Design of a Cavity Beam Position Monitor for the FLASH 2020+ Undulator Intersection Project at DESY

The FLASH 1 beamline at DESY will be upgraded fromfixed to variable gap undulators in the next years. For this thevacuum beamline has to be adapted. This reduces the innerdiameter compared to the existing chamber. The vacuumcomponents should fit to the new dimension to minimizetransitions and therefore reduce wakefields which could interactwith the electron beam and disturb the SASE effect.The electron beam position in the intersection of the undulatorsshould be detected with a high resolution and a largecharge dynamic range. Cavity BPMs are known to fulfillthese requirements. The existing design with 10 mm innerdiameter for the European XFEL is reduced to 6 mm. Additionalimprovements are: widening of the dipole resonatorwaveguide to adapt to the dipole mode and antenna transmission.The resonator frequency of 3.3 GHz and loaded qualityfactor of 70 are maintained to use electronic synergies toother projects. The design considerations and simulationresults of the cavity BPM are presented