Abstract: A method and apparatus for modulating at least one of energy and current of an electron beam in a linac for fast switching of particle beam energy on a time scale comparable with, and shorter than, the interval between linac pulses. Such modulation may be achieved by dividing, in a coupler, a radio-frequency (RF) field into field components and coherently adding these components in a phase shifting section to selectively direct the RF field to a chosen section of the linac. The phase shifting section may include at least one arm containing at least one fast switch and at least one phase changer. In specific embodiments, the phase shifting section may include an electronically controlled plasma switch and a plasma short.