On the thermal stability of multilayer optics for use with high X-ray intensities

High-intensity X-ray free electron laser (XFEL) beams require optics made of materials with minimal radiation absorption, high diffraction efficiency, and high radiation hardness. Multilayer Laue lenses (MLLs) are diffraction-based X-ray optics that can focus XFEL beams, as already demonstrated with tungsten carbide/silicon carbide (WC/SiC)-based MLLs. However, high atomic number materials such as tungsten strongly absorb X-rays, resulting in high heat loads. Numerical simulations predict much lower heat loads in MLLs consisting of low atomic number Z materials, although such MLLs have narrower rocking curve widths. In this paper, we first screen various multilayer candidates and then focus on Mo2C/SiC multilayer due to its high diffraction efficiency. According to numerical simulations, the maximum temperature in this multilayer should remain below 300°C if the MLL made out of this multilayer is exposed to an XFEL beam of 17.5 keV photon energy, 1 mJ energy per pulse and 10 kHz pulse repetition rate. To understand the thermal stability of the Mo2C/SiC multilayer, we performed a study on the multilayers of three different periods (1.5, 5, and 12 nm) and different Mo2C to SiC ratios. We monitored their periods, crystallinity, and stress as a function of annealing temperature for two different heating rates. The results presented in this paper indicate that Mo2C/SiC-based MLLs are viable for focusing XFEL beams without being damaged under these conditions.