A High Power Positron Conversion Target for an Electron Linac

The target consists of a series of annular discs which, when held concentric with the beam, allow the beam to pass unobstructed through the central holes but which, when nutated intercept it to produce electron-positron pairs. Some of the positrons are focussed into the next accelerator section by a magnetic lens, and, a solenoidal field throughout the remainder of the accelerator which is phased for positron accelerator maintains the beam diameter. A novel design feature is that the same water is used to nutate the target via turbine blades and cool the target. Nutation permits bellows to make the vacuum joints so avoiding the use of moving vacuum seals which would be necessary with a rotating target. Target ´heat load´ is discussed with particular reference to the requirement for the Glasgow accelerator where the maximum electron beam energy is 110 MeV and the maximum mean power 30kW. The principle of nutation and the control system are described, and bellows fatigue, operational tests and safety systems are discussed.