Waveguide for a linear accelerator and method of operating a linear accelerator

A waveguide (200, Fig. 2) for a linear accelerator (LINAC) is disclosed comprising: a plurality of accelerating cells 202 arranged to receive and accelerate a beam of charged particles, e.g. electrons, along a beam path (204); and at least one laterally displaced coupling cell 203 arranged to electromagnetically couple a pair of adjacent accelerating cells, wherein the coupling cell comprises a shorting pin 208 oriented substantially parallel to the beam path. A method of controlling a linear accelerator is also disclosed, said method comprising pausing the supply of an electromagnetic field to the waveguide and moving the shorting pin(s) into or out of the coupling cell/cavity before resuming supply of the electromagnetic field. Use of the shorting pin(s) in this way allows the final energy of the particle beam or, for example, x-rays generated therefrom to be varied. The LINAC may be used in a radiotherapy machine.