An einzel lens is a charged particle lens that focuses without changing the energy of the beam. It consists of three or more sets of cylindrical or rectangular tubes in series along an axis. It is used in ion optics to focus ions in flight which is accomplished through manipulation of the electric field in the path of the ions.
The lens elements are symmetric so the ions will regain their initial energy on exiting the lens although the velocity of the outer particles will be altered such that they converge on to the axis. This causes the outer particles to arrive at the focus intersection slightly later than the ones that travel along a straight path as they have to travel an extra distance.
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