Development of Superstructures for High Current Application

Devices for acceleration of electron currents beyond 100 mA are becoming increasingly interesting for high power Free Electron Lasers (FEL) or for Energy Recovering Linacs (ERL). To achieve photon beams of several hundred kW, low emittance electron beams of up to 1 A have to be delivered to undulators from a driving linear accelerator. High quality beams and stable operation of accelerating sections are only possible if Higher Order Modes (HOM) generated by the beams can be sufficiently damped. The positive experience with the HERA 4-cell cavities [1], in which the dominant monopole modes are damped to Qext ∼ 700 and all dipole modes to Qext < 6000 makes it highly likely that a superstructure (SST) consisting of two weakly coupled subunits and employing coaxial HOM dampers of the DESY type can be successfully adapted to a properly designed cavity for acceleration of a ∼1 A beam. This contribution describes the first approach to design a 750 MHz SST for a 1 A electron beam. The calculate d R/Q values of the HOM's of this SST are quite favorable. The total impedance of the first 16 monopole modes is ∼ 140, approximately a factor of 3 smaller than the impedance of the fundamental mode. It seems very likely that the HOM's can be suppressed to the appropriate levels for stable beam operation. In order to explore achievable damping, a 1500 MHz copper 1:2 model of the SST was built and the Qext-values of the dominant HOM's were measured with various HOM coupler configurations. It can be concluded with some confidence that the necessary damping for a 1 A machine can be achieved with the proposed superstructure configuration. However, it is essential to repeat these measurements on a 1:1 model