Question

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

Answer 1

Charge particle moves inclined to the magnetic field. When there is an angle between velocity of charged particle and magnetic field, then the vertical component of velocity $\left(v\sin \theta \right)$ will rotate the charge particle on circular path, but horizontal component

$\left(v\cos \theta \right)$ will move the charged particle in straight line. Hence path of the charge particle becomes helical.