Abstract |
Devices for acceleration of electron
currents beyond 100 mA are becoming increasingly interesting for high
power FELs or for ERLs. To achieve photon beams of several hundreds of
kWs, 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 the accelerating sections are only possible if the
Higher Order Modes (HOM) generated by the beams can be sufficiently
damped. The positive experience with the HERA cavities [1], in which
the dominant monopole modes are damped to Qext about 700 and all dipole
modes to Qext < 6000 makes it highly likely that a superstructure
(SST) consisting out of two weakly coupled subunits and employing
coaxial HOM dampers of the DESY design can be successfully adapted to a
properly designed cavity for acceleration of a 1 A beam. This
contribution describes a 750 MHz SST design for a 1 A electron
beam. The calculated R/Q ? values of the HOMs of this SST are quite
favorable. The total impedance of the first 16 monopole modes is about
140 Ohm, app. a factor of 3 smaller than the impedance for 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 the achievable damping, a 1500 MHz Cu 1:2 model of the SST was
built and the Qext ? value of the dominant HOM?s were measured with
various HOM coupler configurations. As a result it can be concluded
with some confidence that the necessary damping for a 1A machine can be
achieved with the proposed superstructure configuration. However, in a
next step it is essential to repeat these measurements on a 1:1 model. |