Thermophysical and mechanical characteristics of beryllium are compared with the corresponding characteristics of promising materials that are used for fabrication of precision mirrors working under high-intensity electromagnetic irradiation. Advantages and prospects for application of beryllium in the third- and fourth-generation synchrotrons are discussed. An original method for fabrication of ultrasmooth surfaces of beryllium substrates is presented, and limiting roughnesses are reported. Reflectances at a wavelength of 13.5 nm are determined for a multilayer Mo/Si mirror deposited on the beryllium substrate. Prospects for improving quality of polishing of beryllium substrates are discussed.

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