Extreme-ultraviolet multilayer coatings with high spectral purity for solar imaging

Future solar experiments designed to perform solar plasma diagnostics will also be based on extreme-ultravilet observations. Multilayer (ML) optics are essential in this spectral region since these coatings have high reflectivity at normal incidence. Typically, the reflectivity curve of a ML coating has a small but finite bandwidth, and this can be a serious drawback when several spectral lines fall within the bandwidth. In fact, spectral lines emitted by different ion species can correspond to different plasma conditions. We present the design, realization, and characterization of an innovative ML structure with high reflectivity coupled with a strong rejection ratio for two adjacent spectral features. The key element is an optimized capping layer structure deposited on top of the ML that preserves the performance reflectance at the target wavelength and at the same time suppresses the reflectance at specific adjacent wavelengths. Application to the Fe xv3x10(6) K coronal emission line at 28.4 nm with rejection of the He ii Lyman-alpha line at 30.4 nm is presented.