Analysis of the impact of smoothing on residual error accuracy and high-precision figuring method based on spatial filtering in magnetorheological finishing for high-gradient CPP components

The large-aperture CPP is widely used in inertial confinement fusion systems. It is a key diffractive optical element to achieve beam shaping and smoothing. Successful ignition fusion has placed high requirements on its surface accuracy and gradient. The MRF used to imprint large-aperture CPP will introduce mid-spatial frequency errors, which will seriously affect the performance of CPP and the further convergence of subsequent residual errors. Smooth is often used to control it. However, when facing CPP with a larger gradient, the smoothing effect is not ideal. When smoothing and smoothing mid-spatial frequency errors, the accuracy of the low-frequency surface shape and the high-gradient characteristics are often destroyed, or the suppression of mid-spatial frequency errors is not obvious. At this stage, there is a lack of research on this part. First, this paper explains the limitations of smoothing. To study the influence of smoothing processes with polishing disks of different sizes on introducing mid-spatial frequency, experiments were conducted. The influence of smoothing on each frequency band is systematically analyzed, and it is confirmed that smoothing effect on mid-spatial frequency errors on large-aperture high-gradient CPPs is weakened or even ineffective. Second, a method based on spatial filtering in MRF is proposed and applied to the imprinting of large-aperture high-gradient CPPs, achieving a residual error RMS of 31 nm within the clear aperture. Compared to the 45 nm obtained using traditional methods, the precision is improved by over 30%. This study reveals the mechanism and limitations of the smoothing process on error control in various frequency bands of high-gradient CPP and verifies the effectiveness of what we believe to be is a newly proposed method. It also provides a reliable technical approach for the manufacture of large-aperture high-gradient CPP and solves the problems of fairing surfaces and the failure to suppress intermediate frequency errors and destruction of surface shape when facing high-gradient CPP.