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Beam halo experiment at IHEP* PENG Jun( ) 1) HUANG Tao( ) LIU Hua-Chang( ) JIANG Hong-Ping( ) LI Peng( ) LI Fang( ) LI Jian( ) LIU Mei-Fei( ) MU Zhen-Cheng( ) MENG Cai( ) MENG Ming( ) OUYANG Hua-Fu( ) RONG Lin-Yan( ) TIAN Jian-Min( ) WANG Biao( ) WANG Bo( ) XU Tao-Guang( ) XU Xin-An( ) YAO Yuan( ) XIN Wen-Qu( ) ZHAO Fu-Xiang( ) ZENG Lei( ) ZHOU Wen-Zhong( )

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Alignment of superconducting cavities is one of the important issues for the China Spallation Neutron Source Phase II (CSNS II) linac. In order to obtain the cavity displacement in the process of cooling down to the liquid helium temperature, a laser-based Poisson Spot Monitor (PSM) system was newly proposed and a verification system in the laboratory was built. The PSM system uses the diffraction spot formed on the CMOS camera after a beam of parallel laser passes through a spherical target to monitor the position of the measured object, where the spherical target is fixed on the monitored object. The displacement during the cooling process of the cryomodule is replaced by the movement of the high-precision two-axis motorized translation stage. A spherical target is fixed on the translation stage as the monitoring object. A beam of parallel laser passes through the spherical monitoring target to form a Poisson spot image on a CMOS camera. The coordinates of the Poisson spot center are obtained through image processing. Through experiments, the PSM system obtained a high accuracy within 5μm, which meets the displacement monitoring requirement of the CSNS II cryomodule components. The system is fairly simple and able to be constructed without highly specialized parts and can also be used in other high-precision alignment and monitoring fields.