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Negative ion source photo-neutralization has extremely high neutralizing efficiency (>80%) and wall-plug efficiency (>60%) theoretically, making it one of the most promising technologies for neutral beam injection (NBI) in magnetic confinement fusion in the future. Due to the small absorption cross-section of laser on plasma (mainly H-), the neutralization process requires a MW level laser flux; The increase in thickness of the laser target increases the volume of the neutralizer and the instability of the entire system; high power laser causes thermo-elastic deformation of the reflector and increases cavity loss. Based on the transmission characteristics and system layout of the NBI negative ion source, this paper designs a simplified resonant cavity containing multiple sets of mirrors to minimize the projection area of the entire optical path. Based on the diffraction theory of wave optics and the paraxial optical transmission matrix, the stability and laser transmission properties of this optical path are theoretically analyzed. Finally, the ray trajectory of the optical path is simulated using finite element software, and the deposited light power on the reflecting surface is calculated. The above results indicate the feasibility of the simplified resonant cavity and provide a theoretical basis for small-scale experiments of photo-neutralization.
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