Highlights of the XM-3 Cryomodule Tests at DESY

To investigate the feasibility of the continuous wave (CW) upgrade [1] of the European X-ray free electron laser (XFEL), on-going tests are performed on XFEL prototype and production cryomodules since 2011. For these studies, DESY’s cryomodule test bench (CMTB) has been equipped with a 105 kW CW capable inductive output tube (IOT) in addition to the 10 MW pulsed klystron, making CMTB a very flexible test stand, enabling both CW and pulsed operation. XFEL-like low level radio frequency (LLRF) electronics is used for these tests to stabilize amplitude and phase of the voltage vector sum (VS) of all 8 cavities of the cryomodule under test. The cryomodule most often tested is the pre-series XM-3, unique since it is housing one fine grain niobium and seven large grain niobium cavities. Modified flanges were installed in autumn 2017 on all 8 input couplers to increase the maximum reachable loaded quality factor (QL) beyond 2E7. With higher QL, up to 6E7 for 6 cavities and 3E7 for 2 cavities, we have investigated the VS stability and SRF-performance of this cryomodule under various conditions of cooling down rate and operation temperature 1.65K, 1.8K and 2K, at gradients up to 21.5 MV/m. The results of these tests are presented in this paper.