Fig. 1. Brightness comparison of different light sources^[12]

Fig. 2. Effect of carbon contamination in soft X-ray range. (a) Reflectivity curves under different carbon contamination thicknesses and grazing angles; (b) reflectivity curves of carbon contamination with different roughness

Fig. 3. Effect of carbon contamination in hard X-ray range. (a) Reflectivity curves under different carbon contamination thicknesses; (b) reflectivity curves of carbon contamination with different roughness

Fig. 4. Reflectivity curves of B₄C coating layer and carbon contamination layer at same grazing angle in soft X-ray band and hard X-ray range

Fig. 5. Carbon contamination of KB mirror at LCLS AMO end station^[24]

Fig. 6. Atomic force microscope scanning images of nanodots generated by FLASH optics irradiated by free-electron laser^[25]. (a) Scanning area size is 2 μm×2 μm; (b) scanning area size is 10 μm×10 μm

Fig. 7. Reflectivity curves of carbon contamination of B₄C under different roughness in hard X-ray range and soft X-ray range. (a) Hard X-ray range; (b) soft X-ray range

Fig. 8. S_R varying with height error

Fig. 9. Damage threshold and absorbed dose of different material particles

Fig. 10. Analysis of melting situation of different material particle contamination. (a) Power density at 1 MHz; (b) temperature at 10 kHz

Fig. 11. Schematic diagram of circular particle blocking

Fig. 12. Effect of particle blocking. (a) Blocking effect of particles with diameter of 50 μm in beam with full width at half maximum of 250 μm; (b) transmission change directly caused by blocking of particles with different sizes

Fig. 13. Effect of beam propagation after 100 m under situation of particle with diameter of 50 μm on optics. (a) Two-dimensional beam distribution; (b) profile of beam

Fig. 14. Speckle enhancement of beam propagation after 3.5 m under situation of particle with diameter of 50 μm on optics. (a) Two-dimensional beam distribution; (b) profile of beam

Fig. 15. Effect of beam propagation at different distances under situation of 100 particles with diameter of 5 μm on optics. (a) Two-dimensional beam distribution after 1.5 m propagation; (b) profile of beam after 1.5 m propagation; (c) two-dimensional beam distribution after 100 m propagation; (d) profile of beam after 100 m propagation

Fig. 16. Spot distribution and profile at focal position under situations of no particles and 100 particles with diameter of 5 μm on optics. (a) Spot distribution at focal position under situation of 100 particles with diameter of 5 μm on optics; (b) profile at focal position under situation of 100 particles with diameter of 5 μm on optics; (c) spot distribution at focal position under situation of no particles on optics; (d) profile at focal position under situation of no particles on optics

Fig. 17. Effect of particle contamination on spot before and after cleaning at SACLA^[38]. (a) Spot under situation of particle contamination; (b) spot after ultrasonic cleaning