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Solar activity is the source of variations in the solar-terrestrial space environment and disturbs the Earth's magnetic field in various forms. The corona is the most active region of the sun, and its impact on the earth and human beings is also the most intense. The temperature of the corona is 2~3 orders of magnitude higher than the temperature of the photosphere in its inner layer, and the reason how it heats has constantly been a mystery in the field of solar physics. Direct imaging observations of the corona are of great significance for the study of coronal mass ejection (CME) and coronal heating mechanisms. Measurement of the abundance of elemental helium, which is the largest contributor to the density of the coronal plasma next only to hydrogen, could further study the corona and understand the origin and acceleration mechanism of the solar wind sufficiently, however, implementing high-precision measurement of helium abundance on the global scale currently is a severe technical challenge. In this paper, the helium abundance of the sun served as the research object, comparing the international mainstream coronagraph models and basic optical architecture, designed an extreme ultraviolet external occulter (EO) coronagraph, which has a field of view of 1.4~4R☉, with a working wave of 30.4nm for observing helium abundance, aperture of 40mm, focal length of 420mm, and spatial resolution of 7.37arcsec.In this paper, the diffraction of stray light caused by the external occulter of the coronagraph system is studied. Aiming at the external occulter of a single disk and a triple disk, The uniform boundary wave theory is used to study the theory, and the simulation experiments are carried out. it is obtained that the external occulter of a triple disk can effectively reduce the diffraction intensity by 2 ~ 3 orders of magnitude compared with the external occulter of a single disk. On the basis that the external occulter is a triple disk, a method to suppress the diffracted stray light is proposed in this paper, by which the diffraction stray light is blocked by the feasible design of the position and size of the inner occulter (IO). The diffraction stray light formed by the aperture opening of the heat rejection mirror is also suppressed, to suppress the diffracted light of the coronagraph system effectively, which lays an excellent foundation for the prototype production of coronagraph and the experimental measurement of stray light in the immediate future.
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