Linear accelerator

Abstract

A linear accelerator is disclosed, having a series of interconnected cavities through at least some of which an rf signal and an electron beam are sent, comprising at least one variable coupler projecting into the a cavity of the series, a control apparatus adapted to interpret an electrical signal from the coupler and derive diagnostic information as to the electron beam therefrom, wherein the control apparatus is further adapted to vary the interaction of the at least one coupler with the rf signal in dependence on the diagnostic information. Thus, the accelerator properties can be adjusted by encouraging or inciting an Higher-Order Mode ('HOM') having a desired effect such as bunching and/or deflecting. The coupler could be rotateable, and partially or fully retractable, to allow its influence to be adjusted and/or for it to be removed from service when not needed. Several such probes could be available, approaching the cavity from different directions or at different locations, or approaching different cavities. The coupler can be asymmetric, in order to exert an appropriate influence on the cavity and provoke a useful HOM. For example, it can be elongate with at least one directional deviation, such as a hockey stick. Generally, however, the appropriate shape for the coupler will be dependent on the shape of the cavity with which it is working and the specific HOMs that are to be excited.