An intense beam of electrons is accelerated to a comparatively low energy (2.4 GeV) using conventional klystrons. This ‘drive beam’ is injected into a series of Power Extraction and Transfer Structures (PETS), which decelerate the dense beam and extract its energy. This energy is fed via an RF field in a waveguide to a second beam, which is much less intense. Since there are far fewer particles in this ‘main beam’, each one is accelerated to higher energy.