First studies for a low temperature higher-order-mode absorber for the Cornell ERL prototype

Cornell University, in collaboration with Jefferson Laboratory, has proposed the construction of a prototype energy-recovery linac (ERL) to study the energy recovery concept with high current, low emittance beams. The beam with a current of up to 100 mA will excite significant higher-order-mode (HOM) power in the superconducting (s.c.) RF cavities with frequencies up to 100 GHz. Strong damping of the HOMs is essential for beam stability and to reduce the HOM losses to a few hundred Watts per meter. To achieve this demanding goal we plan to place RF absorbing material in the beam tubes between the cavities in the linac. However, this will require operating the HOM absorbers at temperatures below 80 K to simplify the thermal transition to the cavities at 2 K with low static losses to 2 K. One possible material candidate is ferrite, as it is used at room temperature in the HOM absorbers in the s.c. CESR cavities. In this paper we present experiments performed to study the RF absorption properties of ferrite at cryogenic temperatures in the frequency range from 1 GHz to 15 GHz. First results are shown and the resulting HOM damping is evaluated and discussed.