Fig. 1. Working principle of a free-electron laser

Fig. 2. Schematic diagram of energy modulation scheme

Fig. 3. Schematic diagram of current modulation scheme

Fig. 4. Schematic illustration of the ESASE experiment at LCLS^[47]

Fig. 5. Schematic diagram of slotted foil scheme

Fig. 6. Fresh-slice technique based on the transverse wakefields of a dechirper^[68]

Fig. 7. Schematic illustration of nonlinear bunch compression at LCLS^[78]

Fig. 8. Schematic diagram of the mode-locked FEL scheme proposed by Thompson et al.^[80]

Fig. 9. Schematic diagram of the mode-locked FEL scheme proposed by Dunning et al.^[81]

Fig. 10. Schematic diagram of attosecond soft X-ray cascade amplification scheme^[87]

Fig. 11. Schematic diagram of chirped microbunching scheme^[88]

Fig. 12. Comparison of various ultrafast XFEL pulse generation schemes. The blue markers represent hard X-ray generation schemes and magenta markers represent soft X-ray generation schemes. The orange filled markers indicate the schemes have been validated on FEL facilities. The shaded blue area indicates the parameter space that can be achieved currently

Table 1. Main parameters of the schemes presented in Fig.12

Table 2. A summary of various ultrafast XFEL pulse generation schemes