A hybrid linear accelerator comprising: a source of charged particles configured to provide an input beam of charged particles; a standing wave linear accelerator section configured to receive the input beam of charged particles and accelerate the charged particles, the standing wave linear accelerator section providing an intermediate beam of accelerated charged particles; a travelling wave linear accelerator section configured to receive the intermediate beam of accelerated charged particles, and to further increase the momentum and energy of the intermediate beam of accelerated charged particles, the travelling wave linear accelerator section providing an output beam of charged particles; an RF source configured to provide RF power to the travelling wave linear accelerator section; a drift tube configured to provide a path for passage of the intermediate beam of accelerated charged particles from the standing wave linear accelerator section to the travelling wave linear accelerator section, the drift tube configured to RF decouple the standing wave linear accelerator section from the travelling wave linear accelerator section; a first RF waveguide; first RF coupler configured to couple an output of the standing wave linear accelerator section to an input of the drift tube; a second RF coupler configured to couple an output of the drift tube to an input of the travelling wave linear accelerator section, wherein the RF source is configured to provide RF power into the second RF coupler via the first RF waveguide, and wherein the second RF coupler is configured to couple the first RF waveguide to the travelling wave linear accelerator section; and a third RF coupler provided at an output of the travelling wave linear accelerator section, the accelerated charged particles passing through a first output of the third RF coupler; and a second RF waveguide having an input coupled to a second output of the third RF coupler and an output coupled to a further input of the first RF coupler and configured to feed RF power remaining after attenuation in the travelling wave linear accelerator section to the standing wave linear accelerator section to accelerate the charged particles. a source of charged particles configured to provide an input beam of charged particles; a standing wave linear accelerator section configured to receive the input beam of charged particles and accelerate the charged particles, the standing wave linear accelerator section providing an intermediate beam of accelerated charged particles; a travelling wave linear accelerator section configured to receive the intermediate beam of accelerated charged particles, and to further increase the momentum and energy of the intermediate beam of accelerated charged particles, the travelling wave linear accelerator section providing an output beam of charged particles; an RF source configured to provide RF power to the travelling wave linear accelerator section; a drift tube configured to provide a path for passage of the intermediate beam of accelerated charged particles from the standing wave linear accelerator section to the travelling wave linear accelerator section, the drift tube configured to RF decouple the standing wave linear accelerator section from the travelling wave linear accelerator section; a first RF waveguide; first RF coupler configured to couple an output of the standing wave linear accelerator section to an input of the drift tube; a second RF coupler configured to couple an output of the drift tube to an input of the travelling wave linear accelerator section, wherein the RF source is configured to provide RF power into the second RF coupler via the first RF waveguide, and wherein the second RF coupler is configured to couple the first RF waveguide to the travelling wave linear accelerator section; and a third RF coupler provided at an output of the travelling wave linear accelerator section, the accelerated charged particles passing through a first output of the third RF coupler; and a second RF waveguide having an input coupled to a second output of the third RF coupler and an output coupled to a further input of the first RF coupler and configured to feed RF power remaining after attenuation in the travelling wave linear accelerator section to the standing wave linear accelerator section to accelerate the charged particles.