Heat load measurements of XFEL cryomodules using helium evaporation method

The European XFEL Free Electron Laser (EuXFEL) at DESY is at 2K operation since January 2017. User operation with a maximum beam energy of 17,5 GeV began in September 2017. Studies are ongoing for a possible upgrade to operate XFEL at higher duty factors and lower cavity gradients. For this purpose, dynamic heat loads should be measured precisely to evaluate the cryomodules performances and the needed cooling capacity of refrigeration plant. This paper describes a heat loads measurement method based on measurements of the amount of helium evaporated from LHe II bath during a certain time period. A distinctive feature of this method is its insensitivity to eventual leaks across the seats of JT-valves. Furthermore, possible errors in the LHeII level readings can be minimized using this method. This paper summarizes the experience gained so far with this method at the EuXFEL linac and the CryoModule Test Bench (CMTB). Issues that have arisen during the measurements are discussed and conclusions are drawn. Results of the heat load measurements in CW mode are presented for a single cryomodule at CMTB.