Development of Beam Position Monitor Using HOM Coupler

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Abeam position monitor was developed using HOM coupler attached to JAEA ERL-FEL, superconducting linac. A shifter magnet was designed and made to control the electron beam position. Analysis of electro-magnetic field with 3D code MAFIA was used to calculate the beam trajectory. The shifter magnet was installed at the upstream of the first main accelerator. HOM power was measured by changing the shifter magnet current The HOM power was found to be represented by the sum of the square of beam position (sum of the initial beam position and beam shift proportional to shifter magnet current) and the square of the beam position fluctuation. The measured HOM powers and shifter magnet currents were fitted with the analytic formula to estimate the beam position and beam position fluctuation. JAEA ERL-FEL was estimated to have the beam offset of 0.23 mm and -0.31mm and the beam position fluctuation of 0.21 mm and 0.60 mm in horizontal and vertical reprehensively. When the beam position fluctuates, HOM power is not zero even if the beam passes through the axis. Scanning the beam position enable to estimate the beam position and the beam position fluctuation precisely.