What can be the cause of helical motion of charged particle ?

Explanation

A particle moves along a curved path if moves in the presence of magnetic field .
Now, when there is an angular projection, i.e., when the velocity of the particle is at an angle with respect to the magnetic field, then the particle will move in a helical path. the velocity component `(v sin theta)` perpendicular to the field will rotate the particle in a circular path. But, the component `(v cos theta)` along this field will move the particle in a striaght line path. so the cumulative motion of the particles is a helix.