R-F cavities are the preferred means of accelerating particles. Typically a few tens of centimetres in length, they use a standing wave whose frequency is set such that it gives particles an accelerating push as they pass through. For example, if a series of electron bunches are being accelerated then the sign of the wave will flip from positive to negative as the bunch passes through the cavity, returning to positive as the next bunch arrives. The size of a cavity is very important because it must be related to the frequency of the standing wave in order that there is an integer number of nodes (usually one) across it.
Click here to accelerate a particle through an R-F Cavity (thanks to Microcosm)
A very useful feature of R-F cavities is that the standing waves give an equal boost to electrons passing through the cavity in one direction as to positrons (anti-electrons) traveling through the other way. Positrons can be produced by the process of pair production. A high energy photon is absorbed in the electric field around an atomic nucleus and an electron - positron pair is created.