**Figure 6.** Filter bandwidths using the (2,0)_{n} absorption profiles as functions of helium column density. For *n* = 2 (red) the two half-widths (dashed lines) are shown along with the full-width at half-maximum. Only the FWHMs are shown for *n* = 3 (blue), *n* = 4 (green), and *n* = 5 (purple). The dashed vertical lines show the maximum column density used in this work (68 **×** 10^{17} cm^{−2}) and the column density which would correspond to 2 m of gas at atmospheric pressure (5 **×** 10^{21} cm^{−2}). The solid black circles show the bandwidths obtained in the spectra plotted in figures 3 and 4.

**Abstract**

Using the third harmonic of the FEL radiation from the SPring-8 compact SASE (self-amplified stimulated emission) source SCSS we have studied the effects on SASE pulses with central wavelengths near 20 nm due to passage through a helium gas cell. The positions of zero ionization cross-section close to wavelengths corresponding to double-excitations allow operation as an efficient wavelength filter, with effectively 100% transmitted peak intensity until the Doppler-broadening limit is reached. We discuss how the time profile of the SASE pulses is affected, and discuss potential applications.