3.9 GHz Cavity Module for Linear Bunch Compression at FLASH

3.9 GHz superconducting rf cavities have been developed at Fermilab as part of a DESY Fermilab collaboration.Four such 3.9 GHz cavities have been integrated into a module at Fermilab, shipped, and installed in the DESY FLASH XUV FEL at the downstream end of the 1.3 GHz injector module (ACC1).These cavities provide linearization correction to the sinusoidal accelerating field produced by the 1.3 GHz cavities in the injector module.This paper provides an overview of the cavity performance and discusses first operating experience at FLASH.