Status of higher order mode beam position monitors in 3.9 GHz superconducting accelerating cavities at FLASH

Higher order mode (HOM) beam position monitors (BPM) are being developed for the 3.9 GHz third harmonic superconducting accelerating cavities at FLASH. The transverse beam position in a cavity can be determined utilizing beam-excited HOMs based on dipole components. The existing couplers used for HOM suppression provide necessary signals. The diagnostics principle is similar to a cavity BPM, but requires no additional vacuum instruments on the linac. The challenges of HOM-BPM for 3.9 GHz cavities lie in the dense HOM spectrum arising from the coupling of the majority HOMs amongst the four cavities in the cryo-module ACC39. HOMs with particularly promising diagnostics features were evaluated using a spectrum analyzer and custom-built test electronics with various data analysis techniques, data reduction was focused on. After careful theoretical and experimental assessment of the HOM spectrum, multi-cavity modes in the region of 5 GHz were chosen to provide a global position over the complete module with superior resolution (~20 um) while trapped modes in the 9 GHz region provide local position in each cavity with comparable resolution (~50 um). A similar HOM-BPM system has been planned for the European XFEL 3.9 GHz module which encompasses eight cavities. This paper reviews both the current status and the future prospects of HOM-BPMs in 3.9 GHz cavities at FLASH.