Basic research of beam instability with high current ERL beam by newly developed position monitor of superconducting cavities precisely

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Alignment of superconducting cavities is one of the important issues for next generation accelerator like energy recovery linac (ERL). To measure the cavity displacement under cooling to liquid He temperature more precisely, we newly developed the position monitor by using white light interferometer. This monitor is based on the measurement of the interference of light between the measurement target and the reference point. It can measure the position from the outside of the cryomodule. We applied this monitor to the main linac cryomodule at ERL test facility and successfully measured the displacement during 2K cooling with the resolution of +-5um. And by monitoring these displacements in detail, we achieved the energy recovery with 6uA CW beam.