a vacuum with magnetic field?
Assume the energy of the electron is 1keV: try to calculate it's velocity.
Assume the magnetic field is 1kG(Gauss)=0.1T(Tesla) and velocity is perpendicular to the magnetic field.
: try to calculate it radius and frequency.
With the above calculated data: try to think of the motion of the electron.
If the velocity of the electron is not perpendicular: assume 1% of velocity is parallel to the magnetic field.
What do you think about the average motion of the electron will be look like?