Up-scaling High Harmonic Generation in capillary waveguides based on increasing capillary diameter

The optical properties of EUV free-electron lasers can be improved by seeding the laser. Capillary waveguide based high-order harmonic generation (HHG) sources have shown to provide the required beam quality for seeding such lasers: ultrashort pulses of coherent light with extremely short wavelengths. Traditionally those systems cannot produce the high pulse energy required for seeding an FEL. Therefore, experimental research is conducted in increasing the capillary diameter to increase high harmonic power output. However, as the diameter is increased, gas load onto the UHV undulator of the FEL grows problematically. Also, the particle density and thus the EUV absorption behind the capillary is increased, reducing the power output. The solution of differential pumping via slits in the capillary wall is investigated in this report via experiments as well as a theoretical study and numerical analysis.