The Wien Filter

In a Wien filter an electric field is applied perpendicular to a magnetic field.

If we inject charged particles into this field such that they are perpendicular to the electric field and to the magnetic field they will feel a force:

F = q(E + (v×B))

For ions with a velocity, v0, given by:

v0 =  E


The net force on the ions will be zero and hence they will be unperturbed as they travel through the crossed fields. All other ions will feel a non-zero force and hence will be perturbed from their straight line path. By placing a slit such that only ions that feel zero force may pass through the crossed electric and magnetic fields they can be used as a velocity filter (known as the Wien velocity filter). If a mono-energetic ion beam is passed through a Wien filter the beam will be separated according to the ion velocities and hence their masses (E = [1/2]mv2)