Simulation analysis and design of electron deflector for eXTP focusing telescope
Authors: Zheng Renzhou  , Qiang Pengfei  , Yang Yanji  , Yan Yongqing  , Li Yue  , Sheng Lizhi  , Chen Yong  ,   1 State Key Laboratory of Transient Optics and Photonics, Xi'an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi'an 710119, China;,   2 Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
Published: January 2025

X-ray focusing telescope is the core equipment for space X-ray observation. In order to ensure the accuracy of the observation results, it is necessary to deflect the low-energy electrons entering the focusing telescope effectively to reduce the background noise. In this paper, the electron deflector for enhanced X-ray Timing and Polarimetry Mission (eXTP) focusing telescope was developed to meet the deflection requirements of low-energy electrons in the focusing telescope optical system, and to take into account the lightweight, electron deflection ability and electromagnetic compatibility. The finite element analysis software COMSOL Multiphysics was used to establish the full physical simulation model of the electron deflector and focusing telescope mirrors. The magnetic flux density distribution, electron deflection trajectories and the effect of magnetic field on focusing telescope mirrors were analyzed, and the electromagnetic parameters of the electron deflector were designed. The simulation results show that the closer to the magnet and the center of electron deflector, the greater the magnetic flux density, and the maximum magnetic flux density in the middle of the two spokes can reach 0.027 T. When the radius is larger than 280 mm, the longitudinal distance is larger than 60 mm, the magnetic flux density is less than 5×10<sup>-5</sup> T (0.5Gs), that is, the geomagnetic intensity, which meets the design requirements of electromagnetic compatibility performance. When the incidence angle is ≤10°, the electron deflection efficiency decreases with the increase of electron energy and incidence angle, and the deflection efficiency of electrons below 50 keV energy can reach 100%, which meets the design requirements of electron deflection. In addition, as the focusing telescope mirrors are away from the electron deflector, the area of mirrors affected by the magnetic field becomes smaller and smaller. When the distance between the mirror bottom and electron deflector is 130 mm, the magnetic flux density at the mirror bottom only reaches 10<sup>-4</sup> T. Similarly, as the focusing telescope mirrors are away from the electron deflector, the stress at the mirror bottom decreases from 10<sup>3</sup> N/m<sup>2</sup> at 10 mm to 10<sup>-2</sup> N/m<sup>2</sup> at 60 mm, and the deformation at mirror bottom decreases from ~nm at 10 mm to 10<sup>-4</sup> nm at 60 mm. When the distance between the mirror bottom and electron deflector is 130 mm, the stress is only 10<sup>-3</sup> N/m<sup>2</sup>, and the deformation is only 10<sup>-5</sup> nm, indicating that the magnetic field does not affect the optical properties of the focusing telescope. The above simulation analyses show that the design parameters of NdFeB magnet structure of the electron deflector fully meet the requirements of the eXTP focusing telescope optical system for the deflection of low-energy electrons. And the deflection efficiency of electrons with 25 keV energy, incidence angle within ±5°, and deflection distance of 5250 mm is 100%. It provides an important reference for the development of electron deflector for eXTP focusing telescope.